kcm: fix zero-frag skb in frag_list on partial sendmsg error

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Summary

CVECVE-2026-43244
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-06 12:16:44 UTC
Updated2026-05-11 14:12:18 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: kcm: fix zero-frag skb in frag_list on partial sendmsg error Syzkaller reported a warning in kcm_write_msgs() when processing a message with a zero-fragment skb in the frag_list. When kcm_sendmsg() fills MAX_SKB_FRAGS fragments in the current skb, it allocates a new skb (tskb) and links it into the frag_list before copying data. If the copy subsequently fails (e.g. -EFAULT from user memory), tskb remains in the frag_list with zero fragments: head skb (msg being assembled, NOT yet in sk_write_queue) +-----------+ | frags[17] | (MAX_SKB_FRAGS, all filled with data) | frag_list-+--> tskb +-----------+ +----------+ | frags[0] | (empty! copy failed before filling) +----------+ For SOCK_SEQPACKET with partial data already copied, the error path saves this message via partial_message for later completion. For SOCK_SEQPACKET, sock_write_iter() automatically sets MSG_EOR, so a subsequent zero-length write(fd, NULL, 0) completes the message and queues it to sk_write_queue. kcm_write_msgs() then walks the frag_list and hits: WARN_ON(!skb_shinfo(skb)->nr_frags) TCP has a similar pattern where skbs are enqueued before data copy and cleaned up on failure via tcp_remove_empty_skb(). KCM was missing the equivalent cleanup. Fix this by tracking the predecessor skb (frag_prev) when allocating a new frag_list entry. On error, if the tail skb has zero frags, use frag_prev to unlink and free it in O(1) without walking the singly-linked frag_list. frag_prev is safe to dereference because the entire message chain is only held locally (or in kcm->seq_skb) and is not added to sk_write_queue until MSG_EOR, so the send path cannot free it underneath us. Also change the WARN_ON to WARN_ON_ONCE to avoid flooding the log if the condition is somehow hit repeatedly. There are currently no KCM selftests in the kernel tree; a simple reproducer is available at [1]. [1] https://gist.github.com/mrpre/a94d431c757e8d6f168f4dd1a3749daa

Risk And Classification

Primary CVSS: v3.1 5.5 MEDIUM from [email protected]

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

Problem Types: CWE-401

CVSS v3.1 Breakdown

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

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Operating System Linux Linux Kernel All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected ab7ac4eb9832e32a09f4e8042705484d2fb0aad3 9ea3671d70ee07480d80bebe86696397c4e99fb7 git Not specified
CNA Linux Linux affected ab7ac4eb9832e32a09f4e8042705484d2fb0aad3 b1e3edf688a88c1a3ac41657055d9c136a08cd25 git Not specified
CNA Linux Linux affected ab7ac4eb9832e32a09f4e8042705484d2fb0aad3 7af58f76e4b404a74c836881a845e6652db8a09f git Not specified
CNA Linux Linux affected ab7ac4eb9832e32a09f4e8042705484d2fb0aad3 ca220141fa8ebae09765a242076b2b77338106b0 git Not specified
CNA Linux Linux affected 4.6 Not specified
CNA Linux Linux unaffected 4.6 semver Not specified
CNA Linux Linux unaffected 6.12.75 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.16 6.18.* semver Not specified
CNA Linux Linux unaffected 6.19.6 6.19.* semver Not specified
CNA Linux Linux unaffected 7.0 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/b1e3edf688a88c1a3ac41657055d9c136a08cd25 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/ca220141fa8ebae09765a242076b2b77338106b0 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/7af58f76e4b404a74c836881a845e6652db8a09f 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/9ea3671d70ee07480d80bebe86696397c4e99fb7 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
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