Subject: [syzbot] [mptcp?] KASAN: stack-out-of-bounds Read in mptcp_pm_nl_get_local_id

From: syzbot <[email-masked]>
Date: Wed, 21 Jan 2026 02:03:25 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    22cc16c04b78 riscv, bpf: Fix incorrect usage of BPF_TRAMP_..
git tree:       bpf
console output: https://syzkaller.appspot.com/x/log.txt?x=10be528a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a94030c847137a18
dashboard link: https://syzkaller.appspot.com/bug?extid=5498a510ff9de39d37da
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/43a53493cb5f/disk-22cc16c0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/9726fb9e1980/vmlinux-22cc16c0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/efd2bc050ab6/bzImage-22cc16c0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

==================================================================
BUG: KASAN: stack-out-of-bounds in __lookup_addr net/mptcp/pm_kernel.c:283 [inline]
BUG: KASAN: stack-out-of-bounds in mptcp_pm_nl_get_local_id+0x183/0x330 net/mptcp/pm_kernel.c:905
Read of size 2 at addr ffffc90003237354 by task syz.8.3557/19241

CPU: 0 UID: 0 PID: 19241 Comm: syz.8.3557 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xca/0x240 mm/kasan/report.c:482
 kasan_report+0x118/0x150 mm/kasan/report.c:595
 __lookup_addr net/mptcp/pm_kernel.c:283 [inline]
 mptcp_pm_nl_get_local_id+0x183/0x330 net/mptcp/pm_kernel.c:905
 mptcp_pm_get_local_id+0x4ab/0x5b0 net/mptcp/pm.c:925
 subflow_chk_local_id+0x101/0x220 net/mptcp/subflow.c:641
 subflow_v6_rebuild_header+0x16/0x50 net/mptcp/subflow.c:664
 tcp_connect+0x205/0x4f50 net/ipv4/tcp_output.c:4312
 tcp_v6_connect+0x1222/0x18a0 net/ipv6/tcp_ipv6.c:336
 mptcp_connect+0x56b/0x830 net/mptcp/protocol.c:3977
 __inet_stream_connect+0x2ae/0xe70 net/ipv4/af_inet.c:679
 tcp_sendmsg_fastopen+0x3a7/0x5e0 net/ipv4/tcp.c:1064
 mptcp_sendmsg_fastopen+0x17d/0x580 net/mptcp/protocol.c:1780
 mptcp_sendmsg+0x179f/0x19b0 net/mptcp/protocol.c:1880
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0xe5/0x270 net/socket.c:742
 ____sys_sendmsg+0x505/0x820 net/socket.c:2592
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2646
 __sys_sendmsg net/socket.c:2678 [inline]
 __do_sys_sendmsg net/socket.c:2683 [inline]
 __se_sys_sendmsg net/socket.c:2681 [inline]
 __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2681
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f5218b8f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f5219a1c038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f5218de6090 RCX: 00007f5218b8f749
RDX: e07e872420dfef8a RSI: 0000200000000780 RDI: 0000000000000003
RBP: 00007f5218c13f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f5218de6128 R14: 00007f5218de6090 R15: 00007ffd434acc78
 </TASK>

The buggy address belongs to a 8-page vmalloc region starting at 0xffffc90003230000 allocated at copy_process+0x4ea/0x3950 kernel/fork.c:2052
The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x7f2bc8e1d pfn:0x6bcdf
memcg:ffff88801df3ec02
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 0000000000000000 dead000000000122 0000000000000000
raw: 00000007f2bc8e1d 0000000000000000 00000001ffffffff ffff88801df3ec02
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x29c2(GFP_NOWAIT|__GFP_HIGHMEM|__GFP_IO|__GFP_FS|__GFP_ZERO), pid 19239, tgid 19239 (syz.8.3557), ts 697879712329, free_ts 697878067160
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x234/0x290 mm/page_alloc.c:1846
 prep_new_page mm/page_alloc.c:1854 [inline]
 get_page_from_freelist+0x2365/0x2440 mm/page_alloc.c:3915
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5210
 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2486
 alloc_frozen_pages_noprof mm/mempolicy.c:2557 [inline]
 alloc_pages_noprof+0xa9/0x190 mm/mempolicy.c:2577
 vm_area_alloc_pages mm/vmalloc.c:3649 [inline]
 __vmalloc_area_node mm/vmalloc.c:3863 [inline]
 __vmalloc_node_range_noprof+0x795/0x16a0 mm/vmalloc.c:4051
 __vmalloc_node_noprof+0xc2/0x110 mm/vmalloc.c:4111
 alloc_thread_stack_node kernel/fork.c:354 [inline]
 dup_task_struct+0x228/0x9a0 kernel/fork.c:923
 copy_process+0x4ea/0x3950 kernel/fork.c:2052
 kernel_clone+0x21e/0x820 kernel/fork.c:2651
 __do_sys_clone3 kernel/fork.c:2953 [inline]
 __se_sys_clone3+0x256/0x2d0 kernel/fork.c:2932
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 19240 tgid 19239 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1395 [inline]
 __free_frozen_pages+0xbc8/0xd30 mm/page_alloc.c:2943
 bpf_check+0x164a7/0x1c300 kernel/bpf/verifier.c:25396
 bpf_prog_load+0x13ba/0x1a10 kernel/bpf/syscall.c:3088
 __sys_bpf+0x507/0x860 kernel/bpf/syscall.c:6164
 __do_sys_bpf kernel/bpf/syscall.c:6274 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:6272 [inline]
 __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6272
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffffc90003237200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffffc90003237280: 00 00 00 00 f1 f1 f1 f1 00 01 f2 f2 00 01 f2 f2
>ffffc90003237300: 00 f2 f2 f2 00 f2 f2 f2 00 00 f3 f3 00 00 00 00
                                                 ^
 ffffc90003237380: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffffc90003237400: f1 f1 f1 f1 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

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