From: Josh Poimboeuf jpoimboe@kernel.org
[ Upstream commit b388e57d4628eb22782bdad4cd5b83ca87a1b7c9 ]
For CONFIG_RETHUNK kernels, objtool annotates all the function return sites so they can be patched during boot. By design, after apply_returns() is called, all tail-calls to the compiler-generated default return thunk (__x86_return_thunk) should be patched out and replaced with whatever's needed for any mitigations (or lack thereof).
The commit
4461438a8405 ("x86/retpoline: Ensure default return thunk isn't used at runtime")
adds a runtime check and a WARN_ONCE() if the default return thunk ever gets executed after alternatives have been applied. This warning is a sanity check to make sure objtool and apply_returns() are doing their job.
As Nathan reported, that check found something:
Unpatched return thunk in use. This should not happen! WARNING: CPU: 0 PID: 1 at arch/x86/kernel/cpu/bugs.c:2856 __warn_thunk+0x27/0x40 RIP: 0010:__warn_thunk+0x27/0x40 Call Trace: <TASK> ? show_regs ? __warn ? __warn_thunk ? report_bug ? console_unlock ? handle_bug ? exc_invalid_op ? asm_exc_invalid_op ? ia32_binfmt_init ? __warn_thunk warn_thunk_thunk do_one_initcall kernel_init_freeable ? __pfx_kernel_init kernel_init ret_from_fork ? __pfx_kernel_init ret_from_fork_asm </TASK>
Boris debugged to find that the unpatched return site was in init_vdso_image_64(), and its translation unit wasn't being analyzed by objtool, so it never got annotated. So it got ignored by apply_returns().
This is only a minor issue, as this function is only called during boot. Still, objtool needs full visibility to the kernel. Fix it by enabling objtool on vdso-image-{32,64}.o.
Note this problem can only be seen with !CONFIG_X86_KERNEL_IBT, as that requires objtool to run individually on all translation units rather on vmlinux.o.
[ bp: Massage commit message. ]
Reported-by: Nathan Chancellor nathan@kernel.org Signed-off-by: Josh Poimboeuf jpoimboe@kernel.org Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Link: https://lore.kernel.org/r/20240215032049.GA3944823@dev-arch.thelio-3990X Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/entry/vdso/Makefile | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile index b1b8dd1608f7e..4ee59121b9053 100644 --- a/arch/x86/entry/vdso/Makefile +++ b/arch/x86/entry/vdso/Makefile @@ -34,8 +34,12 @@ obj-y += vma.o extable.o KASAN_SANITIZE_vma.o := y UBSAN_SANITIZE_vma.o := y KCSAN_SANITIZE_vma.o := y -OBJECT_FILES_NON_STANDARD_vma.o := n -OBJECT_FILES_NON_STANDARD_extable.o := n + +OBJECT_FILES_NON_STANDARD_extable.o := n +OBJECT_FILES_NON_STANDARD_vdso-image-32.o := n +OBJECT_FILES_NON_STANDARD_vdso-image-64.o := n +OBJECT_FILES_NON_STANDARD_vdso32-setup.o := n +OBJECT_FILES_NON_STANDARD_vma.o := n
# vDSO images to build vdso_img-$(VDSO64-y) += 64 @@ -43,7 +47,6 @@ vdso_img-$(VDSOX32-y) += x32 vdso_img-$(VDSO32-y) += 32
obj-$(VDSO32-y) += vdso32-setup.o -OBJECT_FILES_NON_STANDARD_vdso32-setup.o := n
vobjs := $(foreach F,$(vobjs-y),$(obj)/$F) vobjs32 := $(foreach F,$(vobjs32-y),$(obj)/$F)