--- id: CVE-2024-26804 title: |- In the Linux kernel, the following vulnerability has been resolved: net: ip_tunnel: prevent perpetual headroom growth syzkaller triggered following kasan splat: BUG: KASAN: use-after-free in __skb_flow_dissect+0x19d1/0x7a50 net/core/fl… summary: |- In the Linux kernel, the following vulnerability has been resolved: net: ip_tunnel: prevent perpetual headroom growth syzkaller triggered following kasan splat: BUG: KASAN: use-after-free in __skb_flow_dissect+0x19d1/0x7a50 net/core/fl… severity: high cvss: 7.8 cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H' cwe: - CWE-416 vendor: linux product: linux_kernel affected: - 'linux_kernel >= 2.6.34, < 5.4.271' - 'linux_kernel >= 5.5, < 5.10.212' - 'linux_kernel >= 5.11, < 5.15.151' - 'linux_kernel >= 5.16, < 6.1.81' - 'linux_kernel >= 6.2, < 6.6.21' - 'linux_kernel >= 6.7, < 6.7.9' - linux_kernel = 6.8 - debian_linux = 10.0 patched: - linux_kernel 6.7.9 published: '2024-04-04' updated: '2026-08-04' source: NVD sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2024-26804' references: - url: 'https://git.kernel.org/stable/c/049d7989c67e8dd50f07a2096dbafdb41331fb9b' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/2e95350fe9db9d53c701075060ac8ac883b68aee' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/5ae1e9922bbdbaeb9cfbe91085ab75927488ac0f' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/a0a1db40b23e8ff86dea2786c5ea1470bb23ecb9' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/ab63de24ebea36fe73ac7121738595d704b66d96' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/afec0c5cd2ed71ca95a8b36a5e6d03333bf34282' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/f81e94d2dcd2397137edcb8b85f4c5bed5d22383' label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67 - url: 'https://git.kernel.org/stable/c/049d7989c67e8dd50f07a2096dbafdb41331fb9b' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/2e95350fe9db9d53c701075060ac8ac883b68aee' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/5ae1e9922bbdbaeb9cfbe91085ab75927488ac0f' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/a0a1db40b23e8ff86dea2786c5ea1470bb23ecb9' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/ab63de24ebea36fe73ac7121738595d704b66d96' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/afec0c5cd2ed71ca95a8b36a5e6d03333bf34282' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://git.kernel.org/stable/c/f81e94d2dcd2397137edcb8b85f4c5bed5d22383' label: af854a3a-2127-422b-91ae-364da2661108 - url: 'https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html' label: af854a3a-2127-422b-91ae-364da2661108 tags: - nvd epss: 0.00245 epssPercentile: 0.13942 ingestedAt: '2026-08-05T10:46:49.211Z' --- ## Overview In the Linux kernel, the following vulnerability has been resolved: net: ip_tunnel: prevent perpetual headroom growth syzkaller triggered following kasan splat: BUG: KASAN: use-after-free in __skb_flow_dissect+0x19d1/0x7a50 net/core/flow_dissector.c:1170 Read of size 1 at addr ffff88812fb4000e by task syz-executor183/5191 [..] kasan_report+0xda/0x110 mm/kasan/report.c:588 __skb_flow_dissect+0x19d1/0x7a50 net/core/flow_dissector.c:1170 skb_flow_dissect_flow_keys include/linux/skbuff.h:1514 [inline] ___skb_get_hash net/core/flow_dissector.c:1791 [inline] __skb_get_hash+0xc7/0x540 net/core/flow_dissector.c:1856 skb_get_hash include/linux/skbuff.h:1556 [inline] ip_tunnel_xmit+0x1855/0x33c0 net/ipv4/ip_tunnel.c:748 ipip_tunnel_xmit+0x3cc/0x4e0 net/ipv4/ipip.c:308 __netdev_start_xmit include/linux/netdevice.h:4940 [inline] netdev_start_xmit include/linux/netdevice.h:4954 [inline] xmit_one net/core/dev.c:3548 [inline] dev_hard_start_xmit+0x13d/0x6d0 net/core/dev.c:3564 __dev_queue_xmit+0x7c1/0x3d60 net/core/dev.c:4349 dev_queue_xmit include/linux/netdevice.h:3134 [inline] neigh_connected_output+0x42c/0x5d0 net/core/neighbour.c:1592 ... ip_finish_output2+0x833/0x2550 net/ipv4/ip_output.c:235 ip_finish_output+0x31/0x310 net/ipv4/ip_output.c:323 .. iptunnel_xmit+0x5b4/0x9b0 net/ipv4/ip_tunnel_core.c:82 ip_tunnel_xmit+0x1dbc/0x33c0 net/ipv4/ip_tunnel.c:831 ipgre_xmit+0x4a1/0x980 net/ipv4/ip_gre.c:665 __netdev_start_xmit include/linux/netdevice.h:4940 [inline] netdev_start_xmit include/linux/netdevice.h:4954 [inline] xmit_one net/core/dev.c:3548 [inline] dev_hard_start_xmit+0x13d/0x6d0 net/core/dev.c:3564 ... The splat occurs because skb->data points past skb->head allocated area. This is because neigh layer does: __skb_pull(skb, skb_network_offset(skb)); ... but skb_network_offset() returns a negative offset and __skb_pull() arg is unsigned. IOW, we skb->data gets "adjusted" by a huge value. The negative value is returned because skb->head and skb->data distance is more than 64k and skb->network_header (u16) has wrapped around. The bug is in the ip_tunnel infrastructure, which can cause dev->needed_headroom to increment ad infinitum. The syzkaller reproducer consists of packets getting routed via a gre tunnel, and route of gre encapsulated packets pointing at another (ipip) tunnel. The ipip encapsulation finds gre0 as next output device. This results in the following pattern: 1). First packet is to be sent out via gre0. Route lookup found an output device, ipip0. 2). ip_tunnel_xmit for gre0 bumps gre0->needed_headroom based on the future output device, rt.dev->needed_headroom (ipip0). 3). ip output / start_xmit moves skb on to ipip0. which runs the same code path again (xmit recursion). 4). Routing step for the post-gre0-encap packet finds gre0 as output device to use for ipip0 encapsulated packet. tunl0->needed_headroom is then incremented based on the (already bumped) gre0 device headroom. This repeats for every future packet: gre0->needed_headroom gets inflated because previous packets' ipip0 step incremented rt->dev (gre0) headroom, and ipip0 incremented because gre0 needed_headroom was increased. For each subsequent packet, gre/ipip0->needed_headroom grows until post-expand-head reallocations result in a skb->head/data distance of more than 64k. Once that happens, skb->network_header (u16) wraps around when pskb_expand_head tries to make sure that skb_network_offset() is unchanged after the headroom expansion/reallocation. After this skb_network_offset(skb) returns a different (and negative) result post headroom expansion. The next trip to neigh layer (or anything else that would __skb_pull the network header) makes skb->data point to a memory location outside skb->head area. v2: Cap the needed_headroom update to an arbitarily chosen upperlimit to prevent perpetual increase instead of dropping the headroom increment completely. ## Affected - `linux_kernel >= 2.6.34, < 5.4.271` - `linux_kernel >= 5.5, < 5.10.212` - `linux_kernel >= 5.11, < 5.15.151` - `linux_kernel >= 5.16, < 6.1.81` - `linux_kernel >= 6.2, < 6.6.21` - `linux_kernel >= 6.7, < 6.7.9` - `linux_kernel = 6.8` - `debian_linux = 10.0` ## Remediation Upgrade past the affected range: - `linux_kernel 6.7.9`