[v5.15] general protection fault in ax25_send_control

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 3330a8d33e08 Linux 5.15.201
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=134c955a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=353ae28c40b35af5
dashboard link: https://syzkaller.appspot.com/bug?extid=fc560ce39424d5e16dbe
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b65fe19af112/disk-3330a8d3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/351948519a0d/vmlinux-3330a8d3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a3b3f8477595/bzImage-3330a8d3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+fc560c...@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc000000001a: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000000d0-0x00000000000000d7]
CPU: 0 PID: 9028 Comm: syz.4.1026 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:ax25_send_control+0x78/0x4e0 net/ax25/ax25_subr.c:152
Code: 48 83 c3 08 48 89 d8 48 c1 e8 03 42 80 3c 30 00 74 08 48 89 df e8 88 67 ea f8 41 bf d6 00 00 00 4c 03 3b 4c 89 f8 48 c1 e8 03 <42> 0f b6 04 30 84 c0 0f 85 03 03 00 00 41 0f b7 3f 83 c7 02 be 20
RSP: 0018:ffffc9000320f9f0 EFLAGS: 00010203
RAX: 000000000000001a RBX: ffff888026b2d008 RCX: 0000000000080000
RDX: ffffc90004004000 RSI: 000000000000389e RDI: 000000000000389f
RBP: 000000000000002f R08: ffff888017abbb80 R09: 0000000000000003
R10: 0000000000000002 R11: 0000000000000002 R12: ffff88805b31e828
R13: 1ffff1100b663d05 R14: dffffc0000000000 R15: 00000000000000d6
FS: 00007faad38f16c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b34f17ff8 CR3: 0000000048596000 CR4: 00000000003506f0
Call Trace:
<TASK>
ax25_std_establish_data_link+0x97/0x110 net/ax25/ax25_std_subr.c:-1
ax25_send_frame+0x313/0x490 net/ax25/ax25_out.c:78
rose_send_frame+0x111/0x220 net/rose/rose_link.c:106
rose_transmit_restart_request net/rose/rose_link.c:198 [inline]
rose_transmit_link+0x463/0x730 net/rose/rose_link.c:284
rose_write_internal+0x680/0x1e90 net/rose/rose_subr.c:198
rose_connect+0xada/0x13b0 net/rose/af_rose.c:875
__sys_connect_file net/socket.c:1932 [inline]
__sys_connect+0x3cb/0x450 net/socket.c:1949
__do_sys_connect net/socket.c:1959 [inline]
__se_sys_connect net/socket.c:1956 [inline]
__x64_sys_connect+0x76/0x80 net/socket.c:1956
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7faad5697629
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007faad38f1028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 00007faad5910fa0 RCX: 00007faad5697629
RDX: 000000000000001c RSI: 0000200000000240 RDI: 0000000000000009
RBP: 00007faad572db39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007faad5911038 R14: 00007faad5910fa0 R15: 00007ffee3246448
</TASK>
Modules linked in:
---[ end trace 3de9a202cad3efec ]---
RIP: 0010:ax25_send_control+0x78/0x4e0 net/ax25/ax25_subr.c:152
Code: 48 83 c3 08 48 89 d8 48 c1 e8 03 42 80 3c 30 00 74 08 48 89 df e8 88 67 ea f8 41 bf d6 00 00 00 4c 03 3b 4c 89 f8 48 c1 e8 03 <42> 0f b6 04 30 84 c0 0f 85 03 03 00 00 41 0f b7 3f 83 c7 02 be 20
RSP: 0018:ffffc9000320f9f0 EFLAGS: 00010203
RAX: 000000000000001a RBX: ffff888026b2d008 RCX: 0000000000080000
RDX: ffffc90004004000 RSI: 000000000000389e RDI: 000000000000389f
RBP: 000000000000002f R08: ffff888017abbb80 R09: 0000000000000003
R10: 0000000000000002 R11: 0000000000000002 R12: ffff88805b31e828
R13: 1ffff1100b663d05 R14: dffffc0000000000 R15: 00000000000000d6
FS: 00007faad38f16c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4f945f72f8 CR3: 0000000048596000 CR4: 00000000003506f0
----------------
Code disassembly (best guess):
0: 48 83 c3 08 add $0x8,%rbx
4: 48 89 d8 mov %rbx,%rax
7: 48 c1 e8 03 shr $0x3,%rax
b: 42 80 3c 30 00 cmpb $0x0,(%rax,%r14,1)
10: 74 08 je 0x1a
12: 48 89 df mov %rbx,%rdi
15: e8 88 67 ea f8 call 0xf8ea67a2
1a: 41 bf d6 00 00 00 mov $0xd6,%r15d
20: 4c 03 3b add (%rbx),%r15
23: 4c 89 f8 mov %r15,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 0f b6 04 30 movzbl (%rax,%r14,1),%eax <-- trapping instruction
2f: 84 c0 test %al,%al
31: 0f 85 03 03 00 00 jne 0x33a
37: 41 0f b7 3f movzwl (%r15),%edi
3b: 83 c7 02 add $0x2,%edi
3e: be .byte 0xbe
3f: 20 .byte 0x20


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