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@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/16d19d2249d4485d8380fb215ffaae81e6b8119e.1693889988.git.jpoimboe@kernel.org

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index a5ead6a6d233b2ef4223d5ba54579a69f3042f6f..c850f5a9b1bb318ac6766ed8eec69bbed6cc3a57 100644 (file)
--- 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;