Re: [PATCH V2] fs: fix guard_bio_eod to check for real EOD errors

[Ming Lei <tom.leiming@gmail.com>]

On Tue, Feb 26, 2019 at 6:52 PM Carlos Maiolino <cmaiolino@redhat.com> wrote:
>
> guard_bio_eod() can truncate a segment in bio to allow it to do IO on
> odd last sectors of a device.
>
> It already checks if the IO starts past EOD, but it does not consider
> the possibility of an IO request starting within device boundaries can
> contain more than one segment past EOD.
>
> In such cases, truncated_bytes can be bigger than PAGE_SIZE, and will
> underflow bvec->bv_len.
>
> Fix this by checking if truncated_bytes is lower than PAGE_SIZE.
>
> This situation has been found on filesystems such as isofs and vfat,
> which doesn't check the device size before mount, if the device is
> smaller than the filesystem itself, a readahead on such filesystem,
> which spans EOD, can trigger this situation, leading a call to
> zero_user() with a wrong size possibly corrupting memory.
>
> I didn't see any crash, or didn't let the system run long enough to
> check if memory corruption will be hit somewhere, but adding
> instrumentation to guard_bio_end() to check truncated_bytes size, was
> enough to see the error.
>
> The following script can trigger the error.
>
> MNT=/mnt
> IMG=./DISK.img
> DEV=/dev/loop0
>
> mkfs.vfat $IMG
> mount $IMG $MNT
> cp -R /etc $MNT &> /dev/null
> umount $MNT
>
> losetup -D
>
> losetup --find --show --sizelimit 16247280 $IMG
> mount $DEV $MNT
>
> find $MNT -type f -exec cat {} + >/dev/null
>
> Kudos to Eric Sandeen for coming up with the reproducer above
>
> Changelog:
>
>         V2: Compare truncated_bytes agains bvec->bv_len instead of
>             PAGE_SIZE
>
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
>  fs/buffer.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 784de3dbcf28..b43347896ce0 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3063,6 +3063,13 @@ void guard_bio_eod(int op, struct bio *bio)
>         /* Uhhuh. We've got a bio that straddles the device size! */
>         truncated_bytes = bio->bi_iter.bi_size - (maxsector << 9);
>
> +       /*
> +        * The bio contains more than one segment which spans EOD, just return
> +        * and let IO layer turn it into an EIO
> +        */
> +       if (truncated_bytes > bvec->bv_len)
> +               return;
> +
>         /* Truncate the bio.. */
>         bio->bi_iter.bi_size -= truncated_bytes;
>         bvec->bv_len -= truncated_bytes;

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming Lei