linux-kernel - [PATCH v2] bfs: Reconstruct file type when loading from disk

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Message-ID: <1007e06d-c2d1-4905-ab93-9d66ed7da505@I-love.SAKURA.ne.jp>
Date: Mon, 13 Oct 2025 14:44:49 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Tigran Aivazian <aivazian.tigran@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: [PATCH v2] bfs: Reconstruct file type when loading from disk

syzbot is reporting that S_IFMT bits of inode->i_mode can become bogus when
the S_IFMT bits of the 32bits "mode" field loaded from disk are corrupted
or when the 32bits "attributes" field loaded from disk are corrupted.

A documentation says that BFS uses only lower 9 bits of the "mode" field.
But I can't find an explicit explanation that the unused upper 23 bits
(especially, the S_IFMT bits) are initialized with 0.

Therefore, ignore the S_IFMT bits of the "mode" field loaded from disk.
Also, verify that the value of the "attributes" field loaded from disk is
either BFS_VREG or BFS_VDIR (because BFS supports only regular files and
the root directory).

Reported-by: syzbot+895c23f6917da440ed0d@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=895c23f6917da440ed0d
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Reviewed-by: Tigran Aivazian <aivazian.tigran@...il.com>
---
 fs/bfs/inode.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/fs/bfs/inode.c b/fs/bfs/inode.c
index 1d41ce477df5..984b365df046 100644
--- a/fs/bfs/inode.c
+++ b/fs/bfs/inode.c
@@ -61,7 +61,19 @@ struct inode *bfs_iget(struct super_block *sb, unsigned long ino)
 	off = (ino - BFS_ROOT_INO) % BFS_INODES_PER_BLOCK;
 	di = (struct bfs_inode *)bh->b_data + off;
 
-	inode->i_mode = 0x0000FFFF & le32_to_cpu(di->i_mode);
+	/*
+	 * https://martin.hinner.info/fs/bfs/bfs-structure.html explains that
+	 * BFS in SCO UnixWare environment used only lower 9 bits of di->i_mode
+	 * value. This means that, although bfs_write_inode() saves whole
+	 * inode->i_mode bits (which include S_IFMT bits and S_IS{UID,GID,VTX}
+	 * bits), middle 7 bits of di->i_mode value can be garbage when these
+	 * bits were not saved by bfs_write_inode().
+	 * Since we can't tell whether middle 7 bits are garbage, use only
+	 * lower 12 bits (i.e. tolerate S_IS{UID,GID,VTX} bits possibly being
+	 * garbage) and reconstruct S_IFMT bits for Linux environment from
+	 * di->i_vtype value.
+	 */
+	inode->i_mode = 0x00000FFF & le32_to_cpu(di->i_mode);
 	if (le32_to_cpu(di->i_vtype) == BFS_VDIR) {
 		inode->i_mode |= S_IFDIR;
 		inode->i_op = &bfs_dir_inops;
@@ -71,6 +83,11 @@ struct inode *bfs_iget(struct super_block *sb, unsigned long ino)
 		inode->i_op = &bfs_file_inops;
 		inode->i_fop = &bfs_file_operations;
 		inode->i_mapping->a_ops = &bfs_aops;
+	} else {
+		brelse(bh);
+		printf("Unknown vtype=%u %s:%08lx\n",
+		       le32_to_cpu(di->i_vtype), inode->i_sb->s_id, ino);
+		goto error;
 	}
 
 	BFS_I(inode)->i_sblock =  le32_to_cpu(di->i_sblock);
-- 
2.47.3