FAILED: patch "[PATCH] KVM: SVM: Don't skip unrelated instruction if INT3/INTO is" failed to apply to 6.1-stable tree
The patch below does not apply to the 6.1-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y git checkout FETCH_HEAD git cherry-pick -x 4da3768e1820cf15cced390242d8789aed34f54d # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025120802-remedy-glimmer-fc9d@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 4da3768e1820cf15cced390242d8789aed34f54d Mon Sep 17 00:00:00 2001 From: Omar Sandoval osandov@fb.com Date: Tue, 4 Nov 2025 09:55:26 -0800 Subject: [PATCH] KVM: SVM: Don't skip unrelated instruction if INT3/INTO is replaced
When re-injecting a soft interrupt from an INT3, INT0, or (select) INTn instruction, discard the exception and retry the instruction if the code stream is changed (e.g. by a different vCPU) between when the CPU executes the instruction and when KVM decodes the instruction to get the next RIP.
As effectively predicted by commit 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction"), failure to verify that the correct INTn instruction was decoded can effectively clobber guest state due to decoding the wrong instruction and thus specifying the wrong next RIP.
The bug most often manifests as "Oops: int3" panics on static branch checks in Linux guests. Enabling or disabling a static branch in Linux uses the kernel's "text poke" code patching mechanism. To modify code while other CPUs may be executing that code, Linux (temporarily) replaces the first byte of the original instruction with an int3 (opcode 0xcc), then patches in the new code stream except for the first byte, and finally replaces the int3 with the first byte of the new code stream. If a CPU hits the int3, i.e. executes the code while it's being modified, then the guest kernel must look up the RIP to determine how to handle the #BP, e.g. by emulating the new instruction. If the RIP is incorrect, then this lookup fails and the guest kernel panics.
The bug reproduces almost instantly by hacking the guest kernel to repeatedly check a static branch[1] while running a drgn script[2] on the host to constantly swap out the memory containing the guest's TSS.
[1]: https://gist.github.com/osandov/44d17c51c28c0ac998ea0334edf90b5a [2]: https://gist.github.com/osandov/10e45e45afa29b11e0c7209247afc00b
Fixes: 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction") Cc: stable@vger.kernel.org Co-developed-by: Sean Christopherson seanjc@google.com Signed-off-by: Omar Sandoval osandov@fb.com Link: https://patch.msgid.link/1cc6dcdf36e3add7ee7c8d90ad58414eeb6c3d34.1762278762... Signed-off-by: Sean Christopherson seanjc@google.com
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 48598d017d6f..974d64bf0a4d 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -2143,6 +2143,11 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); * the gfn, i.e. retrying the instruction will hit a * !PRESENT fault, which results in a new shadow page * and sends KVM back to square one. + * + * EMULTYPE_SKIP_SOFT_INT - Set in combination with EMULTYPE_SKIP to only skip + * an instruction if it could generate a given software + * interrupt, which must be encoded via + * EMULTYPE_SET_SOFT_INT_VECTOR(). */ #define EMULTYPE_NO_DECODE (1 << 0) #define EMULTYPE_TRAP_UD (1 << 1) @@ -2153,6 +2158,10 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); #define EMULTYPE_PF (1 << 6) #define EMULTYPE_COMPLETE_USER_EXIT (1 << 7) #define EMULTYPE_WRITE_PF_TO_SP (1 << 8) +#define EMULTYPE_SKIP_SOFT_INT (1 << 9) + +#define EMULTYPE_SET_SOFT_INT_VECTOR(v) ((u32)((v) & 0xff) << 16) +#define EMULTYPE_GET_SOFT_INT_VECTOR(e) (((e) >> 16) & 0xff)
static inline bool kvm_can_emulate_event_vectoring(int emul_type) { diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 1ae7b3c5a7c5..459db8154929 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -272,6 +272,7 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask) }
static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, + int emul_type, bool commit_side_effects) { struct vcpu_svm *svm = to_svm(vcpu); @@ -293,7 +294,7 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, if (unlikely(!commit_side_effects)) old_rflags = svm->vmcb->save.rflags;
- if (!kvm_emulate_instruction(vcpu, EMULTYPE_SKIP)) + if (!kvm_emulate_instruction(vcpu, emul_type)) return 0;
if (unlikely(!commit_side_effects)) @@ -311,11 +312,13 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
static int svm_skip_emulated_instruction(struct kvm_vcpu *vcpu) { - return __svm_skip_emulated_instruction(vcpu, true); + return __svm_skip_emulated_instruction(vcpu, EMULTYPE_SKIP, true); }
-static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu) +static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu, u8 vector) { + const int emul_type = EMULTYPE_SKIP | EMULTYPE_SKIP_SOFT_INT | + EMULTYPE_SET_SOFT_INT_VECTOR(vector); unsigned long rip, old_rip = kvm_rip_read(vcpu); struct vcpu_svm *svm = to_svm(vcpu);
@@ -331,7 +334,7 @@ static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu) * in use, the skip must not commit any side effects such as clearing * the interrupt shadow or RFLAGS.RF. */ - if (!__svm_skip_emulated_instruction(vcpu, !nrips)) + if (!__svm_skip_emulated_instruction(vcpu, emul_type, !nrips)) return -EIO;
rip = kvm_rip_read(vcpu); @@ -367,7 +370,7 @@ static void svm_inject_exception(struct kvm_vcpu *vcpu) kvm_deliver_exception_payload(vcpu, ex);
if (kvm_exception_is_soft(ex->vector) && - svm_update_soft_interrupt_rip(vcpu)) + svm_update_soft_interrupt_rip(vcpu, ex->vector)) return;
svm->vmcb->control.event_inj = ex->vector @@ -3642,11 +3645,12 @@ static bool svm_set_vnmi_pending(struct kvm_vcpu *vcpu)
static void svm_inject_irq(struct kvm_vcpu *vcpu, bool reinjected) { + struct kvm_queued_interrupt *intr = &vcpu->arch.interrupt; struct vcpu_svm *svm = to_svm(vcpu); u32 type;
- if (vcpu->arch.interrupt.soft) { - if (svm_update_soft_interrupt_rip(vcpu)) + if (intr->soft) { + if (svm_update_soft_interrupt_rip(vcpu, intr->nr)) return;
type = SVM_EVTINJ_TYPE_SOFT; @@ -3654,12 +3658,10 @@ static void svm_inject_irq(struct kvm_vcpu *vcpu, bool reinjected) type = SVM_EVTINJ_TYPE_INTR; }
- trace_kvm_inj_virq(vcpu->arch.interrupt.nr, - vcpu->arch.interrupt.soft, reinjected); + trace_kvm_inj_virq(intr->nr, intr->soft, reinjected); ++vcpu->stat.irq_injections;
- svm->vmcb->control.event_inj = vcpu->arch.interrupt.nr | - SVM_EVTINJ_VALID | type; + svm->vmcb->control.event_inj = intr->nr | SVM_EVTINJ_VALID | type; }
void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode, diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 42ecd093bb4c..8e14c0292809 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -9338,6 +9338,23 @@ static bool is_vmware_backdoor_opcode(struct x86_emulate_ctxt *ctxt) return false; }
+static bool is_soft_int_instruction(struct x86_emulate_ctxt *ctxt, + int emulation_type) +{ + u8 vector = EMULTYPE_GET_SOFT_INT_VECTOR(emulation_type); + + switch (ctxt->b) { + case 0xcc: + return vector == BP_VECTOR; + case 0xcd: + return vector == ctxt->src.val; + case 0xce: + return vector == OF_VECTOR; + default: + return false; + } +} + /* * Decode an instruction for emulation. The caller is responsible for handling * code breakpoints. Note, manually detecting code breakpoints is unnecessary @@ -9448,6 +9465,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, * injecting single-step #DBs. */ if (emulation_type & EMULTYPE_SKIP) { + if (emulation_type & EMULTYPE_SKIP_SOFT_INT && + !is_soft_int_instruction(ctxt, emulation_type)) + return 0; + if (ctxt->mode != X86EMUL_MODE_PROT64) ctxt->eip = (u32)ctxt->_eip; else
From: Sean Christopherson seanjc@google.com
[ Upstream commit 258d985f6eb360c9c7aacd025d0dbc080a59423f ]
Use a new EMULTYPE flag, EMULTYPE_WRITE_PF_TO_SP, to track page faults on self-changing writes to shadowed page tables instead of propagating that information to the emulator via a semi-persistent vCPU flag. Using a flag in "struct kvm_vcpu_arch" is confusing, especially as implemented, as it's not at all obvious that clearing the flag only when emulation actually occurs is correct.
E.g. if KVM sets the flag and then retries the fault without ever getting to the emulator, the flag will be left set for future calls into the emulator. But because the flag is consumed if and only if both EMULTYPE_PF and EMULTYPE_ALLOW_RETRY_PF are set, and because EMULTYPE_ALLOW_RETRY_PF is deliberately not set for direct MMUs, emulated MMIO, or while L2 is active, KVM avoids false positives on a stale flag since FNAME(page_fault) is guaranteed to be run and refresh the flag before it's ultimately consumed by the tail end of reexecute_instruction().
Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20230202182817.407394-2-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Stable-dep-of: 4da3768e1820 ("KVM: SVM: Don't skip unrelated instruction if INT3/INTO is replaced") Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/include/asm/kvm_host.h | 37 ++++++++++++++++++--------------- arch/x86/kvm/mmu/mmu.c | 5 +++-- arch/x86/kvm/mmu/mmu_internal.h | 12 ++++++++++- arch/x86/kvm/mmu/paging_tmpl.h | 4 +--- arch/x86/kvm/x86.c | 15 ++----------- 5 files changed, 37 insertions(+), 36 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index d0229323ca634..dd7b4c5f5436c 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -890,23 +890,6 @@ struct kvm_vcpu_arch {
u64 msr_kvm_poll_control;
- /* - * Indicates the guest is trying to write a gfn that contains one or - * more of the PTEs used to translate the write itself, i.e. the access - * is changing its own translation in the guest page tables. KVM exits - * to userspace if emulation of the faulting instruction fails and this - * flag is set, as KVM cannot make forward progress. - * - * If emulation fails for a write to guest page tables, KVM unprotects - * (zaps) the shadow page for the target gfn and resumes the guest to - * retry the non-emulatable instruction (on hardware). Unprotecting the - * gfn doesn't allow forward progress for a self-changing access because - * doing so also zaps the translation for the gfn, i.e. retrying the - * instruction will hit a !PRESENT fault, which results in a new shadow - * page and sends KVM back to square one. - */ - bool write_fault_to_shadow_pgtable; - /* set at EPT violation at this point */ unsigned long exit_qualification;
@@ -1825,6 +1808,25 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); * EMULTYPE_COMPLETE_USER_EXIT - Set when the emulator should update interruptibility * state and inject single-step #DBs after skipping * an instruction (after completing userspace I/O). + * + * EMULTYPE_WRITE_PF_TO_SP - Set when emulating an intercepted page fault that + * is attempting to write a gfn that contains one or + * more of the PTEs used to translate the write itself, + * and the owning page table is being shadowed by KVM. + * If emulation of the faulting instruction fails and + * this flag is set, KVM will exit to userspace instead + * of retrying emulation as KVM cannot make forward + * progress. + * + * If emulation fails for a write to guest page tables, + * KVM unprotects (zaps) the shadow page for the target + * gfn and resumes the guest to retry the non-emulatable + * instruction (on hardware). Unprotecting the gfn + * doesn't allow forward progress for a self-changing + * access because doing so also zaps the translation for + * the gfn, i.e. retrying the instruction will hit a + * !PRESENT fault, which results in a new shadow page + * and sends KVM back to square one. */ #define EMULTYPE_NO_DECODE (1 << 0) #define EMULTYPE_TRAP_UD (1 << 1) @@ -1834,6 +1836,7 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); #define EMULTYPE_VMWARE_GP (1 << 5) #define EMULTYPE_PF (1 << 6) #define EMULTYPE_COMPLETE_USER_EXIT (1 << 7) +#define EMULTYPE_WRITE_PF_TO_SP (1 << 8)
int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type); int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu, diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 2fa130c4c1734..04d060f370535 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -4152,7 +4152,7 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work) work->arch.cr3 != kvm_mmu_get_guest_pgd(vcpu, vcpu->arch.mmu)) return;
- kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true); + kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true, NULL); }
static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) @@ -5580,7 +5580,8 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
if (r == RET_PF_INVALID) { r = kvm_mmu_do_page_fault(vcpu, cr2_or_gpa, - lower_32_bits(error_code), false); + lower_32_bits(error_code), false, + &emulation_type); if (KVM_BUG_ON(r == RET_PF_INVALID, vcpu->kvm)) return -EIO; } diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h index 0a9d5f2925c33..5e4be3bb3624c 100644 --- a/arch/x86/kvm/mmu/mmu_internal.h +++ b/arch/x86/kvm/mmu/mmu_internal.h @@ -223,6 +223,13 @@ struct kvm_page_fault { kvm_pfn_t pfn; hva_t hva; bool map_writable; + + /* + * Indicates the guest is trying to write a gfn that contains one or + * more of the PTEs used to translate the write itself, i.e. the access + * is changing its own translation in the guest page tables. + */ + bool write_fault_to_shadow_pgtable; };
int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault); @@ -256,7 +263,7 @@ enum { };
static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, - u32 err, bool prefetch) + u32 err, bool prefetch, int *emulation_type) { struct kvm_page_fault fault = { .addr = cr2_or_gpa, @@ -290,6 +297,9 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, else r = vcpu->arch.mmu->page_fault(vcpu, &fault);
+ if (fault.write_fault_to_shadow_pgtable && emulation_type) + *emulation_type |= EMULTYPE_WRITE_PF_TO_SP; + /* * Similar to above, prefetch faults aren't truly spurious, and the * async #PF path doesn't do emulation. Do count faults that are fixed diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index 685560a45bf60..1c7d73b8081c6 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -829,10 +829,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault if (r) return r;
- vcpu->arch.write_fault_to_shadow_pgtable = false; - is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu, - &walker, fault->user, &vcpu->arch.write_fault_to_shadow_pgtable); + &walker, fault->user, &fault->write_fault_to_shadow_pgtable);
if (is_self_change_mapping) fault->max_level = PG_LEVEL_4K; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index b349e1fbf9dbc..562ca89ceb638 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8527,7 +8527,6 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type) }
static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, - bool write_fault_to_shadow_pgtable, int emulation_type) { gpa_t gpa = cr2_or_gpa; @@ -8598,7 +8597,7 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, * be fixed by unprotecting shadow page and it should * be reported to userspace. */ - return !write_fault_to_shadow_pgtable; + return !(emulation_type & EMULTYPE_WRITE_PF_TO_SP); }
static bool retry_instruction(struct x86_emulate_ctxt *ctxt, @@ -8869,20 +8868,12 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, int r; struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt; bool writeback = true; - bool write_fault_to_spt;
if (unlikely(!kvm_can_emulate_insn(vcpu, emulation_type, insn, insn_len))) return 1;
vcpu->arch.l1tf_flush_l1d = true;
- /* - * Clear write_fault_to_shadow_pgtable here to ensure it is - * never reused. - */ - write_fault_to_spt = vcpu->arch.write_fault_to_shadow_pgtable; - vcpu->arch.write_fault_to_shadow_pgtable = false; - if (!(emulation_type & EMULTYPE_NO_DECODE)) { kvm_clear_exception_queue(vcpu);
@@ -8903,7 +8894,6 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, return 1; } if (reexecute_instruction(vcpu, cr2_or_gpa, - write_fault_to_spt, emulation_type)) return 1;
@@ -8989,8 +8979,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, return 1;
if (r == EMULATION_FAILED) { - if (reexecute_instruction(vcpu, cr2_or_gpa, write_fault_to_spt, - emulation_type)) + if (reexecute_instruction(vcpu, cr2_or_gpa, emulation_type)) return 1;
return handle_emulation_failure(vcpu, emulation_type);
From: Omar Sandoval osandov@fb.com
[ Upstream commit 4da3768e1820cf15cced390242d8789aed34f54d ]
When re-injecting a soft interrupt from an INT3, INT0, or (select) INTn instruction, discard the exception and retry the instruction if the code stream is changed (e.g. by a different vCPU) between when the CPU executes the instruction and when KVM decodes the instruction to get the next RIP.
As effectively predicted by commit 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction"), failure to verify that the correct INTn instruction was decoded can effectively clobber guest state due to decoding the wrong instruction and thus specifying the wrong next RIP.
The bug most often manifests as "Oops: int3" panics on static branch checks in Linux guests. Enabling or disabling a static branch in Linux uses the kernel's "text poke" code patching mechanism. To modify code while other CPUs may be executing that code, Linux (temporarily) replaces the first byte of the original instruction with an int3 (opcode 0xcc), then patches in the new code stream except for the first byte, and finally replaces the int3 with the first byte of the new code stream. If a CPU hits the int3, i.e. executes the code while it's being modified, then the guest kernel must look up the RIP to determine how to handle the #BP, e.g. by emulating the new instruction. If the RIP is incorrect, then this lookup fails and the guest kernel panics.
The bug reproduces almost instantly by hacking the guest kernel to repeatedly check a static branch[1] while running a drgn script[2] on the host to constantly swap out the memory containing the guest's TSS.
[1]: https://gist.github.com/osandov/44d17c51c28c0ac998ea0334edf90b5a [2]: https://gist.github.com/osandov/10e45e45afa29b11e0c7209247afc00b
Fixes: 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction") Cc: stable@vger.kernel.org Co-developed-by: Sean Christopherson seanjc@google.com Signed-off-by: Omar Sandoval osandov@fb.com Link: https://patch.msgid.link/1cc6dcdf36e3add7ee7c8d90ad58414eeb6c3d34.1762278762... Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/include/asm/kvm_host.h | 9 +++++++++ arch/x86/kvm/svm/svm.c | 24 +++++++++++++----------- arch/x86/kvm/x86.c | 21 +++++++++++++++++++++ 3 files changed, 43 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index dd7b4c5f5436c..fe5c0f86ae389 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1827,6 +1827,11 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); * the gfn, i.e. retrying the instruction will hit a * !PRESENT fault, which results in a new shadow page * and sends KVM back to square one. + * + * EMULTYPE_SKIP_SOFT_INT - Set in combination with EMULTYPE_SKIP to only skip + * an instruction if it could generate a given software + * interrupt, which must be encoded via + * EMULTYPE_SET_SOFT_INT_VECTOR(). */ #define EMULTYPE_NO_DECODE (1 << 0) #define EMULTYPE_TRAP_UD (1 << 1) @@ -1837,6 +1842,10 @@ u64 vcpu_tsc_khz(struct kvm_vcpu *vcpu); #define EMULTYPE_PF (1 << 6) #define EMULTYPE_COMPLETE_USER_EXIT (1 << 7) #define EMULTYPE_WRITE_PF_TO_SP (1 << 8) +#define EMULTYPE_SKIP_SOFT_INT (1 << 9) + +#define EMULTYPE_SET_SOFT_INT_VECTOR(v) ((u32)((v) & 0xff) << 16) +#define EMULTYPE_GET_SOFT_INT_VECTOR(e) (((e) >> 16) & 0xff)
int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type); int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu, diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 00fb1c18e23a8..a2706b49ffa3d 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -370,6 +370,7 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type, void *insn, int insn_len);
static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, + int emul_type, bool commit_side_effects) { struct vcpu_svm *svm = to_svm(vcpu); @@ -399,7 +400,7 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu, if (unlikely(!commit_side_effects)) old_rflags = svm->vmcb->save.rflags;
- if (!kvm_emulate_instruction(vcpu, EMULTYPE_SKIP)) + if (!kvm_emulate_instruction(vcpu, emul_type)) return 0;
if (unlikely(!commit_side_effects)) @@ -417,11 +418,13 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
static int svm_skip_emulated_instruction(struct kvm_vcpu *vcpu) { - return __svm_skip_emulated_instruction(vcpu, true); + return __svm_skip_emulated_instruction(vcpu, EMULTYPE_SKIP, true); }
-static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu) +static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu, u8 vector) { + const int emul_type = EMULTYPE_SKIP | EMULTYPE_SKIP_SOFT_INT | + EMULTYPE_SET_SOFT_INT_VECTOR(vector); unsigned long rip, old_rip = kvm_rip_read(vcpu); struct vcpu_svm *svm = to_svm(vcpu);
@@ -437,7 +440,7 @@ static int svm_update_soft_interrupt_rip(struct kvm_vcpu *vcpu) * in use, the skip must not commit any side effects such as clearing * the interrupt shadow or RFLAGS.RF. */ - if (!__svm_skip_emulated_instruction(vcpu, !nrips)) + if (!__svm_skip_emulated_instruction(vcpu, emul_type, !nrips)) return -EIO;
rip = kvm_rip_read(vcpu); @@ -473,7 +476,7 @@ static void svm_inject_exception(struct kvm_vcpu *vcpu) kvm_deliver_exception_payload(vcpu, ex);
if (kvm_exception_is_soft(ex->vector) && - svm_update_soft_interrupt_rip(vcpu)) + svm_update_soft_interrupt_rip(vcpu, ex->vector)) return;
svm->vmcb->control.event_inj = ex->vector @@ -3523,11 +3526,12 @@ static void svm_inject_nmi(struct kvm_vcpu *vcpu)
static void svm_inject_irq(struct kvm_vcpu *vcpu, bool reinjected) { + struct kvm_queued_interrupt *intr = &vcpu->arch.interrupt; struct vcpu_svm *svm = to_svm(vcpu); u32 type;
- if (vcpu->arch.interrupt.soft) { - if (svm_update_soft_interrupt_rip(vcpu)) + if (intr->soft) { + if (svm_update_soft_interrupt_rip(vcpu, intr->nr)) return;
type = SVM_EVTINJ_TYPE_SOFT; @@ -3535,12 +3539,10 @@ static void svm_inject_irq(struct kvm_vcpu *vcpu, bool reinjected) type = SVM_EVTINJ_TYPE_INTR; }
- trace_kvm_inj_virq(vcpu->arch.interrupt.nr, - vcpu->arch.interrupt.soft, reinjected); + trace_kvm_inj_virq(intr->nr, intr->soft, reinjected); ++vcpu->stat.irq_injections;
- svm->vmcb->control.event_inj = vcpu->arch.interrupt.nr | - SVM_EVTINJ_VALID | type; + svm->vmcb->control.event_inj = intr->nr | SVM_EVTINJ_VALID | type; }
void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode, diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 562ca89ceb638..355c3e52f68cb 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8835,6 +8835,23 @@ static bool is_vmware_backdoor_opcode(struct x86_emulate_ctxt *ctxt) return false; }
+static bool is_soft_int_instruction(struct x86_emulate_ctxt *ctxt, + int emulation_type) +{ + u8 vector = EMULTYPE_GET_SOFT_INT_VECTOR(emulation_type); + + switch (ctxt->b) { + case 0xcc: + return vector == BP_VECTOR; + case 0xcd: + return vector == ctxt->src.val; + case 0xce: + return vector == OF_VECTOR; + default: + return false; + } +} + /* * Decode an instruction for emulation. The caller is responsible for handling * code breakpoints. Note, manually detecting code breakpoints is unnecessary @@ -8925,6 +8942,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, * injecting single-step #DBs. */ if (emulation_type & EMULTYPE_SKIP) { + if (emulation_type & EMULTYPE_SKIP_SOFT_INT && + !is_soft_int_instruction(ctxt, emulation_type)) + return 0; + if (ctxt->mode != X86EMUL_MODE_PROT64) ctxt->eip = (u32)ctxt->_eip; else
On Mon, Dec 08, 2025, Sasha Levin wrote:
From: Sean Christopherson seanjc@google.com
[ Upstream commit 258d985f6eb360c9c7aacd025d0dbc080a59423f ]
Use a new EMULTYPE flag, EMULTYPE_WRITE_PF_TO_SP, to track page faults on self-changing writes to shadowed page tables instead of propagating that information to the emulator via a semi-persistent vCPU flag. Using a flag in "struct kvm_vcpu_arch" is confusing, especially as implemented, as it's not at all obvious that clearing the flag only when emulation actually occurs is correct.
E.g. if KVM sets the flag and then retries the fault without ever getting to the emulator, the flag will be left set for future calls into the emulator. But because the flag is consumed if and only if both EMULTYPE_PF and EMULTYPE_ALLOW_RETRY_PF are set, and because EMULTYPE_ALLOW_RETRY_PF is deliberately not set for direct MMUs, emulated MMIO, or while L2 is active, KVM avoids false positives on a stale flag since FNAME(page_fault) is guaranteed to be run and refresh the flag before it's ultimately consumed by the tail end of reexecute_instruction().
Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20230202182817.407394-2-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Stable-dep-of: 4da3768e1820 ("KVM: SVM: Don't skip unrelated instruction if INT3/INTO is replaced") Signed-off-by: Sasha Levin sashal@kernel.org
Acked-by: Sean Christopherson seanjc@google.com
On Mon, Dec 08, 2025, Sasha Levin wrote:
From: Omar Sandoval osandov@fb.com
[ Upstream commit 4da3768e1820cf15cced390242d8789aed34f54d ]
When re-injecting a soft interrupt from an INT3, INT0, or (select) INTn instruction, discard the exception and retry the instruction if the code stream is changed (e.g. by a different vCPU) between when the CPU executes the instruction and when KVM decodes the instruction to get the next RIP.
As effectively predicted by commit 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction"), failure to verify that the correct INTn instruction was decoded can effectively clobber guest state due to decoding the wrong instruction and thus specifying the wrong next RIP.
The bug most often manifests as "Oops: int3" panics on static branch checks in Linux guests. Enabling or disabling a static branch in Linux uses the kernel's "text poke" code patching mechanism. To modify code while other CPUs may be executing that code, Linux (temporarily) replaces the first byte of the original instruction with an int3 (opcode 0xcc), then patches in the new code stream except for the first byte, and finally replaces the int3 with the first byte of the new code stream. If a CPU hits the int3, i.e. executes the code while it's being modified, then the guest kernel must look up the RIP to determine how to handle the #BP, e.g. by emulating the new instruction. If the RIP is incorrect, then this lookup fails and the guest kernel panics.
The bug reproduces almost instantly by hacking the guest kernel to repeatedly check a static branch[1] while running a drgn script[2] on the host to constantly swap out the memory containing the guest's TSS.
Fixes: 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction") Cc: stable@vger.kernel.org Co-developed-by: Sean Christopherson seanjc@google.com Signed-off-by: Omar Sandoval osandov@fb.com Link: https://patch.msgid.link/1cc6dcdf36e3add7ee7c8d90ad58414eeb6c3d34.1762278762... Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Sasha Levin sashal@kernel.org
Acked-by: Sean Christopherson seanjc@google.com
linux-stable-mirror@lists.linaro.org
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gregkh＠linuxfoundation.org -
Sasha Levin -
Sean Christopherson