CVE-2026-98158 in Linux
Summary
by MITRE • 09/25/2026
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.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability resides within the Point-to-Point Protocol asynchronous driver implementation in the Linux kernel, specifically affecting the handling of received network buffers during error conditions. The core issue stems from an incorrect management of socket buffer headroom when processing frames that fail validation checks. In normal operation, the function ppp_receive_nonmp_frame is responsible for preparing incoming packets by prepending a two-byte direction tag before applying BPF filters. This preparation requires sufficient space in the packet's headroom to safely write data without overwriting existing payload or causing memory corruption. However, the error handling path within process_input_packet fails to maintain this critical invariant. When an error occurs during frame processing, such as a bad Frame Check Sequence, the code attempts to reset the socket buffer for reuse by trimming its length and adjusting its headroom pointer. The implementation incorrectly assumes that resetting the headroom to zero effectively restores the buffer to a fresh state suitable for subsequent operations.
This assumption is fundamentally flawed because freshly allocated sockets from dev_alloc_skb are initialized with NET_SKB_PAD, which reserves specific space at the beginning of the buffer for network headers and alignment purposes. By forcing the headroom to zero, the driver violates this guarantee established by the kernel's networking subsystem. Consequently, when a subsequent frame begins reassembly into the same reused socket buffer, there is insufficient space to prepend the required two-byte tag. If an attacker sends a malformed packet with a bad Frame Check Sequence followed immediately by another valid-looking frame starting with specific bytes such as ff 03, the kernel attempts to push data into memory below the allocated start of the buffer. This results in a critical underflow condition where the write operation targets addresses lower than skb->head, triggering an immediate kernel panic via skb_under_panic and leading to a fatal exception in interrupt context.
The operational impact of this vulnerability is severe, manifesting as a remote denial-of-service attack that can crash the entire system with root privileges required only for initial PPP configuration on the target machine. Beyond the catastrophic kernel panic caused by the filter underflow, there exists an additional risk when CCP compression is enabled. In such configurations, ppp_decompress_frame may attempt to access memory at skb->data minus two bytes. Due to the same headroom violation, this operation results in a read-out-of-bounds condition before the start of the allocated buffer region. This not only compromises system stability but also introduces potential information disclosure vectors where sensitive kernel memory contents could be leaked through malformed packet processing. The vulnerability is exploitable remotely by any peer capable of sending crafted PPP frames over an asynchronous serial link or pseudo-terminal interface, making it a significant security concern for systems relying on PPP connections.
The resolution involves correcting the root cause in the transport layer rather than attempting to patch symptoms downstream. Instead of resetting the headroom pointer which leads to invalid memory states, the fix dictates dropping the errored frame entirely and clearing the reference to the reused packet buffer ap->rpkt. This forces the next incoming frame to be reassembled into a newly allocated socket buffer that correctly possesses NET_SKB_PAD guarantees. This approach aligns with existing practices in ppp_synctty which already handles errors by discarding bad frames rather than attempting complex state restoration on potentially corrupted buffers. The fix is simpler, more robust, and eliminates both the filter panic and the potential out-of-bounds read associated with CCP decompression.
From a classification perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input in C/C++ as it involves writing data beyond allocated boundaries due to incorrect buffer management. It also relates to CWE-787 Out-of-bounds Write and CWE-125 Out-of-bounds Read depending on the specific execution path triggered by the attacker's input sequence. In terms of attack patterns, this represents a classic remote denial-of-service scenario where an unauthenticated or authenticated peer can disrupt service availability through malformed packet injection. The exploit relies on precise timing and frame sequencing to trigger the state corruption, highlighting the importance of strict buffer lifecycle management in network protocol implementations. Mitigation requires applying kernel updates that include this specific patch for ppp_async, ensuring that error paths properly discard corrupted buffers rather than attempting unsafe reuse without proper reallocation or headroom restoration.