[PATCH v2 3/4] arm64: entry: improve data abort handling of tagged pointers

[Dave Martin]

On Wed, May 03, 2017 at 04:37:47PM +0100, Kristina Martsenko wrote:
> When handling a data abort from EL0, we currently zero the top byte of
> the faulting address, as we assume the address is a TTBR0 address, which
> may contain a non-zero address tag. However, the address may be a TTBR1
> address, in which case we should not zero the top byte. This patch fixes
> that. The effect is that the full TTBR1 address is passed to the task's
> signal handler (or printed out in the kernel log).
> 
> When handling a data abort from EL1, we leave the faulting address
> intact, as we assume it's either a TTBR1 address or a TTBR0 address with
> tag 0x00. This is true as far as I'm aware, we don't seem to access a
> tagged TTBR0 address anywhere in the kernel. Regardless, it's easy to
> forget about address tags, and code added in the future may not always
> remember to remove tags from addresses before accessing them. So add tag
> handling to the EL1 data abort handler as well. This also makes it
> consistent with the EL0 data abort handler.

FWIW,
Reviewed-by: Dave Martin <Dave.Martin at arm.com>

(Looks like Will beat me to it, though.)

Cheers
---Dave

> 
> Fixes: d50240a5f6ce ("arm64: mm: permit use of tagged pointers at EL0")
> Signed-off-by: Kristina Martsenko <kristina.martsenko at arm.com>
> ---
>  arch/arm64/include/asm/asm-uaccess.h | 9 +++++++++
>  arch/arm64/kernel/entry.S            | 5 +++--
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/asm-uaccess.h b/arch/arm64/include/asm/asm-uaccess.h
> index df411f3e083c..ecd9788cd298 100644
> --- a/arch/arm64/include/asm/asm-uaccess.h
> +++ b/arch/arm64/include/asm/asm-uaccess.h
> @@ -62,4 +62,13 @@ alternative_if ARM64_ALT_PAN_NOT_UAO
>  alternative_else_nop_endif
>  	.endm
>  
> +/*
> + * Remove the address tag from a virtual address, if present.
> + */
> +	.macro	clear_address_tag, dst, addr
> +	tst	\addr, #(1 << 55)
> +	bic	\dst, \addr, #(0xff << 56)
> +	csel	\dst, \dst, \addr, eq
> +	.endm
> +
>  #endif
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 43512d4d7df2..b738880350f9 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -428,12 +428,13 @@ el1_da:
>  	/*
>  	 * Data abort handling
>  	 */
> -	mrs	x0, far_el1
> +	mrs	x3, far_el1
>  	enable_dbg
>  	// re-enable interrupts if they were enabled in the aborted context
>  	tbnz	x23, #7, 1f			// PSR_I_BIT
>  	enable_irq
>  1:
> +	clear_address_tag x0, x3
>  	mov	x2, sp				// struct pt_regs
>  	bl	do_mem_abort
>  
> @@ -594,7 +595,7 @@ el0_da:
>  	// enable interrupts before calling the main handler
>  	enable_dbg_and_irq
>  	ct_user_exit
> -	bic	x0, x26, #(0xff << 56)
> +	clear_address_tag x0, x26
>  	mov	x1, x25
>  	mov	x2, sp
>  	bl	do_mem_abort
> -- 
> 2.1.4
> 
> 
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