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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Oct 21, 2021 at 04:30:30PM -1000, Linus Torvalds wrote:
> On Thu, Oct 21, 2021 at 4:42 AM Andreas Gruenbacher <agruenba@redhat.com> wrote:
> > 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
> 
> That's probably fine.
> 
> Although it should be more than one byte - "copy_from_user()" might do
> word-at-a-time optimizations, so you could have an infinite loop of
> 
>  (a) copy_from_user() fails because the chunk it tried to get failed partly
> 
>  (b) fault_in() probing succeeds, because the beginning part is fine
> 
> so I agree that the fault-in code doesn't need to do the whole area,
> but it needs to at least do some <N bytes, up to length> thing, to
> handle the situation where the copy_to/from_user requires more than a
> single byte.

From a discussion with Al some months ago, if there are bytes still
accessible, copy_from_user() is not allowed to fail fully (i.e. return
the requested copy size) even when it uses word-at-a-time. In the worst
case, it should return size - 1. If the fault_in() then continues
probing from uaddr + 1, it should eventually hit the faulty address.

The problem appears when fault_in() restarts from uaddr rather than
where copy_from_user() stopped. That's what the btrfs search_ioctl()
does. I also need to check the direct I/O cases that Andreas mentioned,
maybe they can be changed not to attempt the fault_in() from the
beginning of the block.

-- 
Catalin