[PATCH v3] nilfs2: Fix potential block overflow that cause system hang

[Edward Adam Davis <eadavis@qq.com>]

When a user executes the FITRIM command, an underflow can occur when
calculating nblocks if end_block is too small. Since nblocks is of
type sector_t, which is u64, a negative nblocks value will become a
very large positive integer. This ultimately leads to the block layer
function __blkdev_issue_discard() taking an excessively long time to
process the bio chain, and the ns_segctor_sem lock remains held for a
long period. This prevents other tasks from acquiring the ns_segctor_sem
lock, resulting in the hang reported by syzbot in [1].

If the ending block is too small, for example, smaller than first data
block, this poses a risk of corrupting the filesystem's superblock.
Here, I check if the segment's ending block number is 0 to determine
if the previously calculated ending block is too small.

Although the start and len values in the user input range are too small,
a conservative strategy is adopted here to safely ignore them, which is
equivalent to a no-op; it will not perform any trimming and will not
throw an error.

[1]
task:segctord state:D stack:28968 pid:6093 tgid:6093  ppid:2 task_flags:0x200040 flags:0x00080000
Call Trace:
 rwbase_write_lock+0x3dd/0x750 kernel/locking/rwbase_rt.c:272
 nilfs_transaction_lock+0x253/0x4c0 fs/nilfs2/segment.c:357
 nilfs_segctor_thread_construct fs/nilfs2/segment.c:2569 [inline]
 nilfs_segctor_thread+0x6ec/0xe00 fs/nilfs2/segment.c:2684

Fixes: 82e11e857be3 ("nilfs2: add nilfs_sufile_trim_fs to trim clean segs")
Reported-by: syzbot+7eedce5eb281acd832f0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7eedce5eb281acd832f0
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
v2 -> v3: change to segment end check and update comments
v1 -> v2: continue do discard and comments

 fs/nilfs2/sufile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
index 83f93337c01b..fa612d5ec726 100644
--- a/fs/nilfs2/sufile.c
+++ b/fs/nilfs2/sufile.c
@@ -1095,6 +1095,8 @@ int nilfs_sufile_trim_fs(struct inode *sufile, struct fstrim_range *range)
 
 	segnum = nilfs_get_segnum_of_block(nilfs, start_block);
 	segnum_end = nilfs_get_segnum_of_block(nilfs, end_block);
+	if (!segnum_end)
+		return 0;
 
 	down_read(&NILFS_MDT(sufile)->mi_sem);
 
-- 
2.43.0