Re: [PATCH] Btrfs: fix out of bounds array access while reading extent buffer

[Liu Bo <bo.li.liu@oracle.com>]

Hi Filipe,

On Tue, Aug 08, 2017 at 09:47:21AM +0100, Filipe Manana wrote:
> On Mon, Aug 7, 2017 at 8:39 PM, Liu Bo <bo.li.liu@oracle.com> wrote:
> > There is a cornel case that slip through the checkers in functions
> > reading extent buffer, ie.
> >
> > if (start < eb->len) and (start + len > eb->len),
> > then
> >
> > a) map_private_extent_buffer() returns immediately because
> > it's thinking the range spans across two pages,
> >
> > b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
> > and WARN_ON(start + len > eb->start + eb->len), both are OK in this
> > corner case, but it'd actually try to access the eb->pages out of
> > bounds because of (start + len > eb->len).
> >
> > The case is found by switching extent inline ref type from shared data
> > ref to non-shared data ref.
> >
> > This is adding proper checks in order to avoid invalid memory access,
> > ie. 'general protection', before it's too late.
> 
> Hi Bo,
> 
> I don't understand these 2 last paragraphs.
> How do you fix the invalid memory access? All the change does is make
> sure that attempts to read from invalid regions of an extent buffer
> result in a warning and returning an error code. Those paragraphs give
> the idea that the problem is that some caller is passing a wrong
> offset/length pair, however you aren't fixing any caller. can you
> clarify?
>

I see your doubt, that wrong offset/length pair comes from one of
btrfs_setget helpers, eg. btrfs_extent_data_ref_root.

The invalid memory access happens when the pointer it's using to
access fields in "struct btrfs_extent_data_ref" is actually a "struct
btrfs_shared_data_ref", this is caused by switching those types.

So the offset/length pair is correct from btrfs_setget helper's point
of view, but it's just using the wrong helper due to a wrong ref type
(in v2 this'll be added to the commit log to clarity).

> Also when you say " The case is found by switching extent inline ref
> type from shared data ref to non-shared data ref", it gives the idea
> this is a deterministic problem that always happens when doing that
> switch. If so, can we have a test case?
>

It is deterministic, but it depends on patching btrfs-corrupt-block
and last time I posted a corrupt-block related case, I was told that
tool is gonna change a lot and maybe get deleted in the near future,
so it's not yet suitable for fstests cases.

> Also missing the word "fault" after 'general protection'.
>

Yeah, there is a 'fault', the full text is
"general protection fault: 0000 [#1] SMP KASAN",
will update.

Thanks,

-liubo

> Thanks.
> 
> >
> > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > ---
> >  fs/btrfs/extent_io.c | 22 ++++++++++++++--------
> >  1 file changed, 14 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index 0aff9b2..d198e87 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -5416,13 +5416,19 @@ void read_extent_buffer(struct extent_buffer *eb, void *dstv,
> >         char *dst = (char *)dstv;
> >         size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
> >         unsigned long i = (start_offset + start) >> PAGE_SHIFT;
> > +       unsigned long num_pages = num_extent_pages(eb->start, eb->len);
> >
> > -       WARN_ON(start > eb->len);
> > -       WARN_ON(start + len > eb->start + eb->len);
> > +       if (start + len > eb->len) {
> > +               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
> > +                    eb->start, eb->len, start, len);
> > +               memset(dst, 0, len);
> > +               return;
> > +       }
> >
> >         offset = (start_offset + start) & (PAGE_SIZE - 1);
> >
> >         while (len > 0) {
> > +               ASSERT(i < num_pages);
> >                 page = eb->pages[i];
> >
> >                 cur = min(len, (PAGE_SIZE - offset));
> > @@ -5491,6 +5497,12 @@ int map_private_extent_buffer(struct extent_buffer *eb, unsigned long start,
> >         unsigned long end_i = (start_offset + start + min_len - 1) >>
> >                 PAGE_SHIFT;
> >
> > +       if (start + min_len > eb->len) {
> > +               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
> > +                      eb->start, eb->len, start, min_len);
> > +               return -EINVAL;
> > +       }
> > +
> >         if (i != end_i)
> >                 return 1;
> >
> > @@ -5502,12 +5514,6 @@ int map_private_extent_buffer(struct extent_buffer *eb, unsigned long start,
> >                 *map_start = ((u64)i << PAGE_SHIFT) - start_offset;
> >         }
> >
> > -       if (start + min_len > eb->len) {
> > -               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
> > -                      eb->start, eb->len, start, min_len);
> > -               return -EINVAL;
> > -       }
> > -
> >         p = eb->pages[i];
> >         kaddr = page_address(p);
> >         *map = kaddr + offset;
> > --
> > 2.9.4
> >
> > --
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> 
> 
> 
> -- 
> Filipe David Manana,
> 
> “Whether you think you can, or you think you can't — you're right.”
> --
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