Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

[Andreas Gruenbacher <agruenba@redhat.com>]

On Sat, Nov 27, 2021 at 12:06 AM Catalin Marinas
<catalin.marinas@arm.com> wrote:
> On Fri, Nov 26, 2021 at 11:29:45PM +0100, Andreas Gruenbacher wrote:
> > On Thu, Nov 25, 2021 at 11:42 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > > As per Linus' reply, we can work around this by doing
> > > a sub-page fault_in_writable(point_of_failure, align) where 'align'
> > > should cover the copy_to_user() impreciseness.
> > >
> > > (of course, fault_in_writable() takes the full size argument but behind
> > > the scene it probes the 'align' prefix at sub-page fault granularity)
> >
> > That doesn't make sense; we don't want fault_in_writable() to fail or
> > succeed depending on the alignment of the address range passed to it.
>
> If we know that the arch copy_to_user() has an error of say maximum 16
> bytes (or 15 rather on arm64), we can instead get fault_in_writeable()
> to probe the first 16 bytes rather than 1.

That isn't going to help one bit: [raw_]copy_to_user() is allowed to
copy as little or as much as it wants as long as it follows the rules
documented in include/linux/uaccess.h:

[] 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.

When fault_in_writeable() tells us that an address range is accessible
in principle, that doesn't mean that copy_to_user() will allow us to
access it in arbitrary chunks. It's also not the case that
fault_in_writeable(addr, size) is always followed by
copy_to_user(addr, ..., size) for the exact same address range, not
even in this case.

These alignment restrictions have nothing to do with page or sub-page faults.

I'm also fairly sure that passing in an unaligned buffer will send
search_ioctl into an endless loop on architectures with copy_to_user()
alignment restrictions; there don't seem to be any buffer alignment
checks.

> > Have a look at the below code to see what I mean.  Function
> > copy_to_user_nofault_unaligned() should be further optimized, maybe as
> > mm/maccess.c:copy_from_kernel_nofault() and/or per architecture
> > depending on the actual alignment rules; I'm not sure.
> [...]
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -2051,13 +2051,30 @@ static noinline int key_in_sk(struct btrfs_key *key,
> >       return 1;
> >  }
> >
> > +size_t copy_to_user_nofault_unaligned(void __user *to, void *from, size_t size)
> > +{
> > +     size_t rest = copy_to_user_nofault(to, from, size);
> > +
> > +     if (rest) {
> > +             size_t n;
> > +
> > +             for (n = size - rest; n < size; n++) {
> > +                     if (copy_to_user_nofault(to + n, from + n, 1))
> > +                             break;
> > +             }
> > +             rest = size - n;
> > +     }
> > +     return rest;
>
> That's what I was trying to avoid. That's basically a fall-back to byte
> at a time copy (we do this in copy_mount_options(); at some point we
> even had a copy_from_user_exact() IIRC).

We could try 8/4/2 byte chunks if both buffers are 8/4/2-byte aligned.
It's just not clear that it's worth it.

> Linus' idea (if I got it correctly) was instead to slightly extend the
> probing in fault_in_writeable() for the beginning of the buffer from 1
> byte to some per-arch range.
>
> I attempted the above here and works ok:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=devel/btrfs-live-lock-fix
>
> but too late to post it this evening, I'll do it in the next day or so
> as an alternative to this series.
>
> --
> Catalin
>

Thanks,
Andreas