Re: [PATCH] x86/retpoline: Fix a missing return thunk warning (was: Re: [linus:master] [x86/bugs] 4535e1a417: WARNING:at_arch/x86/kernel/alternative.c:#apply_returns)

From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kernel test robot <oliver.sang@intel.com>,
	oe-lkp@lists.linux.dev, lkp@intel.com,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] x86/retpoline: Fix a missing return thunk warning (was: Re: [linus:master] [x86/bugs] 4535e1a417: WARNING:at_arch/x86/kernel/alternative.c:#apply_returns)
Date: Wed, 3 Apr 2024 19:05:34 +0200	[thread overview]
Message-ID: <20240403170534.GHZg2MXmwFRv-x8usY@fat_crate.local> (raw)
In-Reply-To: <CAHk-=wj+Q_LXV0Y5+kBvv-5sTxT3Y7E=8wJ2sX4vzWksd3LWzA@mail.gmail.com>

On Wed, Apr 03, 2024 at 09:45:25AM -0700, Linus Torvalds wrote:
> On Wed, 3 Apr 2024 at 05:24, Borislav Petkov <bp@alien8.de> wrote:
> >
> > Subject: [PATCH] x86/retpoline: Fix a missing return thunk warning
> 
> Thanks, applied directly,

Can you pls replace it with the below one?

There's more breakage:

https://bugzilla.kernel.org/show_bug.cgi?id=218679

and I think it needs an __EXPORT_THUNK too.

Yeah, I know. Damn ifdeffery. :-\

I've changed it:

https://bugzilla.kernel.org/attachment.cgi?id=306084&action=diff

and this should be the correct fix(tm). Famous last words.

Thx.

---
From ca5b34498f997cba9c24d450f7ccab4b37377560 Mon Sep 17 00:00:00 2001
From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Tue, 2 Apr 2024 16:05:49 +0200
Subject: [PATCH] x86/retpoline: Do the necessary fixup to the Zen3/4 srso
 return thunk for !SRSO

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@atlas.cz, https://bugzilla.kernel.org/show_bug.cgi?id=218679.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202404020901.da75a60f-oliver.sang@intel.com
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/all/202404020901.da75a60f-oliver.sang@intel.com/
---
 arch/x86/lib/retpoline.S | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 02cde194a99e..0795b3464058 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -228,8 +228,11 @@ SYM_CODE_END(srso_return_thunk)
 #else /* !CONFIG_MITIGATION_SRSO */
 /* Dummy for the alternative in CALL_UNTRAIN_RET. */
 SYM_CODE_START(srso_alias_untrain_ret)
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_FUNC_END(srso_alias_untrain_ret)
+__EXPORT_THUNK(srso_alias_untrain_ret)
 #define JMP_SRSO_UNTRAIN_RET "ud2"
 #endif /* CONFIG_MITIGATION_SRSO */
 
-- 
2.43.0


-- 
Regards/Gruss,
    Boris.

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