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Message-ID: <20231019143951.GEZTE/t/wECKBxMSjl@fat_crate.local>
Date: Thu, 19 Oct 2023 16:39:51 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Kaplan, David" <David.Kaplan@....com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
Ingo Molnar <mingo@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-tip-commits@...r.kernel.org"
<linux-tip-commits@...r.kernel.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
"x86@...nel.org" <x86@...nel.org>,
David Howells <dhowells@...hat.com>
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
used at runtime
On Thu, Oct 19, 2023 at 02:21:40PM +0000, Kaplan, David wrote:
> The return thunk is used for all functions though, including assembly
> coded functions which may use non-standard calling conventions and
> aren't visible to gcc. I think the only safe thing would be to
> preserve all GPRs across the call to check_thunks. Something like
> PUSH_REGS/call check_thunks/POP_REGS.
That call nop will be inside the return thunk. I.e., something like
this:
SYM_CODE_START(__x86_return_thunk)
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
ANNOTATE_UNRET_SAFE
ALTERNATIVE CALL nop, check_thunks, X86_FEATURE_ALWAYS
ret
int3
SYM_CODE_END(__x86_return_thunk)
EXPORT_SYMBOL(__x86_return_thunk)
I suspect that gcc doesn't know that there is a function call in the asm
there, which is also what you hint at - I need to ask a compiler guy.
But yeah, if it doesn't, then we'll need to push/pop regs as you
suggest.
--
Regards/Gruss,
Boris.
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