[AMD Official Use Only - General] > -----Original Message----- > From: Borislav Petkov <bp@xxxxxxxxx> > Sent: Thursday, October 19, 2023 9:15 AM > To: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-tip- > commits@xxxxxxxxxxxxxxx; Kaplan, David <David.Kaplan@xxxxxxx>; Peter > Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>; x86@xxxxxxxxxx; David Howells > <dhowells@xxxxxxxxxx> > 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
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