6.18-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
commit 9935df5333aa503a18de5071f53762b65c783c4c upstream.
Reject attempts to disable KVM_MEM_GUEST_MEMFD on a memslot that was initially created with a guest_memfd binding, as KVM doesn't support toggling KVM_MEM_GUEST_MEMFD on existing memslots. KVM prevents enabling KVM_MEM_GUEST_MEMFD, but doesn't prevent clearing the flag.
Failure to reject the new memslot results in a use-after-free due to KVM not unbinding from the guest_memfd instance. Unbinding on a FLAGS_ONLY change is easy enough, and can/will be done as a hardening measure (in anticipation of KVM supporting dirty logging on guest_memfd at some point), but fixing the use-after-free would only address the immediate symptom.
================================================================== BUG: KASAN: slab-use-after-free in kvm_gmem_release+0x362/0x400 [kvm] Write of size 8 at addr ffff8881111ae908 by task repro/745
CPU: 7 UID: 1000 PID: 745 Comm: repro Not tainted 6.18.0-rc6-115d5de2eef3-next-kasan #3 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 Call Trace: <TASK> dump_stack_lvl+0x51/0x60 print_report+0xcb/0x5c0 kasan_report+0xb4/0xe0 kvm_gmem_release+0x362/0x400 [kvm] __fput+0x2fa/0x9d0 task_work_run+0x12c/0x200 do_exit+0x6ae/0x2100 do_group_exit+0xa8/0x230 __x64_sys_exit_group+0x3a/0x50 x64_sys_call+0x737/0x740 do_syscall_64+0x5b/0x900 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f581f2eac31 </TASK>
Allocated by task 745 on cpu 6 at 9.746971s: kasan_save_stack+0x20/0x40 kasan_save_track+0x13/0x50 __kasan_kmalloc+0x77/0x90 kvm_set_memory_region.part.0+0x652/0x1110 [kvm] kvm_vm_ioctl+0x14b0/0x3290 [kvm] __x64_sys_ioctl+0x129/0x1a0 do_syscall_64+0x5b/0x900 entry_SYSCALL_64_after_hwframe+0x4b/0x53
Freed by task 745 on cpu 6 at 9.747467s: kasan_save_stack+0x20/0x40 kasan_save_track+0x13/0x50 __kasan_save_free_info+0x37/0x50 __kasan_slab_free+0x3b/0x60 kfree+0xf5/0x440 kvm_set_memslot+0x3c2/0x1160 [kvm] kvm_set_memory_region.part.0+0x86a/0x1110 [kvm] kvm_vm_ioctl+0x14b0/0x3290 [kvm] __x64_sys_ioctl+0x129/0x1a0 do_syscall_64+0x5b/0x900 entry_SYSCALL_64_after_hwframe+0x4b/0x53
Reported-by: Alexander Potapenko glider@google.com Fixes: a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20251202020334.1171351-2-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- virt/kvm/kvm_main.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2085,7 +2085,7 @@ static int kvm_set_memory_region(struct return -EINVAL; if ((mem->userspace_addr != old->userspace_addr) || (npages != old->npages) || - ((mem->flags ^ old->flags) & KVM_MEM_READONLY)) + ((mem->flags ^ old->flags) & (KVM_MEM_READONLY | KVM_MEM_GUEST_MEMFD))) return -EINVAL;
if (base_gfn != old->base_gfn)