Bug 2461561 (CVE-2026-31577)

Summary: CVE-2026-31577 kernel: nilfs2: fix NULL i_assoc_inode dereference in nilfs_mdt_save_to_shadow_map
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the `nilfs2` filesystem within the Linux kernel. A local user can trigger a null pointer dereference by calling the `NILFS_IOCTL_CLEAN_SEGMENTS` operation immediately after mounting the filesystem, but before any btree operations have occurred on the Data Allocation Table (DAT) inode. This can lead to a general protection fault, resulting in a system crash and a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-04-24 15:08:33 UTC 
In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix NULL i_assoc_inode dereference in nilfs_mdt_save_to_shadow_map

The DAT inode's btree node cache (i_assoc_inode) is initialized lazily
during btree operations. However, nilfs_mdt_save_to_shadow_map()
assumes i_assoc_inode is already initialized when copying dirty pages
to the shadow map during GC.

If NILFS_IOCTL_CLEAN_SEGMENTS is called immediately after mount before
any btree operation has occurred on the DAT inode, i_assoc_inode is
NULL leading to a general protection fault.

Fix this by calling nilfs_attach_btree_node_cache() on the DAT inode
in nilfs_dat_read() at mount time, ensuring i_assoc_inode is always
initialized before any GC operation can use it.
Comment 1 Keith Grant 2026-04-24 18:18:24 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026042410-CVE-2026-31577-5e81@gregkh/T