[PATCH 2/2] x86/retpoline: Do the necessary fixup to the Zen3/4 srso return thunk for !SRSO

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From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Date: Tue, 2 Apr 2024 16:05:49 +0200

Commit 0e110732473e14d6520e49d75d2c88ef7d46fe67 upstream.

The srso_alias_untrain_ret() dummy thunk in the !CONFIG_MITIGATION_SRSO
case is there only for the altenative in CALL_UNTRAIN_RET to have
a symbol to resolve.

However, testing with kernels which don't have CONFIG_MITIGATION_SRSO
enabled, leads to the warning in patch_return() to fire:

  missing return thunk: srso_alias_untrain_ret+0x0/0x10-0x0: eb 0e 66 66 2e
  WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:826 apply_returns (arch/x86/kernel/alternative.c:826

Put in a plain "ret" there so that gcc doesn't put a return thunk in
in its place which special and gets checked.

In addition:

  ERROR: modpost: "srso_alias_untrain_ret" [arch/x86/kvm/kvm-amd.ko] undefined!
  make[2]: *** [scripts/Makefile.modpost:145: Module.symvers] Chyba 1
  make[1]: *** [/usr/src/linux-6.8.3/Makefile:1873: modpost] Chyba 2
  make: *** [Makefile:240: __sub-make] Chyba 2

since !SRSO builds would use the dummy return thunk as reported by
petr.pisar@xxxxxxxx, https://bugzilla.kernel.org/show_bug.cgi?id=218679.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202404020901.da75a60f-oliver.sang@xxxxxxxxx
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/all/202404020901.da75a60f-oliver.sang@xxxxxxxxx/
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 arch/x86/lib/retpoline.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 24c850e1e239..a96e816e5ccd 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -110,6 +110,7 @@ SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
 	ret
 	int3
 SYM_FUNC_END(srso_alias_untrain_ret)
+__EXPORT_THUNK(srso_alias_untrain_ret)
 #endif
 
 SYM_START(srso_alias_safe_ret, SYM_L_GLOBAL, SYM_A_NONE)
-- 
2.43.0


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette




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