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Message-ID: <20230621084802.GA2048237@hirez.programming.kicks-ass.net>
Date: Wed, 21 Jun 2023 10:48:02 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <keescook@...omium.org>
Cc: x86@...nel.org, alyssa.milburn@...ux.intel.com,
linux-kernel@...r.kernel.org, samitolvanen@...gle.com,
jpoimboe@...nel.org, joao@...rdrivepizza.com,
tim.c.chen@...ux.intel.com
Subject: Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0
On Wed, Jun 21, 2023 at 10:18:57AM +0200, Peter Zijlstra wrote:
> (and I need to write a better Changelog).
Updated changelog...
---
Subject: x86/fineibt: Poison ENDBR at +0
From: Peter Zijlstra <peterz@...radead.org>
Date: Thu, 15 Jun 2023 21:35:48 +0200
Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
booting on IBT enabled hardware 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@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>
Acked-by: Kees Cook <keescook@...omium.org>
Link: https://lore.kernel.org/r/20230615193722.194131053@infradead.org
---
arch/x86/kernel/alternative.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1063,6 +1063,17 @@ static int cfi_rewrite_preamble(s32 *sta
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)
{
@@ -1157,14 +1168,19 @@ static void __apply_fineibt(s32 *start_r
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;
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