Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()

[Andreas Gruenbacher]

On Thu, Oct 21, 2021 at 7:09 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> On Thu, Oct 21, 2021 at 04:42:33PM +0200, Andreas Gruenbacher wrote:
> > On Thu, Oct 21, 2021 at 12:06 PM Catalin Marinas
> > <catalin.marinas@xxxxxxx> wrote:
> > > On Thu, Oct 21, 2021 at 02:46:10AM +0200, Andreas Gruenbacher wrote:
> > > > When a page fault would occur, we
> > > > get back an error instead, and then we try to fault in the offending
> > > > pages. If a page is resident and we still get a fault trying to access
> > > > it, trying to fault in the same page again isn't going to help and we
> > > > have a true error.
> > >
> > > You can't be sure the second fault is a true error. The unlocked
> > > fault_in_*() may race with some LRU scheme making the pte not accessible
> > > or a write-back making it clean/read-only. copy_to_user() with
> > > pagefault_disabled() fails again but that's a benign fault. The
> > > filesystem should re-attempt the fault-in (gup would correct the pte),
> > > disable page faults and copy_to_user(), potentially in an infinite loop.
> > > If you bail out on the second/third uaccess following a fault_in_*()
> > > call, you may get some unexpected errors (though very rare). Maybe the
> > > filesystems avoid this problem somehow but I couldn't figure it out.
> >
> > Good point, we can indeed only bail out if both the user copy and the
> > fault-in fail.
> >
> > But probing the entire memory range in fault domain granularity in the
> > page fault-in functions still doesn't actually make sense. Those
> > functions really only need to guarantee that we'll be able to make
> > progress eventually. From that point of view, it should be enough to
> > probe the first byte of the requested memory range, so when one of
> > those functions reports that the next N bytes should be accessible,
> > this really means that the first byte surely isn't permanently
> > inaccessible and that the rest is likely accessible. Functions
> > fault_in_readable and fault_in_writeable already work that way, so
> > this only leaves function fault_in_safe_writeable to worry about.
>
> I agree, that's why generic_perform_write() works. It does a get_user()
> from the first byte in that range and the subsequent copy_from_user()
> will make progress of at least one byte if it was readable. Eventually
> it will hit the byte that faults. The gup-based fault_in_*() are a bit
> more problematic.
>
> Your series introduces fault_in_safe_writeable() and I think for MTE
> doing a _single_ get_user(uaddr) (in addition to the gup checks for
> write) would be sufficient as long as generic_file_read_iter() advances
> by at least one byte (eventually).
>
> This discussion started with the btrfs search_ioctl() where, even if
> some bytes were written in copy_to_sk(), it always restarts from an
> earlier position, reattempting to write the same bytes. Since
> copy_to_sk() doesn't guarantee forward progress even if some bytes are
> writable, Linus' suggestion was for fault_in_writable() to probe the
> whole range. I consider this overkill since btrfs is the only one that
> needs probing every 16 bytes. The other cases like the new
> fault_in_safe_writeable() can be fixed by probing the first byte only
> followed by gup.

Hmm. Direct I/O request sizes are multiples of the underlying device
block size, so we'll also get stuck there if fault-in won't give us a
full block. This is getting pretty ugly. So scratch that idea; let's
stick with probing the whole range.

Thanks,
Andreas

> I think we need to better define the semantics of the fault_in + uaccess
> sequences. For uaccess, we document "a hard requirement that not storing
> anything at all (i.e. returning size) should happen only when nothing
> could be copied" (from linux/uaccess.h). I think we can add a
> requirement for the new size_t-based fault_in_* variants without
> mandating that the whole range is probed, something like: "returning
> leftover < size guarantees that a subsequent user access at uaddr copies
> at least one byte eventually". I said "eventually" but maybe we can come
> up with some clearer wording for a liveness property.
>
> Such requirement would ensure that infinite loops of fault_in_* +
> uaccess make progress as long as they don't reset the probed range. Code
> like btrfs search_ioctl() would need to be adjusted to avoid such range
> reset and guarantee forward progress.
>
> --
> Catalin
>