linux-kernel - [PATCH] bfs: Fix calculation of offset when moving blocks

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Message-ID: <20251124172815.100111-1-zlatistiv@gmail.com>
Date: Mon, 24 Nov 2025 19:28:15 +0200
From: "Nikola Z. Ivanov" <zlatistiv@...il.com>
To: aivazian.tigran@...il.com
Cc: linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org,
	david.hunter.linux@...il.com,
	khalid@...nel.org,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	"Nikola Z. Ivanov" <zlatistiv@...il.com>,
	syzbot+f51a2a34984e4d8888fd@...kaller.appspotmail.com
Subject: [PATCH] bfs: Fix calculation of offset when moving blocks

bfs_move_blocks performs an incorrect calculation when
looping over the blocks that are to be moved.

When looping over the section of blocks:

for (i = start; i <= end; i++)
        if(bfs_move_block(i, where + i, sb)) {

Here bfs_move_block will not move
blocks "start"-"end" to "where"-"where+size"
but instead it will add "i" (which is initialized to "start")
to "where", adding additional unintended offset.

Example reproducer of the bug:

dd if=/dev/zero of=./bfs.img count=200 bs=1M
mkfs.bfs ./bfs.img
losetup /dev/loop0 ./bfs.img
mount /dev/loop0 /mnt
dd if=/dev/urandom of=/mnt/file0 count=75 bs=1M
dd if=/dev/urandom of=/mnt/file1 count=25 bs=1M
dd if=/dev/urandom of=/mnt/file2 count=25 bs=1M
echo 123 >> /mnt/file1

Essentially this will create 3 files, file1 located
right between file0 and file2 on-disk. Writing more
data to file1 will cause bfs to attempt to move it
after file2, where we have enough space to store it,
but due to the incorrect calculation it will attempt
to move it past the end of device and trigger a NULL
pointer dereference in bfs_move_block.

Even if we have enough space to account for the
unintended offset, the content of file1 will now be
garbage due to bfs_get_block expecting file1 to be
right after file2.

For example:

dd if=/dev/zero of=./bfs.img count=200 bs=1M
mkfs.bfs ./bfs.img
losetup /dev/loop0 ./bfs.img
mount /dev/loop0 /mnt
dd if=/dev/urandom of=/mnt/file0 count=1 bs=512
dd if=/dev/urandom of=/mnt/file1 count=1 bs=512
dd if=/dev/urandom of=/mnt/file2 count=1 bs=512
sync; echo 1 > /proc/sys/vm/drop_caches
xxd /mnt/file1
echo 123 >> /mnt/file1
sync; echo 1 > /proc/sys/vm/drop_caches
xxd /mnt/file1

The hexdump reveals that our file now contains zeroes,
except for the "123\n" we echoed at the end.

Fix this by correcting the calculation. Additionally add
a check for new == NULL inside bfs_move_block and propagate
the return value of bfs_move_block as a return value of
bfs_move_blocks in case of error.

Reported-by: syzbot+f51a2a34984e4d8888fd@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f51a2a34984e4d8888fd
Signed-off-by: Nikola Z. Ivanov <zlatistiv@...il.com>
---
The example reproducers might trigger a WARNING() in mark_buffer_dirty,
which is observed often when blocks are moved around,
syzbot has already hit that bug here:
https://syzkaller.appspot.com/bug?extid=2327bccb02eef9291c1c
I will follow up with a patch for that soon.

 fs/bfs/file.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/fs/bfs/file.c b/fs/bfs/file.c
index d33d6bde992b..649105018014 100644
--- a/fs/bfs/file.c
+++ b/fs/bfs/file.c
@@ -40,6 +40,10 @@ static int bfs_move_block(unsigned long from, unsigned long to,
 	if (!bh)
 		return -EIO;
 	new = sb_getblk(sb, to);
+	if (!new) {
+		brelse(bh);
+		return -EIO;
+	}
 	memcpy(new->b_data, bh->b_data, bh->b_size);
 	mark_buffer_dirty(new);
 	bforget(bh);
@@ -51,14 +55,17 @@ static int bfs_move_blocks(struct super_block *sb, unsigned long start,
 				unsigned long end, unsigned long where)
 {
 	unsigned long i;
+	int err;
 
 	dprintf("%08lx-%08lx->%08lx\n", start, end, where);
-	for (i = start; i <= end; i++)
-		if(bfs_move_block(i, where + i, sb)) {
-			dprintf("failed to move block %08lx -> %08lx\n", i,
-								where + i);
-			return -EIO;
+	for (i = 0; i <= end - start; i++) {
+		err = bfs_move_block(start + i, where + i, sb);
+		if (err) {
+			dprintf("failed to move block %08lx -> %08lx\n",
+				start + i, where + i);
+			return err;
 		}
+	}
 	return 0;
 }
 
-- 
2.51.0