CVE-2024-26726: 
Linux Kernel vulnerability analysis and mitigation

CVE-2024-26726 affects the Linux kernel's BTRFS filesystem implementation. The vulnerability was discovered during CI testing and involves a bug in handling extent maps for free space inodes during write errors. The issue was identified and fixed in early 2024, with the initial commit being made by Josef Bacik on January 31, 2024 (Kernel Commit).

The vulnerability occurs when writing out the free space cache fails and the system attempts to write it again. During the second attempt, btrfs_get_extent() is called on the inode to get the extent mapping. Due to searching the commit root to avoid deadlocking with the tree for new block groups, nothing is found and EXTENT_MAP_HOLE is returned. The issue arises because the first failed write attempt drops the extent mapping, which is normal for regular files but problematic for free space cache inodes where the extent map must always be correct (Kernel Commit).

When triggered, this vulnerability results in a kernel panic with an assertion failure: 'block_start != EXTENT_MAP_HOLE' in fs/btrfs/extent_io.c:1385. This leads to system instability and potential denial of service (Kernel Commit).

The vulnerability can be triggered during normal filesystem operations when writing to the free space cache fails. It was reproducible through error injection testing in the filesystem's CI environment (Kernel Commit).

The issue has been fixed by modifying the code to skip dropping the extent map range for failed ranges specifically in the case of free space inodes. The fix has been implemented in the Linux kernel and backported to stable kernel versions 4.14 and newer (Kernel Commit).