[syzbot] [net?] memory leak in build_skb

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    65ba8f34ec88 Merge tag 'xtensa-20261027' of https://github..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1592353e580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=fbf447a1375a1948
dashboard link: https://syzkaller.appspot.com/bug?extid=a3f21220cbbb6e14ee2e
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=142ee469580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b4e738ce5f2b/disk-65ba8f34.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1de9d1ecbbb6/vmlinux-65ba8f34.xz
kernel image: https://storage.googleapis.com/syzbot-assets/8a3d79b65181/bzImage-65ba8f34.xz

AI-generated root cause summary:
In IPsec IP-TFS (XFRM_MODE_IPTFS) processing, when inner packets (including the
reused outer skb) are parsed and added to a temporary local 'sublist' during
__input_process_payload(), any error encountering invalid header data or
truncated payload jumps to 'done:'. At 'done:', packets on 'sublist' are neither
delivered nor freed, and the reused outer skb is neither placed in 'defer' nor
freed by kfree_skb(), permanently leaking the sk_buff and its attached sec_path
extension.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=a3f21220cbbb6e14ee2e

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a3f21220cbbb6e14ee2e@xxxxxxxxxxxxxxxxxxxxxxxxx

2026/10/28 09:56:01 executed programs: 5
BUG: memory leak
unreferenced object 0xffff888104c73c00 (size 240):
  comm "syz.0.17", pid 5970, jiffies 4294947974
  hex dump (first 32 bytes):
    90 38 1f 02 00 c9 ff ff 90 38 1f 02 00 c9 ff ff  .8.......8......
    00 00 6a 24 81 88 ff ff 00 00 00 00 00 00 00 00  ..j$............
  backtrace (crc 84fdfeb5):
    kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
    slab_post_alloc_hook mm/slub.c:4733 [inline]
    slab_alloc_node mm/slub.c:5033 [inline]
    kmem_cache_alloc_noprof+0x35b/0x3e0 mm/slub.c:5047
    __build_skb net/core/skbuff.c:495 [inline]
    build_skb+0x25/0x120 net/core/skbuff.c:511
    __tun_build_skb drivers/net/tun.c:1631 [inline]
    tun_build_skb+0x489/0x770 drivers/net/tun.c:1726
    tun_get_user+0xd2e/0x2040 drivers/net/tun.c:1860
    tun_chr_write_iter+0x71/0x120 drivers/net/tun.c:2100
    new_sync_write fs/read_write.c:595 [inline]
    vfs_write+0x3ae/0x720 fs/read_write.c:687
    ksys_write+0xa5/0x170 fs/read_write.c:739
    do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
    do_syscall_64+0xe9/0x530 arch/x86/entry/syscall_64.c:84
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

BUG: memory leak
unreferenced object 0xffff888104d139c0 (size 192):
  comm "syz.0.17", pid 5970, jiffies 4294947974
  hex dump (first 32 bytes):
    01 00 00 00 00 02 00 00 00 00 0b 00 00 00 00 00  ................
    80 9f 7b 24 81 88 ff ff 00 00 00 00 00 00 00 00  ..{$............
  backtrace (crc 420634a7):
    kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
    slab_post_alloc_hook mm/slub.c:4733 [inline]
    slab_alloc_node mm/slub.c:5033 [inline]
    kmem_cache_alloc_noprof+0x35b/0x3e0 mm/slub.c:5047
    __skb_ext_alloc+0x1a/0x50 net/core/skbuff.c:7246
    skb_ext_add+0x1a6/0x280 net/core/skbuff.c:7350
    secpath_set+0x7c/0xc0 net/xfrm/xfrm_input.c:127
    xfrm_input+0x381/0x28a0 net/xfrm/xfrm_input.c:571
    vti_input+0xf4/0x160 net/ipv4/ip_vti.c:69
    xfrm4_esp_rcv+0x46/0xb0 net/ipv4/xfrm4_protocol.c:104
    ip_protocol_deliver_rcu+0x259/0x260 net/ipv4/ip_input.c:207
    ip_local_deliver_finish+0x115/0x1e0 net/ipv4/ip_input.c:241
    NF_HOOK include/linux/netfilter.h:325 [inline]
    NF_HOOK include/linux/netfilter.h:319 [inline]
    ip_local_deliver+0x97/0x140 net/ipv4/ip_input.c:262
    dst_input include/net/dst.h:481 [inline]
    ip_rcv_finish net/ipv4/ip_input.c:492 [inline]
    NF_HOOK include/linux/netfilter.h:325 [inline]
    NF_HOOK include/linux/netfilter.h:319 [inline]
    ip_rcv+0x212/0x230 net/ipv4/ip_input.c:612
    __netif_receive_skb_one_core+0xce/0x100 net/core/dev.c:6405
    __netif_receive_skb+0x1d/0x80 net/core/dev.c:6518
    netif_receive_skb_internal net/core/dev.c:6604 [inline]
    netif_receive_skb+0x49/0x200 net/core/dev.c:6663
    tun_rx_batched drivers/net/tun.c:1575 [inline]
    tun_get_user+0x1ccc/0x2040 drivers/net/tun.c:2053
    tun_chr_write_iter+0x71/0x120 drivers/net/tun.c:2100

connection error: failed to recv *flatrpc.ExecutorMessageRawT: EOF


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