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x86/fineibt: Poison ENDBR at +0

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Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
booting on IBT enabled hardware to obtain FineIBT, the indirect
functions look like:

  __cfi_foo:
	endbr64
	subl	$hash, %r10d
	jz	1f
	ud2
	nop
  1:
  foo:
	endbr64

This is because the compiler generates code for kCFI+IBT. In that case
the caller does the hash check and will jump to +0, so there must be
an ENDBR there. The compiler doesn't know about FineIBT at all; also
it is possible to actually use kCFI+IBT when booting with 'cfi=kcfi'
on IBT enabled hardware.

Having this second ENDBR however makes it possible to elide the CFI
check. Therefore, we should poison this second ENDBR when switching to
FineIBT mode.

Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
Reported-by: "Milburn, Alyssa" <alyssa.milburn@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20230615193722.194131053@infradead.org
This commit is contained in:
Peter Zijlstra 2023-06-15 21:35:48 +02:00
parent 3aec4ecb3d
commit 04505bbbbb
1 changed files with 16 additions and 0 deletions
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16
arch/x86/kernel/alternative.c
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@ -1068,6 +1068,17 @@ static int cfi_rewrite_preamble(s32 *start, s32 *end)
return 0;
}
static void cfi_rewrite_endbr(s32 *start, s32 *end)
{
s32 *s;
for (s = start; s < end; s++) {
void *addr = (void *)s + *s;
poison_endbr(addr+16, false);
}
}
/* .retpoline_sites */
static int cfi_rand_callers(s32 *start, s32 *end)
{
@ -1162,14 +1173,19 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
return;
case CFI_FINEIBT:
/* place the FineIBT preamble at func()-16 */
ret = cfi_rewrite_preamble(start_cfi, end_cfi);
if (ret)
goto err;
/* rewrite the callers to target func()-16 */
ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
if (ret)
goto err;
/* now that nobody targets func()+0, remove ENDBR there */
cfi_rewrite_endbr(start_cfi, end_cfi);
if (builtin)
pr_info("Using FineIBT CFI\n");
return;