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

[Filipe Manana <fdmanana@gmail.com>]

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?

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?

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

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.”