[PATCH] nilfs2: no longer save to shadow map if the num of members is too small

The value of argv0.v_nmembs passed from userspace is 0. This prevents
nilfs_iget_for_gc() from being called to initialize the gcinode during
the execution of nilfs_ioctl_move_blocks(). Consequently, this triggers
a null-ptr-deref involving ii->i_assoc_inode within the subsequent call
sequence: nilfs_clean_segments()->nilfs_mdt_save_to_shadow_map() [1].

A check for argv[0].v_nmembs has been added to nilfs_clean_segments()
to prevent this potential null-ptr-deref of ii->i_assoc_inode.

[1]
KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
Call Trace:
 nilfs_clean_segments+0x162/0xa50 fs/nilfs2/segment.c:2521
 nilfs_ioctl_clean_segments fs/nilfs2/ioctl.c:916 [inline]
 nilfs_ioctl+0x261f/0x2780 fs/nilfs2/ioctl.c:1346

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=4b4093b1f24ad789bf37
Signed-off-by: Edward Adam Davis <[email protected]>
---
 fs/nilfs2/segment.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 1491a4d4b1e1..7e0b24361d0b 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2518,9 +2518,11 @@ int nilfs_clean_segments(struct super_block *sb, struct nilfs_argv *argv,
 
 	nilfs_transaction_lock(sb, &ti, 1);
 
-	err = nilfs_mdt_save_to_shadow_map(nilfs->ns_dat);
-	if (unlikely(err))
-		goto out_unlock;
+	if (argv[0].v_nmembs > 0) {
+		err = nilfs_mdt_save_to_shadow_map(nilfs->ns_dat);
+		if (unlikely(err))
+			goto out_unlock;
+	}
 
 	err = nilfs_ioctl_prepare_clean_segments(nilfs, argv, kbufs);
 	if (unlikely(err)) {
-- 
2.43.0