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authorPeter Zijlstra <peterz@infradead.org>2023-06-15 22:35:48 +0300
committerPeter Zijlstra <peterz@infradead.org>2023-07-10 10:52:25 +0300
commit04505bbbbb15da950ea0239e328a76a3ad2376e0 (patch)
treebd85fe01301497d709a8d33ad012e960310c57ea /arch/x86
parent3aec4ecb3d1f313a8ab985df7cab07c4af81f478 (diff)
downloadlinux-04505bbbbb15da950ea0239e328a76a3ad2376e0.tar.xz
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/x86')
-rw-r--r--arch/x86/kernel/alternative.c16
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;