Subject: [tip: x86/bugs] x86/alternatives: Remove faulty optimization

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID: eae1a2bb7dd350195c18f1788f5d687684d8d92d
Author: Josh Poimboeuf <[email protected]>
AuthorDate: Mon, 04 Sep 2023 22:04:54 -07:00
Committer: Ingo Molnar <[email protected]>
CommitterDate: Tue, 19 Sep 2023 11:42:47 +02:00

x86/alternatives: Remove faulty optimization

The following commit

095b8303f383 ("x86/alternative: Make custom return thunk unconditional")

made '__x86_return_thunk' a placeholder value. All code setting
X86_FEATURE_RETHUNK also changes the value of 'x86_return_thunk'. So
the optimization at the beginning of apply_returns() is dead code.

Also, before the above-mentioned commit, the optimization actually had a
bug It bypassed __static_call_fixup(), causing some raw returns to
remain unpatched in static call trampolines. Thus the 'Fixes' tag.

Fixes: d2408e043e72 ("x86/alternative: Optimize returns patching")
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Borislav Petkov (AMD) <[email protected]>
Link:[email protected]
arch/x86/kernel/alternative.c | 8 --------
1 file changed, 8 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index a5ead6a..c850f5a 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -720,14 +720,6 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
s32 *s;

- /*
- * Do not patch out the default return thunks if those needed are the
- * ones generated by the compiler.
- */
- if (cpu_feature_enabled(X86_FEATURE_RETHUNK) &&
- (x86_return_thunk == __x86_return_thunk))
- return;
for (s = start; s < end; s++) {
void *dest = NULL, *addr = (void *)s + *s;
struct insn insn;