linux-kernel - Re: [BUG] arm64: an infinite loop in generic_perform

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Date:   Tue, 6 Jul 2021 18:50:52 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Chen Huang <chenhuang5@...wei.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Randy Dunlap <rdunlap@...radead.org>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mm <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write()

On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote:
> diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
> index 043da90f5dd7..cfb598ae4812 100644
> --- a/arch/arm64/lib/copy_to_user.S
> +++ b/arch/arm64/lib/copy_to_user.S
> @@ -32,7 +32,7 @@
>  	.endm
>  	.macro strh1 reg, ptr, val
> -	user_ldst 9998f, sttrh, \reg, \ptr, \val
> +	user_ldst 9997f, sttrh, \reg, \ptr, \val
>  	.endm
>  	.macro ldr1 reg, ptr, val
> @@ -40,7 +40,7 @@
>  	.endm
>  	.macro str1 reg, ptr, val
> -	user_ldst 9998f, sttr, \reg, \ptr, \val
> +	user_ldst 9997f, sttr, \reg, \ptr, \val
>  	.endm
>  	.macro ldp1 reg1, reg2, ptr, val
> @@ -48,12 +48,14 @@
>  	.endm
>  	.macro stp1 reg1, reg2, ptr, val
> -	user_stp 9998f, \reg1, \reg2, \ptr, \val
> +	user_stp 9997f, \reg1, \reg2, \ptr, \val
>  	.endm
>  end	.req	x5
> +srcin	.req	x15
>  SYM_FUNC_START(__arch_copy_to_user)
>  	add	end, x0, x2
> +	mov	srcin, x1
>  #include "copy_template.S"
>  	mov	x0, #0
>  	ret
> @@ -62,6 +64,12 @@ EXPORT_SYMBOL(__arch_copy_to_user)
>  	.section .fixup,"ax"
>  	.align	2
> +9997:	cmp	dst, dstin
> +	b.ne	9998f
> +	// Before being absolutely sure we couldn't copy anything, try harder
> +	ldrb	tmp1w, [srcin]
> +USER(9998f, sttrb tmp1w, [dstin])
> +	add	dst, dstin, #1
>  9998:	sub	x0, end, dst			// bytes not copied
>  	ret
>  	.previous

I think it's worth doing the copy_to_user() fallback in a loop until it
faults or hits the end of the buffer. This would solve the problem we
currently have with writing more bytes than actually reported. The
copy_from_user() is not necessary, a byte would suffice.

-- 
Catalin