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

ext4: fix bug_on ext4_mb_use_inode_pa

Hulk Robot reported a BUG_ON:

kernel BUG at fs/ext4/mballoc.c:3211! […] RIP: 0010:ext4_mb_mark_diskspace_used.cold+0x85/0x136f […] Call Trace: ext4_mb_new_blocks+0x9df/0x5d30 ext4_ext_map_blocks+0x1803/0x4d80 ext4_map_blocks+0x3a4/0x1a10 ext4_writepages+0x126d/0x2c30 do_writepages+0x7f/0x1b0 __filemap_fdatawrite_range+0x285/0x3b0 file_write_and_wait_range+0xb1/0x140 ext4_sync_file+0x1aa/0xca0 vfs_fsync_range+0xfb/0x260 do_fsync+0x48/0xa0 […]

Above issue may happen as follows:

do_fsync vfs_fsync_range ext4_sync_file file_write_and_wait_range __filemap_fdatawrite_range do_writepages ext4_writepages mpage_map_and_submit_extent mpage_map_one_extent ext4_map_blocks ext4_mb_new_blocks ext4_mb_normalize_request »> start + size <= ac->ac_o_ex.fe_logical ext4_mb_regular_allocator ext4_mb_simple_scan_group ext4_mb_use_best_found ext4_mb_new_preallocation ext4_mb_new_inode_pa ext4_mb_use_inode_pa »> set ac->ac_b_ex.fe_len <= 0 ext4_mb_mark_diskspace_used »> BUG_ON(ac->ac_b_ex.fe_len <= 0);

we can easily reproduce this problem with the following commands: fallocate -l100M disk mkfs.ext4 -b 1024 -g 256 disk mount disk /mnt fsstress -d /mnt -l 0 -n 1000 -p 1

The size must be smaller than or equal to EXT4_BLOCKS_PER_GROUP. Therefore, start + size <= ac->ac_o_ex.fe_logical may occur when the size is truncated. So start should be the start position of the group where ac_o_ex.fe_logical is located after alignment. In addition, when the value of fe_logical or EXT4_BLOCKS_PER_GROUP is very large, the value calculated by start_off is more accurate.

Affected Software

References

• https://git.kernel.org/stable/c/5707d721d1819db57dba57b1d4623034fcb32047
• https://git.kernel.org/stable/c/6880fb2e64331b9fdc85d3f32b1d7e81ad8703f1
• https://git.kernel.org/stable/c/6fdaf31ad5f3d3afab744dfd9a8b0d9142aa881f
• https://git.kernel.org/stable/c/887a3e9ad4b8309a2266bce7ae749b2bf1f7a687
• https://git.kernel.org/stable/c/90f0f9d45dff0128c0fca0d2358c4153b024afa6
• https://git.kernel.org/stable/c/a08f789d2ab5242c07e716baf9a835725046be89
• https://git.kernel.org/stable/c/a37c1359714da42517dd19d36fc3c4d17edba832
• https://git.kernel.org/stable/c/a6b31616e5afe1d3972cb0682a373e50597faf5c