bfs: Reconstruct file type when loading from disk

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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).

References

• https://git.kernel.org/stable/c/d0c5ec1f57d8fbb953f166a27d9d32473dc8f3e4
• https://git.kernel.org/stable/c/aeccd6743ee4fdd1ab8cfcbb5b9a20b613418f6d
• https://git.kernel.org/stable/c/8f73336b75bd3457b6f9410f2a0601a238f32238
• https://git.kernel.org/stable/c/a9f626396bfe66f49b743601e862767928237cc0
• https://git.kernel.org/stable/c/77899444d46162aeb65f229590c26ba266864223
• https://git.kernel.org/stable/c/a8cb796e7e2cb7971311ba236922f5e7e1be77e6
• https://git.kernel.org/stable/c/34ab4c75588c07cca12884f2bf6b0347c7a13872