[PATCH v2] fs: minix: Fix handling of corrupted directories

[Andrey Kriulin <kitotavrik.s@gmail.com>]

If the directory is corrupted and the number of nlinks is less than 2
(valid nlinks have at least 2), then when the directory is deleted, the
minix_rmdir will try to reduce the nlinks(unsigned int) to a negative
value.

Make nlinks validity check for directory.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Andrey Kriulin <kitotavrik.media@gmail.com>
Signed-off-by: Andrey Kriulin <kitotavrik.s@gmail.com>
---
v2: Move nlinks validaty check to V[12]_minix_iget() per Jan Kara
<jack@suse.cz> request. Change return error code to EUCLEAN. Don't block
directory in r/o mode per Al Viro <viro@zeniv.linux.org.uk> request.

 fs/minix/inode.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/fs/minix/inode.c b/fs/minix/inode.c
index f007e389d5d2..d815397b8b0d 100644
--- a/fs/minix/inode.c
+++ b/fs/minix/inode.c
@@ -517,6 +517,14 @@ static struct inode *V1_minix_iget(struct inode *inode)
 		iget_failed(inode);
 		return ERR_PTR(-ESTALE);
 	}
+	if (S_ISDIR(raw_inode->i_mode) && raw_inode->i_nlinks < 2) {
+		printk("MINIX-fs: inode directory with corrupted number of links");
+		if (!sb_rdonly(inode->i_sb)) {
+			brelse(bh);
+			iget_failed(inode);
+			return ERR_PTR(-EUCLEAN);
+		}
+	}
 	inode->i_mode = raw_inode->i_mode;
 	i_uid_write(inode, raw_inode->i_uid);
 	i_gid_write(inode, raw_inode->i_gid);
@@ -555,6 +563,14 @@ static struct inode *V2_minix_iget(struct inode *inode)
 		iget_failed(inode);
 		return ERR_PTR(-ESTALE);
 	}
+	if (S_ISDIR(raw_inode->i_mode) && raw_inode->i_nlinks < 2) {
+		printk("MINIX-fs: inode directory with corrupted number of links");
+		if (!sb_rdonly(inode->i_sb)) {
+			brelse(bh);
+			iget_failed(inode);
+			return ERR_PTR(-EUCLEAN);
+		}
+	}
 	inode->i_mode = raw_inode->i_mode;
 	i_uid_write(inode, raw_inode->i_uid);
 	i_gid_write(inode, raw_inode->i_gid);
-- 
2.47.2