[Devel] [PATCH rh7 1/2] ms/fs: fix guard_bio_eod to check for real EOD errors

[Konstantin Khorenko]

From: Carlos Maiolino <cmaiolino at redhat.com>

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 at redhat.com>
Signed-off-by: Carlos Maiolino <cmaiolino at redhat.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>

https://jira.sw.ru/browse/PSBM-96006
(cherry picked from commit dce30ca9e3b676fb288c33c1f4725a0621361185)

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 fs/buffer.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index 2a0be4ad8ff1..e9a4c65eed73 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -3023,6 +3023,13 @@ void guard_bio_eod(int rw, struct bio *bio)
 	/* Uhhuh. We've got a bio that straddles the device size! */
 	truncated_bytes = bio->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_size -= truncated_bytes;
 	BUG_ON(truncated_bytes > bvec->bv_len);
-- 
2.15.1