[BUG] arm64: an infinite loop in generic_perform_write()

[Catalin Marinas]

On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote:
> From: Robin Murphy <robin.murphy at arm.com>
> Subject: [PATCH] arm64: Avoid premature usercopy failure
> 
> Al reminds us that the usercopy API must only return complete failure
> if absolutely nothing could be copied. Currently, if userspace does
> something silly like giving us an unaligned pointer to Device memory,
> or a size which overruns MTE tag bounds, we may fail to honour that
> requirement when faulting on a multi-byte access even though a smaller
> access could have succeeded.
> 
> Add a mitigation to the fixup routines to fall back to a single-byte
> copy if we faulted on a larger access before anything has been written
> to the destination, to guarantee making *some* forward progress. We
> needn't be too concerned about the overall performance since this should
> only occur when callers are doing something a bit dodgy in the first
> place. Particularly broken userspace might still be able to trick
> generic_perform_write() into an infinite loop by targeting write() at
> an mmap() of some read-only device register where the fault-in load
> succeeds but any store synchronously aborts such that copy_to_user() is
> genuinely unable to make progress, but, well, don't do that...
> 
> Reported-by: Chen Huang <chenhuang5 at huawei.com>
> Suggested-by: Al Viro <viro at zeniv.linux.org.uk>
> Signed-off-by: Robin Murphy <robin.murphy at arm.com>

Thanks Robin for putting this together. I'll write some MTE kselftests
to check for regressions in the future.

> diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> index 95cd62d67371..5b720a29a242 100644
> --- a/arch/arm64/lib/copy_from_user.S
> +++ b/arch/arm64/lib/copy_from_user.S
> @@ -29,7 +29,7 @@
>  	.endm
>  	.macro ldrh1 reg, ptr, val
> -	user_ldst 9998f, ldtrh, \reg, \ptr, \val
> +	user_ldst 9997f, ldtrh, \reg, \ptr, \val
>  	.endm
>  	.macro strh1 reg, ptr, val
> @@ -37,7 +37,7 @@
>  	.endm
>  	.macro ldr1 reg, ptr, val
> -	user_ldst 9998f, ldtr, \reg, \ptr, \val
> +	user_ldst 9997f, ldtr, \reg, \ptr, \val
>  	.endm
>  	.macro str1 reg, ptr, val
> @@ -45,7 +45,7 @@
>  	.endm
>  	.macro ldp1 reg1, reg2, ptr, val
> -	user_ldp 9998f, \reg1, \reg2, \ptr, \val
> +	user_ldp 9997f, \reg1, \reg2, \ptr, \val
>  	.endm
>  	.macro stp1 reg1, reg2, ptr, val
> @@ -53,8 +53,10 @@
>  	.endm
>  end	.req	x5
> +srcin	.req	x15
>  SYM_FUNC_START(__arch_copy_from_user)
>  	add	end, x0, x2
> +	mov	srcin, x1
>  #include "copy_template.S"
>  	mov	x0, #0				// Nothing to copy
>  	ret
> @@ -63,6 +65,12 @@ EXPORT_SYMBOL(__arch_copy_from_user)
>  	.section .fixup,"ax"
>  	.align	2
> +9997:	cmp	dst, dstin
> +	b.ne	9998f
> +	// Before being absolutely sure we couldn't copy anything, try harder
> +USER(9998f, ldtrb tmp1w, [srcin])
> +	strb	tmp1w, [dstin]
> +	add	dst, dstin, #1

Nitpick: can we do just strb tmb1w, [dst], #1? It matches the strb1
macro in this file.

Either way, it looks fine to me.

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>