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SubjectRe: [BUG] arm64: an infinite loop in generic_perform_write()
On Thu, Jun 24, 2021 at 09:36:54PM +0100, Robin Murphy wrote:
> On 2021-06-24 19:55, Catalin Marinas wrote:
> > On Thu, Jun 24, 2021 at 04:27:17PM +0000, Al Viro wrote:
> > > On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote:
> > > > FWIW I think the only way to make the kernel behaviour any more robust here
> > > > would be to make the whole uaccess API more expressive, such that rather
> > > > than simply saying "I only got this far" it could actually differentiate
> > > > between stopping due to a fault which may be recoverable and worth retrying,
> > > > and one which definitely isn't.
> > >
> > > ... and propagate that "more expressive" information through what, 3 or 4
> > > levels in the call chain?
> > >
> > > From include/linux/uaccess.h:
> > >
> > > * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting
> > > * at to must become equal to the bytes fetched from the corresponding area
> > > * starting at from. All data past to + size - N must be left unmodified.
> > > *
> > > * If copying succeeds, the return value must be 0. If some data cannot be
> > > * fetched, it is permitted to copy less than had been fetched; the only
> > > * hard requirement is that not storing anything at all (i.e. returning size)
> > > * should happen only when nothing could be copied. In other words, you don't
> > > * have to squeeze as much as possible - it is allowed, but not necessary.
> > >
> > > arm64 instances violate the aforementioned hard requirement.
> >
> > After reading the above a few more times, I think I get it. The key
> > sentence is: not storing anything at all should happen only when nothing
> > could be copied. In the MTE case, something can still be copied.
> >
> > > Please, fix
> > > it there; it's not hard. All you need is an exception handler in .Ltiny15
> > > that would fall back to (short) byte-by-byte copy if the faulting address
> > > happened to be unaligned. Or just do one-byte copy, not that it had been
> > > considerably cheaper than a loop. Will be cheaper than propagating that extra
> > > information up the call chain, let alone paying for extra ->write_begin()
> > > and ->write_end() for single byte in generic_perform_write().
> >
> > Yeah, it's definitely fixable in the arch code. I misread the above
> > requirements and thought it could be fixed in the core code.
> >
> > Quick hack, though I think in the actual exception handling path in .S
> > more sense (and it needs the copy_to_user for symmetry):
>
> Hmm, if anything the asm version might be even more straightforward; I think
> it's pretty much just this (untested):

That's what I thought but it was too late in the day to think in asm.

> diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
> index 043da90f5dd7..632bf1f9540d 100644
> --- a/arch/arm64/lib/copy_to_user.S
> +++ b/arch/arm64/lib/copy_to_user.S
> @@ -62,6 +62,9 @@ EXPORT_SYMBOL(__arch_copy_to_user)
>
> .section .fixup,"ax"
> .align 2
> -9998: sub x0, end, dst // bytes not copied
> +9998: ldrb w7, [x1]
> +USER(9997f, sttrb w7, [x0])
> + add x0, x0, #1
> +9997: sub x0, end, dst // bytes not copied
> ret
> .previous
>
> If we can get away without trying to finish the whole copy bytewise, (i.e.
> we don't cause any faults of our own by knowingly over-reading in the
> routine itself), I'm more than happy with that.

I don't think we over-read/write in the routine itself as this is based
on the user memcpy() which can't handle faults. And since we got a fault
before the end of the copy, we have at least one byte left in the
buffer (which may or may not trigger a fault).

I wonder whether we should skip the extra byte copy if something was
copied, i.e. start the exception handler with:

cmp dstin, dst
b.ne 9997f

That said, the fall-back to bytewise copying may have some advantage. I
think we still have the issue where we copy some data to user but report
less (STP failing on the second 8-byte when the first had been already
written first 8). A byte copy loop would solve this, unless we pass the
fault address to the exception handler (I thought you had some patch for
this at some point).

--
Catalin

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