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    Subject[PATCH AUTOSEL 5.0 059/262] fs: fix guard_bio_eod to check for real EOD errors
    Date
    From: Carlos Maiolino <cmaiolino@redhat.com>

    [ Upstream commit dce30ca9e3b676fb288c33c1f4725a0621361185 ]

    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

    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/buffer.c | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/fs/buffer.c b/fs/buffer.c
    index 48318fb74938..cab7a026876b 100644
    --- a/fs/buffer.c
    +++ b/fs/buffer.c
    @@ -3027,6 +3027,13 @@ 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;
    --
    2.19.1
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