Re: [PATCH] arm64: mops: Do not dereference src reg for a set operation

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Mar 26, 2025 at 07:02:55AM +0000, Keir Fraser wrote:
> The register is not defined and reading it can result in a UBSAN
> out-of-bounds array access error, specifically when the srcreg field
> value is 31.

I'm assuming this is for a MOPS exception taken from a SET* sequence
with XZR as the source?

It'd be nice to say that explicitly, as this is the only case where any
of the src/dst/size fields in the ESR can be reported as 31. In all
other cases where a CPY* or SET* instruction takes register 31 as an
argument, the behaviour is CONSTRAINED UNPREDICTABLE and cannot generate
a MOPS exception.

Note that in ARM DDI 0487 L.a there's a bug where:

* The prose says that SET* taking XZR as a src is CONSTRAINED
  UNPREDICTABLE, per K1.2.17.1.1 linked from C6.2.332.

  The title for the K1.2.17.1.1 is "Memory Copy and Memory Set CPY*",
  which looks like an editing error.

* The pseudocode is explicit that the CONSTRAINED UNPREDICTABLE
  behaviours differ for CPY* and SET* per J1.1.3.121
  CheckCPYConstrainedUnpredictable() and J1.1.3.125
  CheckSETConstrainedUnpredictable().

... and I'll go file a ticket about that soon if someone doesn't beat me
to it.

> Cc: Kristina Martsenko <kristina.martsenko@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org

Looks like this should have:

Fixes: 2de451a329cf662b ("KVM: arm64: Add handler for MOPS exceptions")

Prior to that, the code in do_el0_mops() was benign as the use of
pt_regs_read_reg() prevented the out-of-bounds access. It'd also be nice
to note that in the commit message.

Mark.

> Signed-off-by: Keir Fraser <keirf@google.com>
> ---
>  arch/arm64/include/asm/traps.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/traps.h b/arch/arm64/include/asm/traps.h
> index d780d1bd2eac..82cf1f879c61 100644
> --- a/arch/arm64/include/asm/traps.h
> +++ b/arch/arm64/include/asm/traps.h
> @@ -109,10 +109,9 @@ static inline void arm64_mops_reset_regs(struct user_pt_regs *regs, unsigned lon
>  	int dstreg = ESR_ELx_MOPS_ISS_DESTREG(esr);
>  	int srcreg = ESR_ELx_MOPS_ISS_SRCREG(esr);
>  	int sizereg = ESR_ELx_MOPS_ISS_SIZEREG(esr);
> -	unsigned long dst, src, size;
> +	unsigned long dst, size;
>  
>  	dst = regs->regs[dstreg];
> -	src = regs->regs[srcreg];
>  	size = regs->regs[sizereg];
>  
>  	/*
> @@ -129,6 +128,7 @@ static inline void arm64_mops_reset_regs(struct user_pt_regs *regs, unsigned lon
>  		}
>  	} else {
>  		/* CPY* instruction */
> +		unsigned long src = regs->regs[srcreg];
>  		if (!(option_a ^ wrong_option)) {
>  			/* Format is from Option B */
>  			if (regs->pstate & PSR_N_BIT) {
> -- 
> 2.49.0.395.g12beb8f557-goog
>