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

ext4: fix double brelse() the buffer of the extents path

In ext4_ext_try_to_merge_up(), set path[1].p_bh to NULL after it has been released, otherwise it may be released twice. An example of what triggers this is as follows:

ext4_ext_map_blocks ext4_ext_handle_unwritten_extents ext4_split_convert_extents // path->p_depth == 0 ext4_split_extent // 1. do split1 ext4_split_extent_at |ext4_ext_insert_extent | ext4_ext_create_new_leaf | ext4_ext_grow_indepth | le16_add_cpu(&neh->eh_depth, 1) | ext4_find_extent | // return -ENOMEM |// get error and try zeroout |path = ext4_find_extent | path->p_depth = 1 |ext4_ext_try_to_merge | ext4_ext_try_to_merge_up | path->p_depth = 0 | brelse(path[1].p_bh) —> not set to NULL here |// zeroout success // 2. update path ext4_find_extent // 3. do split2 ext4_split_extent_at ext4_ext_insert_extent ext4_ext_create_new_leaf ext4_ext_grow_indepth le16_add_cpu(&neh->eh_depth, 1) ext4_find_extent path[0].p_bh = NULL; path->p_depth = 1 read_extent_tree_block —> return err // path[1].p_bh is still the old value ext4_free_ext_path ext4_ext_drop_refs // path->p_depth == 1 brelse(path[1].p_bh) —> brelse a buffer twice

Finally got the following WARRNING when removing the buffer from lru:

============================================ VFS: brelse: Trying to free free buffer WARNING: CPU: 2 PID: 72 at fs/buffer.c:1241 __brelse+0x58/0x90 CPU: 2 PID: 72 Comm: kworker/u19:1 Not tainted 6.9.0-dirty #716 RIP: 0010:__brelse+0x58/0x90 Call Trace: __find_get_block+0x6e7/0x810 bdev_getblk+0x2b/0x480 __ext4_get_inode_loc+0x48a/0x1240 ext4_get_inode_loc+0xb2/0x150 ext4_reserve_inode_write+0xb7/0x230 __ext4_mark_inode_dirty+0x144/0x6a0 ext4_ext_insert_extent+0x9c8/0x3230 ext4_ext_map_blocks+0xf45/0x2dc0 ext4_map_blocks+0x724/0x1700 ext4_do_writepages+0x12d6/0x2a70 […]

Weakness

The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/230ee0535d01478bad9a3037292043f39b9be10b
• https://git.kernel.org/stable/c/32bbb59e3f18facd7201bef110010bf35819b8c3
• https://git.kernel.org/stable/c/68a69cf60660c73990c1875f94a5551600b04775
• https://git.kernel.org/stable/c/7633407ca4ab8be2916ab214eb44ccebc6a50e1a
• https://git.kernel.org/stable/c/78bbc3d15b6f443acb26e94418c445bac940d414
• https://git.kernel.org/stable/c/b6c29c8f3d7cb67b505f3b2f6c242d52298d1f2e
• https://git.kernel.org/stable/c/d4574bda63906bf69660e001470bfe1a0ac524ae
• https://git.kernel.org/stable/c/dcaa6c31134c0f515600111c38ed7750003e1b9c
• https://git.kernel.org/stable/c/f9fd47c9d9548f9e47fa60098eab99dde175401d