Subject: Re: [PATCH v3 10/23] arm64: mte: Handle synchronous and asynchronous tag check faults

From: Dave Martin <[email-masked]>
Date: Mon, 27 Apr 2020 17:58:22 +0100

On Tue, Apr 21, 2020 at 03:25:50PM +0100, Catalin Marinas wrote:
> From: Vincenzo Frascino <[email-masked]>
> 
> The Memory Tagging Extension has two modes of notifying a tag check
> fault at EL0, configurable through the SCTLR_EL1.TCF0 field:
> 
> 1. Synchronous raising of a Data Abort exception with DFSC 17.
> 2. Asynchronous setting of a cumulative bit in TFSRE0_EL1.
> 
> Add the exception handler for the synchronous exception and handling of
> the asynchronous TFSRE0_EL1.TF0 bit setting via a new TIF flag in
> do_notify_resume().
> 
> On a tag check failure in user-space, whether synchronous or
> asynchronous, a SIGSEGV will be raised on the faulting thread.

Has there been any discussion on whether this should be SIGSEGV or
SIGBUS?

Probably neither is much more appropriate than the other.

> Signed-off-by: Vincenzo Frascino <[email-masked]>
> Co-developed-by: Catalin Marinas <[email-masked]>
> Signed-off-by: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>

[...]

> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 339882db5a91..e377d77c065e 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -732,6 +732,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
>  	regs->regs[29] = (unsigned long)&user->next_frame->fp;
>  	regs->pc = (unsigned long)ka->sa.sa_handler;
>  
> +	/* TCO (Tag Check Override) always cleared for signal handlers */
> +	regs->pstate &= ~PSR_TCO_BIT;
> +
>  	if (ka->sa.sa_flags & SA_RESTORER)
>  		sigtramp = ka->sa.sa_restorer;
>  	else
> @@ -923,6 +926,11 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
>  			if (thread_flags & _TIF_UPROBE)
>  				uprobe_notify_resume(regs);
>  
> +			if (thread_flags & _TIF_MTE_ASYNC_FAULT) {
> +				clear_thread_flag(TIF_MTE_ASYNC_FAULT);
> +				force_signal_inject(SIGSEGV, SEGV_MTEAERR, 0);
> +			}
> +

Should this definitely be a force_signal_inject()?

SEGV_MTEAERR is not intrinsically fatal: it must be possible to run past
the error, because that's the whole point -- chances are we already did.

Compare this with MTESERR where running past the signal would lead to a
spin.

If MTEAERR is forced, a martian tag check failure might land in the
middle of a "normal" SIGSEGV, when SIGSEGV would usually be blocked for
good reasons, defeating the process' own handling mechanisms for no good
reason: delivering the MTEAERR when SIGSEGV is next unblocked seems
perfectly reasonable in such a case.

Only braindead software would block or ignore things like SIGSEGV across
exec, so software shouldn't end up ignoring these non-forced signals
unless it does so on purpose.

Alternatively, perhaps asynchronous errors should be delivered via a
different signal.  I don't have a good suggestion though.

[...]

Cheers
---Dave

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