diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2023-06-15 21:35:48 +0200 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2023-07-10 09:52:25 +0200 |
| commit | 04505bbbbb15da950ea0239e328a76a3ad2376e0 (patch) | |
| tree | bd85fe01301497d709a8d33ad012e960310c57ea /arch | |
| parent | 3aec4ecb3d1f313a8ab985df7cab07c4af81f478 (diff) | |
x86/fineibt: Poison ENDBR at +0
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
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kernel/alternative.c | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 04b25a275233..d77aaabf9fe8 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -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; |