Bug 2460683 (CVE-2026-31437)

Summary: CVE-2026-31437 kernel: netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in the Linux kernel's netfs component. When a write operation is retried, the `netfs_unbuffered_write()` function can attempt to access a non-existent function, leading to a NULL pointer dereference. This vulnerability can be triggered by specific filesystem configurations, such as 9P, and could allow a local attacker to cause a system crash, resulting in a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-04-22 15:05:13 UTC 
In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

When a write subrequest is marked NETFS_SREQ_NEED_RETRY, the retry path
in netfs_unbuffered_write() unconditionally calls stream->prepare_write()
without checking if it is NULL.

Filesystems such as 9P do not set the prepare_write operation, so
stream->prepare_write remains NULL. When get_user_pages() fails with
-EFAULT and the subrequest is flagged for retry, this results in a NULL
pointer dereference at fs/netfs/direct_write.c:189.

Fix this by mirroring the pattern already used in write_retry.c: if
stream->prepare_write is NULL, skip renegotiation and directly reissue
the subrequest via netfs_reissue_write(), which handles iterator reset,
IN_PROGRESS flag, stats update and reissue internally.
Comment 1 Keith Grant 2026-04-22 16:58:58 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026042243-CVE-2026-31437-7cba@gregkh/T