[PATCH v2] nilfs2: reject CLEAN_SEGMENTS ioctl with out-of-range segment numbers

[Deepanshu Kartikey <kartikey406@gmail.com>]

Syzbot reported a hung task in nilfs_transaction_begin() where multiple
tasks performing chmod() on a nilfs2 mount blocked for over 143 seconds
waiting to acquire ns_segctor_sem for read:

  INFO: task syz.0.17:5918 blocked for more than 143 seconds.
  Call Trace:
   schedule+0x164/0x360
   rwsem_down_read_slowpath+0x6d9/0x940
   down_read+0x99/0x2e0
   nilfs_transaction_begin+0x364/0x710 fs/nilfs2/segment.c:221
   nilfs_setattr+0x124/0x2c0 fs/nilfs2/inode.c:921
   notify_change+0xc1a/0xf40
   chmod_common+0x273/0x4a0
   do_fchmodat+0x12d/0x230

The writer holding ns_segctor_sem was a concurrent NILFS_IOCTL_CLEAN_SEGMENTS
caller, stuck inside printk while emitting per-element warnings from
nilfs_sufile_updatev():

   __nilfs_msg+0x373/0x450 fs/nilfs2/super.c:78
   nilfs_sufile_updatev+0x21c/0x6d0 fs/nilfs2/sufile.c:186
   nilfs_sufile_freev fs/nilfs2/sufile.h:93 [inline]
   nilfs_free_segments fs/nilfs2/segment.c:1140 [inline]
   nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1261 [inline]
   nilfs_segctor_do_construct+0x1f55/0x76c0
   nilfs_clean_segments+0x3bd/0xa50
   nilfs_ioctl_clean_segments fs/nilfs2/ioctl.c:922 [inline]
   nilfs_ioctl+0x261f/0x2780

The root cause is that nilfs_ioctl_clean_segments() does not validate
the user-supplied segment numbers in kbufs[4] before calling
nilfs_clean_segments(), which acquires ns_segctor_sem for write.  The
range check on each segnum is performed deep inside the call chain by
nilfs_sufile_updatev(), which emits a nilfs_warn() per invalid entry
while still under the segctor lock and the sufile mi_sem.  Under load
(repeated invocations across multiple mounts saturating the global
printk path), the cumulative printk latency keeps ns_segctor_sem held
long enough to trip the hung_task watchdog, blocking concurrent
operations such as chmod() that need ns_segctor_sem for read.

Fix by validating the contents of kbufs[4] in the ioctl entry path,
before any FS-wide lock is acquired.  Out-of-range segment numbers are
rejected with -EINVAL synchronously, with no work performed under
ns_segctor_sem.

Reported-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=62f0f99d2f2bb8e3bbd7
Fixes: 4f6b828837b4 ("nilfs2: fix lock order reversal in nilfs_clean_segments ioctl")
Cc: stable@vger.kernel.org
Tested-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/20260428040256.84403-1-kartikey406@gmail.com/T/ [v1]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Changes in v2:
  - Reuse existing 'n' loop variable instead of introducing a new
    one (Slava Dubeyko)
  - Add dedicated out_free_segnums label so the validation-failure
    path falls through the existing cleanup ladder rather than
    duplicating kfree(kbufs[4]) inline (Slava Dubeyko)
---
 fs/nilfs2/ioctl.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
index e0a606643e87..41e47f558600 100644
--- a/fs/nilfs2/ioctl.c
+++ b/fs/nilfs2/ioctl.c
@@ -876,6 +876,20 @@ static int nilfs_ioctl_clean_segments(struct inode *inode, struct file *filp,
 	}
 	nilfs = inode->i_sb->s_fs_info;
 
+	/*
+	 * Validate segment numbers against the filesystem's segment count
+	 * before entering nilfs_clean_segments(), which acquires
+	 * ns_segctor_sem for write.  Catching invalid segnums here avoids
+	 * holding that lock while emitting per-element diagnostics under
+	 * the segment constructor.
+	 */
+	for (n = 0; n < nsegs; n++) {
+		if (((__u64 *)kbufs[4])[n] >= nilfs->ns_nsegments) {
+			ret = -EINVAL;
+			goto out_free_segnums;
+		}
+	}
+
 	for (n = 0; n < 4; n++) {
 		ret = -EINVAL;
 		if (argv[n].v_size != argsz[n])
@@ -928,6 +942,7 @@ static int nilfs_ioctl_clean_segments(struct inode *inode, struct file *filp,
 out_free:
 	while (--n >= 0)
 		kvfree(kbufs[n]);
+out_free_segnums:
 	kfree(kbufs[4]);
 out:
 	mnt_drop_write_file(filp);
-- 
2.43.0