4.14-stable review patch. If anyone has any objections, please let me know.
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[ 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 8086cc8ff0bc..bdca7b10e239 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -3084,6 +3084,13 @@ 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;