• Carlos Maiolino's avatar
    fs: fix guard_bio_eod to check for real EOD errors · 2e5086f3
    Carlos Maiolino authored
    [ 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.
    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: default avatarMing Lei <ming.lei@redhat.com>
    Signed-off-by: default avatarCarlos Maiolino <cmaiolino@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
buffer.c 89.1 KB