CVE-2026-52906

Summary

In the Linux kernel, the following vulnerability has been resolved:

9p: fix access mode flags being ORed instead of replaced

Since commit 1f3e4142c0eb ("9p: convert to the new mount API"), v9fs_apply_options() applies parsed mount flags with |= onto flags already set by v9fs_session_init(). For 9P2000.L, session_init sets V9FS_ACCESS_CLIENT as the default, so when the user mounts with "access=user", both bits end up set. Access mode checks compare against exact values, so having both bits set matches neither mode.

This causes v9fs_fid_lookup() to fall through to the default switch case, using INVALID_UID (nobody/65534) instead of current_fsuid() for all fid lookups. Root is then unable to chown or perform other privileged operations.

Fix by clearing the access mask before applying the user's choice.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/b8f037e87a083291190204b959cda417aaf01058
• https://git.kernel.org/stable/c/da2346a48a5a1fed86c3fe3d73c0b60e7b3027c9

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