linux-kernel - [tip: x86/bugs] x86/alternatives: Remove faulty optimization

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Message-ID: <169390857696.27769.15598358510708203877.tip-bot2@tip-bot2>
Date:   Tue, 05 Sep 2023 10:09:36 -0000
From:   "tip-bot2 for Josh Poimboeuf" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/bugs] x86/alternatives: Remove faulty optimization

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

Commit-ID:     85d5fbfb7deaf3a5b4666e741bb72a68866439e6
Gitweb:        https://git.kernel.org/tip/85d5fbfb7deaf3a5b4666e741bb72a68866439e6
Author:        Josh Poimboeuf <jpoimboe@...nel.org>
AuthorDate:    Mon, 04 Sep 2023 22:04:54 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 05 Sep 2023 12:05:07 +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 <jpoimboe@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/16d19d2249d4485d8380fb215ffaae81e6b8119e.1693889988.git.jpoimboe@kernel.org
---
 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;