In 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
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
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
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| Linux | Linux | affected |
|
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Advisories
No advisories yet.
Fixes
Solution
No solution given by the vendor.
Workaround
No workaround given by the vendor.
References
History
Wed, 06 May 2026 12:15:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In 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 | |
| Title | kcm: fix zero-frag skb in frag_list on partial sendmsg error | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-05-06T11:28:36.946Z
Reserved: 2026-05-01T14:12:55.995Z
Link: CVE-2026-43244
Vulnrichment
No data.
NVD
Status : Awaiting Analysis
Published: 2026-05-06T12:16:44.873
Modified: 2026-05-06T13:07:51.607
Link: CVE-2026-43244
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.