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SubjectRe: [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

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