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x86/vdso: Fix rethunk patching for vdso-image-{32,64}.o

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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

  4461438a84 ("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
This commit is contained in:
Josh Poimboeuf 2024-02-19 21:57:18 -08:00 committed by Borislav Petkov (AMD)
parent 4589f199eb
commit b388e57d46
1 changed files with 6 additions and 3 deletions
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7
arch/x86/entry/vdso/Makefile
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@ -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_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)