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

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

From: 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 user-supplied segment numbers are not validated
before nilfs_clean_segments() begins doing work; 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 holding 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 nilfs_clean_segments()
immediately after acquiring ns_segctor_sem via nilfs_transaction_lock().
Holding ns_segctor_sem serializes the check against
nilfs_ioctl_resize(), which can modify ns_nsegments, so the validation
uses a consistent value.  Out-of-range segment numbers are rejected
with -EINVAL before any segment-cleaning work begins, so the bad
entries never reach the per-element diagnostic path inside
nilfs_sufile_updatev().

Reported-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=62f0f99d2f2bb8e3bbd7
Tested-by: syzbot+62f0f99d2f2bb8e3bbd7@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Fixes: 071cb4b81987 ("nilfs2: eliminate removal list of segments")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
Hi Viacheslav,

Please queue this patch.

This is a fix by Deepanshu that addresses the problem recently
detected by syzbot, a hang-up that can occur when GC ioctl parameters
are invalid (this time, when a segment number to be freed is
out-of-range).

Thanks,
Ryusuke Konishi

 fs/nilfs2/segment.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 1491a4d4b1e1..9332f5ac6083 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2512,12 +2512,33 @@ int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
 	struct nilfs_sc_info *sci = nilfs->ns_writer;
 	struct nilfs_transaction_info ti;
 	int err;
+	size_t i, nfreesegs = argv[4].v_nmembs;
+	__u64 *segnumv = kbufs[4];
 
 	if (unlikely(!sci))
 		return -EROFS;
 
 	nilfs_transaction_lock(sb, &ti, 1);
 
+	/*
+	 * Validate segment numbers under ns_segctor_sem (held for write
+	 * by nilfs_transaction_lock above) so the check is serialized
+	 * against nilfs_ioctl_resize(), which can modify ns_nsegments.
+	 * Rejecting bad input here, before any segment-cleaning work
+	 * begins, avoids the per-element diagnostic path inside
+	 * nilfs_sufile_updatev() that would otherwise run under this
+	 * same lock and stall concurrent readers.
+	 */
+	for (i = 0; i < nfreesegs; i++) {
+		if (segnumv[i] >= nilfs->ns_nsegments) {
+			nilfs_err(sb,
+				 "Segment number %llu to be freed is out of range",
+				 (unsigned long long)segnumv[i]);
+			err = -EINVAL;
+			goto bail_unlock;
+		}
+	}
+
 	err = nilfs_mdt_save_to_shadow_map(nilfs->ns_dat);
 	if (unlikely(err))
 		goto out_unlock;
@@ -2558,6 +2579,7 @@ int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
 	sci->sc_freesegs = NULL;
 	sci->sc_nfreesegs = 0;
 	nilfs_mdt_clear_shadow_map(nilfs->ns_dat);
+ bail_unlock:
 	nilfs_transaction_unlock(sb);
 	return err;
 }
-- 
2.43.0