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

[Edward Adam Davis]

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].

The fix involves adding a check for the end block: if it equals the
start block, the trim operation is exited and -EINVAL is returned.

[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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=7eedce5eb281acd832f0
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 fs/nilfs2/sufile.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
index 83f93337c01b..63a1f0b29066 100644
--- a/fs/nilfs2/sufile.c
+++ b/fs/nilfs2/sufile.c
@@ -1093,6 +1093,9 @@ int nilfs_sufile_trim_fs(struct inode *sufile, struct fstrim_range *range)
 	else
 		end_block = start_block + len - 1;
 
+	if (start_block == end_block)
+		return -EINVAL;
+
 	segnum = nilfs_get_segnum_of_block(nilfs, start_block);
 	segnum_end = nilfs_get_segnum_of_block(nilfs, end_block);
 
-- 
2.43.0