exit: hold a reference to thread_pid across proc_flush_pid

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Summary

CVECVE-2026-97903
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-25 11:17:17 UTC
Updated2026-09-25 11:17:17 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: exit: hold a reference to thread_pid across proc_flush_pid Commit 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") removed the reference around proc_flush_pid(). It assumed that free_pids(post.pids) at the end of release_task() would keep thread_pid alive until then. That assumption is wrong. __change_pid() only records a detached PID in post.pids when pid_has_task() is false for every PIDTYPE. If another task still uses the exiting task's PID as its process group or session ID, __unhash_process() removes the exiting task's PIDTYPE_PID link but leaves the PID out of post.pids. release_task() therefore holds no reference to it after dropping tasklist_lock. The other task can then remove the remaining PIDTYPE links. Its free_pids() call schedules delayed_put_pid(), and the RCU callback can free the PID before the first release_task() reaches proc_flush_pid(). An unprivileged reproducer races wait4(-1) against setsid() to trigger this ordering. Three of three fresh v7.2 KASAN boots reported: BUG: KASAN: slab-use-after-free in proc_invalidate_siblings_dcache+0x3e2/0x3f0 Read of size 8 by task h7_pid_reaper/1921 Call Trace: proc_invalidate_siblings_dcache release_task wait_consider_task __do_wait do_wait kernel_wait4 Freed by task 0: kmem_cache_free put_pid delayed_put_pid rcu_core Last potentially related work creation: __call_rcu_common free_pids ksys_setsid KASAN identified a 144-byte object from the pid cache and located the bad read 80 bytes into the freed object, matching pid->inodes. With an explicit reference, three of three fresh boots completed without a KASAN report. The concurrent RCU callback dropped its reference while proc_flush_pid() was protected, and the balancing put_pid() performed the final free afterward. Take a reference before __unhash_process() clears p->thread_pid and release it after proc_flush_pid() completes. A tested source reproducer is available privately on request. No controlled read or write, information leak, or privilege escalation is claimed. The mainline patch applies directly to v6.19.y and newer; v6.16.y through v6.18.y need a context-adjusted backport.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 0a36bad01731e71568bdd365764d38b6bd576ab0 bcb0936862f2b653114d9bd582ca26990ef54a64 git Not specified
CNA Linux Linux affected 0a36bad01731e71568bdd365764d38b6bd576ab0 96803a12e8ac15f0a1b4b151db59038f12304e58 git Not specified
CNA Linux Linux affected 0a36bad01731e71568bdd365764d38b6bd576ab0 cdd812d0683dee14ead02c9eded568685e61b23f git Not specified
CNA Linux Linux affected 6.16 Not specified
CNA Linux Linux unaffected 6.16 semver Not specified
CNA Linux Linux unaffected 6.18.53 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.7 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc3 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/cdd812d0683dee14ead02c9eded568685e61b23f 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/96803a12e8ac15f0a1b4b151db59038f12304e58 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/bcb0936862f2b653114d9bd582ca26990ef54a64 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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