Bug 2516645 (CVE-2026-72034)

Summary: CVE-2026-72034 kernel: fhandle: reject detached mounts in capable_wrt_mount()
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's file handle (fhandle) mechanism. A local attacker could exploit a vulnerability in the capable_wrt_mount() function, which incorrectly handles detached mounts. By performing specific operations, an attacker could trigger a null pointer dereference, leading to a system panic and causing a Denial of Service (DoS) on the affected system.
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Description OSIDB Bzimport 2026-08-15 06:23:41 UTC 
In the Linux kernel, the following vulnerability has been resolved:

fhandle: reject detached mounts in capable_wrt_mount()

The recent fhandle RCU fix moved the mount namespace capability check
into capable_wrt_mount(), so a non-NULL mnt_namespace survives the
ns_capable() dereference. The helper still assumes the later
READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh()
checked is_mounted() first.

That assumption is not stable. A detached mount from
open_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while
open_by_handle_at() is between those checks, and umount_tree() can
clear mount->mnt_ns. If the helper observes NULL, it dereferences
mnt_ns->user_ns and panics.

Return false when the RCU read observes a detached mount. This keeps
the relaxed permission path conservative: a mount no longer attached
to a namespace cannot authorize open_by_handle_at() access.
Comment 1 Keith Grant 2026-08-17 17:03:51 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026081512-CVE-2026-72034-a89c@gregkh/T