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

[Robin Murphy]

On 2021-06-29 09:30, Catalin Marinas wrote:
> On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote:
> > From: Robin Murphy <robin.murphy@xxxxxxx>
> > 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@xxxxxxxxxx>
> > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
>
> 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.

Oh, of course; I think I befuddled myself there and failed to consider 
that by this point we've already mandated that dstin == dst (so that we 
can use srcin because src may have advanced already), so in fact we 
*can* use dst as the base register to avoid the shuffling, and 
post-index this one. I'll clean that up.

> Either way, it looks fine to me.
>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>

Thanks!

Robin.