Message-ID: <695caa47.050a0220.318c5c.0136.GAE@google.com>
Date: Mon, 05 Jan 2026 22:23:03 -0800
From: syzbot <syzbot+2c29addf92581b410079@...kaller.appspotmail.com>
To: kartikey406@...il.com, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bpf?] KASAN: slab-out-of-bounds Read in strnchr

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
KASAN: slab-out-of-bounds Read in strnchr

==================================================================
BUG: KASAN: slab-out-of-bounds in strnchr+0x5e/0x80 lib/string.c:405
Read of size 1 at addr ffff8880796d9bb0 by task syz.0.17/6447

CPU: 1 UID: 0 PID: 6447 Comm: syz.0.17 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
 strnchr+0x5e/0x80 lib/string.c:405
 bpf_bprintf_prepare+0x167/0x13d0 kernel/bpf/helpers.c:829
 ____bpf_snprintf kernel/bpf/helpers.c:1065 [inline]
 bpf_snprintf+0xd3/0x1b0 kernel/bpf/helpers.c:1049
 bpf_prog_c2925c0a7ac12d80+0x58/0x60
 bpf_dispatcher_nop_func include/linux/bpf.h:1378 [inline]
 __bpf_prog_run include/linux/filter.h:723 [inline]
 bpf_prog_run include/linux/filter.h:730 [inline]
 __bpf_trace_run kernel/trace/bpf_trace.c:2075 [inline]
 bpf_trace_run1+0x27f/0x4c0 kernel/trace/bpf_trace.c:2115
 __bpf_trace_rcu_utilization+0xa1/0xf0 include/trace/events/rcu.h:27
 __do_trace_rcu_utilization include/trace/events/rcu.h:27 [inline]
 trace_rcu_utilization+0x191/0x1c0 include/trace/events/rcu.h:27
 rcu_note_context_switch+0xc9/0x1120 kernel/rcu/tree_plugin.h:330
 __schedule+0x346/0x5000 kernel/sched/core.c:6748
 __schedule_loop kernel/sched/core.c:6945 [inline]
 schedule+0x165/0x360 kernel/sched/core.c:6960
 futex_do_wait kernel/futex/waitwake.c:358 [inline]
 __futex_wait+0x1c3/0x3d0 kernel/futex/waitwake.c:687
 futex_wait+0x104/0x360 kernel/futex/waitwake.c:715
 do_futex+0x333/0x420 kernel/futex/syscalls.c:130
 __do_sys_futex kernel/futex/syscalls.c:207 [inline]
 __se_sys_futex+0x36f/0x400 kernel/futex/syscalls.c:188
 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: 0033:0x7fe67538f749
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:00007fe6762900e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: ffffffffffffffda RBX: 00007fe6755e5fa8 RCX: 00007fe67538f749
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007fe6755e5fa8
RBP: 00007fe6755e5fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe6755e6038 R14: 00007ffdb1d360a0 R15: 00007ffdb1d36188
 </TASK>

Allocated by task 6447:
 kasan_save_stack mm/kasan/common.c:56 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
 poison_kmalloc_redzone mm/kasan/common.c:397 [inline]
 __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:414
 kasan_kmalloc include/linux/kasan.h:262 [inline]
 __do_kmalloc_node mm/slub.c:5657 [inline]
 __kmalloc_node_noprof+0x57a/0x820 mm/slub.c:5663
 kmalloc_node_noprof include/linux/slab.h:987 [inline]
 __bpf_map_area_alloc kernel/bpf/syscall.c:395 [inline]
 bpf_map_area_alloc+0x64/0x180 kernel/bpf/syscall.c:408
 insn_array_alloc+0x52/0x140 kernel/bpf/bpf_insn_array.c:49
 map_create+0xafd/0x16a0 kernel/bpf/syscall.c:1514
 __sys_bpf+0x5f0/0x860 kernel/bpf/syscall.c:6146
 __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

The buggy address belongs to the object at ffff8880796d9800
 which belongs to the cache kmalloc-cg-1k of size 1024
The buggy address is located 0 bytes to the right of
 allocated 944-byte region [ffff8880796d9800, ffff8880796d9bb0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x796d8
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff8880783cfa01
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88813ffb0280 dead000000000122 0000000000000000
raw: 0000000000000000 0000000080100010 00000000f5000000 ffff8880783cfa01
head: 00fff00000000040 ffff88813ffb0280 dead000000000122 0000000000000000
head: 0000000000000000 0000000080100010 00000000f5000000 ffff8880783cfa01
head: 00fff00000000003 ffffea0001e5b601 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 6341, tgid 6341 (syz-executor), ts 133223797627, free_ts 133152404370
 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_slab_page mm/slub.c:3075 [inline]
 allocate_slab+0x86/0x3b0 mm/slub.c:3248
 new_slab mm/slub.c:3302 [inline]
 ___slab_alloc+0xf2b/0x1960 mm/slub.c:4656
 __slab_alloc+0x65/0x100 mm/slub.c:4779
 __slab_alloc_node mm/slub.c:4855 [inline]
 slab_alloc_node mm/slub.c:5251 [inline]
 __do_kmalloc_node mm/slub.c:5656 [inline]
 __kvmalloc_node_noprof+0x6b6/0x920 mm/slub.c:7134
 xt_alloc_table_info+0x40/0xb0 net/netfilter/x_tables.c:1193
 do_replace net/ipv6/netfilter/ip6_tables.c:1143 [inline]
 do_ip6t_set_ctl+0x88a/0xce0 net/ipv6/netfilter/ip6_tables.c:1644
 nf_setsockopt+0x26f/0x290 net/netfilter/nf_sockopt.c:101
 do_sock_setsockopt+0x17c/0x1b0 net/socket.c:2322
 __sys_setsockopt net/socket.c:2347 [inline]
 __do_sys_setsockopt net/socket.c:2353 [inline]
 __se_sys_setsockopt net/socket.c:2350 [inline]
 __x64_sys_setsockopt+0x13f/0x1b0 net/socket.c:2350
 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 6341 tgid 6341 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
 discard_slab mm/slub.c:3346 [inline]
 __put_partials+0x146/0x170 mm/slub.c:3886
 put_cpu_partial+0x1f2/0x2d0 mm/slub.c:3961
 __slab_free+0x288/0x2a0 mm/slub.c:5952
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x97/0x100 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:349
 kasan_slab_alloc include/linux/kasan.h:252 [inline]
 slab_post_alloc_hook mm/slub.c:4953 [inline]
 slab_alloc_node mm/slub.c:5263 [inline]
 __do_kmalloc_node mm/slub.c:5656 [inline]
 __kmalloc_node_track_caller_noprof+0x526/0x820 mm/slub.c:5764
 __kmemdup_nul mm/util.c:64 [inline]
 kstrdup+0x42/0x100 mm/util.c:84
 __kernfs_new_node+0x9c/0x880 fs/kernfs/dir.c:633
 kernfs_new_node+0x102/0x210 fs/kernfs/dir.c:716
 kernfs_create_link+0xa7/0x200 fs/kernfs/symlink.c:39
 sysfs_do_create_link_sd+0x83/0x110 fs/sysfs/symlink.c:44
 driver_sysfs_add+0x89/0x210 drivers/base/dd.c:442
 device_bind_driver+0x17/0x60 drivers/base/dd.c:500
 mac80211_hwsim_new_radio+0x4a0/0x5320 drivers/net/wireless/virtual/mac80211_hwsim.c:5436

Memory state around the buggy address:
 ffff8880796d9a80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8880796d9b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8880796d9b80: 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc
                                     ^
 ffff8880796d9c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8880796d9c80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


Tested on:

commit:         22cc16c0 riscv, bpf: Fix incorrect usage of BPF_TRAMP_..
git tree:       bpf
console output: https://syzkaller.appspot.com/x/log.txt?x=15cb692a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a94030c847137a18
dashboard link: https://syzkaller.appspot.com/bug?extid=2c29addf92581b410079
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=12950e9a580000

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