ppp_async: drop the errored frame instead of resetting its headroom

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-98158
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-25 11:17:47 UTC
Updated2026-09-25 11:17:47 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: ppp_async: drop the errored frame instead of resetting its headroom ppp_receive_nonmp_frame() prepends a two-byte direction tag before running the pass/active BPF filters: *(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_INBOUND_TAG); Nothing on the receive path guarantees those two bytes of headroom. The frame-error path in ppp_async's process_input_packet() resets a reused skb's headroom to zero while claiming to restore it to a freshly allocated state - but a fresh skb from dev_alloc_skb() carries NET_SKB_PAD: err: if (skb) { /* make skb appear as freshly allocated */ skb_trim(skb, 0); skb_reserve(skb, - skb_headroom(skb)); } ap->rpkt still points at that skb, so the next frame is reassembled into it with no headroom at all. A peer that sends a bad-FCS frame followed by one beginning ff 03 then leaves a single byte of headroom by the time the filter tag is pushed, which lands one byte below skb->head: skbuff: skb_under_panic: len:49 put:2 head:ffff888003c10000 data:ffff888003c0ffff tail:0x30 end:0x640 dev:<NULL> kernel BUG at net/core/skbuff.c:214! RIP: 0010:skb_panic+0x13e/0x230 Call Trace: skb_push+0xbd/0x100 ppp_receive_nonmp_frame+0x48a/0x1d10 ppp_input+0x4e9/0x2f80 ppp_async_process+0x2a/0xe0 tasklet_action_common+0x20f/0x8a0 handle_softirqs+0x18e/0x590 Kernel panic - not syncing: Fatal exception in interrupt Zeroing the headroom violates the NET_SKB_PAD guarantee that dev_alloc_skb() gives the rest of the receive path. Besides the filter panic above, when CCP compression is enabled ppp_decompress_frame() hands skb->data - 2 to ->decompress()/->incomp(), which then reads out of bounds before skb->head for the same reason. Rather than restore the headroom, drop the errored frame - as ppp_synctty already does on its error path - and clear ap->rpkt so the next frame is reassembled into a fresh skb with proper headroom. This is simpler and fixes both the filter under-panic and the CCP out-of-bounds read. The original V1 of this patch made room in ppp_receive_nonmp_frame() with skb_cow_head(); Eric pointed out that fixing the root cause in the transport is the right approach. Found by fuzzing the PPP receive path with a mutating peer on a pty; it is an interesting (remote) DoS: root configures PPP, the peer supplies two crashing frames. The reproducer (repro-ppp-skb.c, unchanged from v1) panics in about a second, and returns cleanly with this applied.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 6722e78c90054101e6797d5944cdc81af9897a0a d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da git Not specified
CNA Linux Linux affected 6722e78c90054101e6797d5944cdc81af9897a0a 0c53eb14975f029abd6b26896a460f0d2aaefe6b git Not specified
CNA Linux Linux affected 6722e78c90054101e6797d5944cdc81af9897a0a 717137221c7d90e7c98bda9a370c9da6cbf015e5 git Not specified
CNA Linux Linux affected 6722e78c90054101e6797d5944cdc81af9897a0a 8dc5d98a16fa23c00999aecf10018c9f69fa5bf4 git Not specified
CNA Linux Linux affected 2.6.15 Not specified
CNA Linux Linux unaffected 2.6.15 semver Not specified
CNA Linux Linux unaffected 6.12.111 6.12.* 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/0c53eb14975f029abd6b26896a460f0d2aaefe6b 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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