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

Official 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. ]

Affected packages & fixed versions

Affected products (as reported)

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