smb: server: fix active_num_conn leak on transport allocation failure

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Summary

CVECVE-2026-31711
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-01 14:16:21 UTC
Updated2026-05-03 07:16:18 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: smb: server: fix active_num_conn leak on transport allocation failure Commit 77ffbcac4e56 ("smb: server: fix leak of active_num_conn in ksmbd_tcp_new_connection()") addressed the kthread_run() failure path. The earlier alloc_transport() == NULL path in the same function has the same leak, is reachable pre-authentication via any TCP connect to port 445, and was empirically reproduced on UML (ARCH=um, v7.0-rc7): a small number of forced allocation failures were sufficient to put ksmbd into a state where every subsequent connection attempt was rejected for the remainder of the boot. ksmbd_kthread_fn() increments active_num_conn before calling ksmbd_tcp_new_connection() and discards the return value, so when alloc_transport() returns NULL the socket is released and -ENOMEM returned without decrementing the counter. Each such failure permanently consumes one slot from the max_connections pool; once cumulative failures reach the cap, atomic_inc_return() hits the threshold on every subsequent accept and every new connection is rejected. The counter is only reset by module reload. An unauthenticated remote attacker can drive the server toward the memory pressure that makes alloc_transport() fail by holding open connections with large RFC1002 lengths up to MAX_STREAM_PROT_LEN (0x00FFFFFF); natural transient allocation failures on a loaded host produce the same drift more slowly. Mirror the existing rollback pattern in ksmbd_kthread_fn(): on the alloc_transport() failure path, decrement active_num_conn gated on server_conf.max_connections. Repro details: with the patch reverted, forced alloc_transport() NULL returns leaked counter slots and subsequent connection attempts -- including legitimate connects issued after the forced-fail window had closed -- were all rejected with "Limit the maximum number of connections". With this patch applied, the same connect sequence produces no rejections and the counter cycles cleanly between zero and one on every accept.

Risk And Classification

Primary CVSS: v3.1 7.5 HIGH from 416baaa9-dc9f-4396-8d5f-8c081fb06d67

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS: 0.001150000 probability, percentile 0.298650000 (date 2026-05-02)

VersionSourceTypeScoreSeverityVector
3.1416baaa9-dc9f-4396-8d5f-8c081fb06d67Secondary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
3.1CNADECLARED7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 0d0d4680db22eda1eea785c47bbf66a9b33a8b16 97f8d2648ef4871e4cd335e2d769cb40054a6772 git Not specified
CNA Linux Linux affected 0d0d4680db22eda1eea785c47bbf66a9b33a8b16 295a9fc6789d1011c36ded9f0f2907bb34fa0de4 git Not specified
CNA Linux Linux affected 0d0d4680db22eda1eea785c47bbf66a9b33a8b16 283027aa93380380a0994f35dde3ec95318f2654 git Not specified
CNA Linux Linux affected 0d0d4680db22eda1eea785c47bbf66a9b33a8b16 fb48185bcd946d42de7017cf27f912f8ab26acf0 git Not specified
CNA Linux Linux affected 0d0d4680db22eda1eea785c47bbf66a9b33a8b16 6551300dc452ac16a855a83dbd1e74899542d3b3 git Not specified
CNA Linux Linux affected 4210c3555db4b38bade92331b153e583261f05f9 git Not specified
CNA Linux Linux affected d5d7847e57ac69fa99c18b363a34419bcdb5a281 git Not specified
CNA Linux Linux affected 6.2 Not specified
CNA Linux Linux unaffected 6.2 semver Not specified
CNA Linux Linux unaffected 6.6.136 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.84 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.25 6.18.* semver Not specified
CNA Linux Linux unaffected 7.0.2 7.0.* semver Not specified
CNA Linux Linux unaffected 7.1-rc1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/97f8d2648ef4871e4cd335e2d769cb40054a6772 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/6551300dc452ac16a855a83dbd1e74899542d3b3 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/295a9fc6789d1011c36ded9f0f2907bb34fa0de4 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/fb48185bcd946d42de7017cf27f912f8ab26acf0 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/283027aa93380380a0994f35dde3ec95318f2654 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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