CVE-2026-98158 (GCVE-0-2026-98158)
Vulnerability from cvelistv5
Published
2026-09-25 10:36
Modified
2026-10-19 09:14
Severity ?
Summary
In 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.
References
Impacted products
Vendor Product Version
Linux Linux Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
Version: 6722e78c90054101e6797d5944cdc81af9897a0a
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{
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          "programFiles": [
            "drivers/net/ppp/ppp_async.c"
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      "cpeApplicability": [
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nppp_async: drop the errored frame instead of resetting its headroom\n\nppp_receive_nonmp_frame() prepends a two-byte direction tag before running\nthe pass/active BPF filters:\n\n\t*(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_INBOUND_TAG);\n\nNothing on the receive path guarantees those two bytes of headroom. The\nframe-error path in ppp_async\u0027s process_input_packet() resets a reused skb\u0027s\nheadroom to zero while claiming to restore it to a freshly allocated state -\nbut a fresh skb from dev_alloc_skb() carries NET_SKB_PAD:\n\n\terr:\n\t\tif (skb) {\n\t\t\t/* make skb appear as freshly allocated */\n\t\t\tskb_trim(skb, 0);\n\t\t\tskb_reserve(skb, - skb_headroom(skb));\n\t\t}\n\nap-\u003erpkt still points at that skb, so the next frame is reassembled into it\nwith no headroom at all. A peer that sends a bad-FCS frame followed by one\nbeginning ff 03 then leaves a single byte of headroom by the time the filter\ntag is pushed, which lands one byte below skb-\u003ehead:\n\n  skbuff: skb_under_panic: len:49 put:2 head:ffff888003c10000\n          data:ffff888003c0ffff tail:0x30 end:0x640 dev:\u003cNULL\u003e\n  kernel BUG at net/core/skbuff.c:214!\n  RIP: 0010:skb_panic+0x13e/0x230\n  Call Trace:\n   skb_push+0xbd/0x100\n   ppp_receive_nonmp_frame+0x48a/0x1d10\n   ppp_input+0x4e9/0x2f80\n   ppp_async_process+0x2a/0xe0\n   tasklet_action_common+0x20f/0x8a0\n   handle_softirqs+0x18e/0x590\n  Kernel panic - not syncing: Fatal exception in interrupt\n\nZeroing the headroom violates the NET_SKB_PAD guarantee that dev_alloc_skb()\ngives the rest of the receive path. Besides the filter panic above, when CCP\ncompression is enabled ppp_decompress_frame() hands skb-\u003edata - 2 to\n-\u003edecompress()/-\u003eincomp(), which then reads out of bounds before skb-\u003ehead\nfor the same reason.\n\nRather than restore the headroom, drop the errored frame - as ppp_synctty\nalready does on its error path - and clear ap-\u003erpkt so the next frame is\nreassembled into a fresh skb with proper headroom. This is simpler and fixes\nboth the filter under-panic and the CCP out-of-bounds read.\n\nThe original V1 of this patch made room in ppp_receive_nonmp_frame() with\nskb_cow_head(); Eric pointed out that fixing the root cause in the transport\nis the right approach.\n\nFound by fuzzing the PPP receive path with a mutating peer on a pty; it is an\ninteresting (remote) DoS: root configures PPP, the peer supplies two crashing\nframes. The reproducer (repro-ppp-skb.c, unchanged from v1) panics in about a\nsecond, and returns cleanly with this applied."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-19T09:14:52.068Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/25f354c55b8435d1f51b8a0a05a3cfe429358523"
        },
        {
          "url": "https://git.kernel.org/stable/c/a86a17745c3e1c6fadd4d6e90c03552dfe531c8c"
        },
        {
          "url": "https://git.kernel.org/stable/c/ff035780adb0f49dc2c7ada26b4697849a68bec8"
        },
        {
          "url": "https://git.kernel.org/stable/c/c4bb894362d224b699e2f95c6c26707d9654e4a3"
        },
        {
          "url": "https://git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da"
        },
        {
          "url": "https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b"
        },
        {
          "url": "https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5"
        },
        {
          "url": "https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4"
        }
      ],
      "title": "ppp_async: drop the errored frame instead of resetting its headroom",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-98158",
    "datePublished": "2026-09-25T10:36:27.608Z",
    "dateReserved": "2026-09-25T10:25:14.321Z",
    "dateUpdated": "2026-10-19T09:14:52.068Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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