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 11:00:58AM +0000, Keir Fraser wrote:
> The source register is not used for SET* and reading it can result in
> a UBSAN out-of-bounds array access error, specifically when the MOPS
> exception is taken from a SET* sequence with XZR (reg 31) as the
> source. Architecturally this is the only case where a src/dst/size
> field in the ESR can be reported as 31.
> 
> Prior to 2de451a329cf662b the code in do_el0_mops() was benign as the
> use of pt_regs_read_reg() prevented the out-of-bounds access.
> 
> Fixes: 2de451a329cf662b ("KVM: arm64: Add handler for MOPS exceptions")
> 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: stable@vger.kernel.org
> Reviewed-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Keir Fraser <keirf@google.com>

Thanks!

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  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
>