[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <SN6PR12MB2702AC3C27D25414FE4260F994D4A@SN6PR12MB2702.namprd12.prod.outlook.com>
Date: Thu, 19 Oct 2023 14:21:40 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Borislav Petkov <bp@...en8.de>,
Josh Poimboeuf <jpoimboe@...nel.org>
CC: 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
[AMD Official Use Only - General]
> -----Original Message-----
> From: Borislav Petkov <bp@...en8.de>
> Sent: Thursday, October 19, 2023 9:15 AM
> To: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Ingo Molnar <mingo@...nel.org>; linux-kernel@...r.kernel.org; linux-tip-
> commits@...r.kernel.org; Kaplan, David <David.Kaplan@....com>; Peter
> Zijlstra (Intel) <peterz@...radead.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
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Wed, Oct 18, 2023 at 11:59:28PM -0700, Josh Poimboeuf wrote:
> > One last idea, since the return thunk is used everywhere (even non-ABI
> > compliant functions) it might be possible the "call check_thunks" (and
> > its call to warn_printk) is clobbering some registers which some code
> > (exception handling entry code?) doesn't appreciate.
>
> Yeah, that is still unclean, I'd say. gcc doesn't know that we patch in a CALL insn
> in the alternative. What should work is to have
>
> alternative_call
>
> there which alternates between two calls and gcc knows there's a call so it can
> act accordingly wrt callee-* regs.
>
> Considering how __x86_return_thunk is there only until alternatives have run,
> we could do something like
>
> ALTERNATIVE_CALL nop, check_thunks
>
> where nop is a function which doesn't do anything.
>
> I say "ALTERNATIVE_CALL" because we don't have a _CALL asm macro yet.
>
> And then in check_thunks() we can do all kinds of screaming, tainting and
> setting mitigation status to vulnerable, etc.
>
> Anyway something along those lines.
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.
--David Kaplan
Powered by blists - more mailing lists