Message-ID: <CALW65jac7taVKERiUUymAQ4zqzkHzqqE+RpTJP3wzFj5xvnehg@mail.gmail.com>
Date: Mon, 9 Feb 2026 16:29:43 +0800
From: Qingfang Deng <dqfext@...il.com>
To: syzbot ci <syzbot+ci73cb85ac9652d7d4@...kaller.appspotmail.com>
Cc: andrew@...n.ch, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	linux-kernel@...r.kernel.org, linux-ppp@...r.kernel.org, 
	netdev@...r.kernel.org, pabeni@...hat.com, syzbot@...ts.linux.dev, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot ci] Re: ppp: don't store tx skb in the fastpath

On Mon, Feb 9, 2026 at 3:29 PM syzbot ci
<syzbot+ci73cb85ac9652d7d4@...kaller.appspotmail.com> wrote:
>
> syzbot ci has tested the following series
>
> [v1] ppp: don't store tx skb in the fastpath
> https://lore.kernel.org/all/20260209021134.21194-1-dqfext@gmail.com
> * [RFC PATCH net-next] ppp: don't store tx skb in the fastpath
>
> and found the following issue:
> KASAN: slab-use-after-free Write in sk_skb_reason_drop
>
> Full report is available here:
> https://ci.syzbot.org/series/0307aa1e-223a-4cd7-af71-1a3c3b7b33f0
>
> ***
>
> KASAN: slab-use-after-free Write in sk_skb_reason_drop
>
> tree:      net-next
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net-next.git
> base:      57be33f85e369ce9f69f61eaa34734e0d3bd47a7
> arch:      amd64
> compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> config:    https://ci.syzbot.org/builds/3b8a7fc0-258a-4d96-b4cf-cf828112415c/config
> C repro:   https://ci.syzbot.org/findings/43f9458e-e8a7-4c35-83a9-fd92f2c65af4/c_repro
> syz repro: https://ci.syzbot.org/findings/43f9458e-e8a7-4c35-83a9-fd92f2c65af4/syz_repro
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
> BUG: KASAN: slab-use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
> BUG: KASAN: slab-use-after-free in __refcount_sub_and_test include/linux/refcount.h:389 [inline]
> BUG: KASAN: slab-use-after-free in __refcount_dec_and_test include/linux/refcount.h:432 [inline]
> BUG: KASAN: slab-use-after-free in refcount_dec_and_test include/linux/refcount.h:450 [inline]
> BUG: KASAN: slab-use-after-free in skb_unref include/linux/skbuff.h:1292 [inline]
> BUG: KASAN: slab-use-after-free in __sk_skb_reason_drop net/core/skbuff.c:1226 [inline]
> BUG: KASAN: slab-use-after-free in sk_skb_reason_drop+0x37/0x170 net/core/skbuff.c:1254
> Write of size 4 at addr ffff88811262cce4 by task syz.0.17/5962
>
> CPU: 1 UID: 0 PID: 5962 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xba/0x230 mm/kasan/report.c:482
>  kasan_report+0x117/0x150 mm/kasan/report.c:595
>  check_region_inline mm/kasan/generic.c:-1 [inline]
>  kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
>  instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
>  atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:400 [inline]
>  __refcount_sub_and_test include/linux/refcount.h:389 [inline]
>  __refcount_dec_and_test include/linux/refcount.h:432 [inline]
>  refcount_dec_and_test include/linux/refcount.h:450 [inline]
>  skb_unref include/linux/skbuff.h:1292 [inline]
>  __sk_skb_reason_drop net/core/skbuff.c:1226 [inline]
>  sk_skb_reason_drop+0x37/0x170 net/core/skbuff.c:1254
>  kfree_skb_reason include/linux/skbuff.h:1322 [inline]
>  kfree_skb include/linux/skbuff.h:1331 [inline]
>  ppp_push+0x1284/0x1440 drivers/net/ppp/ppp_generic.c:-1
>  __ppp_xmit_process drivers/net/ppp/ppp_generic.c:1680 [inline]
>  ppp_xmit_process+0x9d7/0x1d90 drivers/net/ppp/ppp_generic.c:1706
>  ppp_write+0x2c6/0x400 drivers/net/ppp/ppp_generic.c:540
>  do_loop_readv_writev fs/read_write.c:850 [inline]
>  vfs_writev+0x4bd/0x990 fs/read_write.c:1059
>  do_pwritev fs/read_write.c:1153 [inline]
>  __do_sys_pwritev fs/read_write.c:1199 [inline]
>  __se_sys_pwritev fs/read_write.c:1194 [inline]
>  __x64_sys_pwritev+0x19f/0x2a0 fs/read_write.c:1194
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f1a1d39acb9
> 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:00007ffc1381da18 EFLAGS: 00000246 ORIG_RAX: 0000000000000128
> RAX: ffffffffffffffda RBX: 00007f1a1d615fa0 RCX: 00007f1a1d39acb9
> RDX: 0000000000000001 RSI: 00002000000002c0 RDI: 0000000000000003
> RBP: 00007f1a1d408bf7 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000007 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f1a1d615fac R14: 00007f1a1d615fa0 R15: 00007f1a1d615fa0
>  </TASK>
>
> Allocated by task 5962:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  unpoison_slab_object mm/kasan/common.c:340 [inline]
>  __kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
>  kasan_slab_alloc include/linux/kasan.h:253 [inline]
>  slab_post_alloc_hook mm/slub.c:4953 [inline]
>  slab_alloc_node mm/slub.c:5263 [inline]
>  kmem_cache_alloc_node_noprof+0x427/0x6f0 mm/slub.c:5315
>  __alloc_skb+0x1d0/0x7d0 net/core/skbuff.c:702
>  alloc_skb include/linux/skbuff.h:1383 [inline]
>  ppp_write+0xb1/0x400 drivers/net/ppp/ppp_generic.c:523
>  do_loop_readv_writev fs/read_write.c:850 [inline]
>  vfs_writev+0x4bd/0x990 fs/read_write.c:1059
>  do_pwritev fs/read_write.c:1153 [inline]
>  __do_sys_pwritev fs/read_write.c:1199 [inline]
>  __se_sys_pwritev fs/read_write.c:1194 [inline]
>  __x64_sys_pwritev+0x19f/0x2a0 fs/read_write.c:1194
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 5962:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
>  poison_slab_object mm/kasan/common.c:253 [inline]
>  __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
>  kasan_slab_free include/linux/kasan.h:235 [inline]
>  slab_free_hook mm/slub.c:2540 [inline]
>  slab_free mm/slub.c:6674 [inline]
>  kmem_cache_free+0x195/0x610 mm/slub.c:6785
>  ppp_prepare_tx_skb drivers/net/ppp/ppp_generic.c:1850 [inline]
>  __ppp_xmit_process drivers/net/ppp/ppp_generic.c:1676 [inline]
>  ppp_xmit_process+0x1147/0x1d90 drivers/net/ppp/ppp_generic.c:1706
>  ppp_write+0x2c6/0x400 drivers/net/ppp/ppp_generic.c:540
>  do_loop_readv_writev fs/read_write.c:850 [inline]
>  vfs_writev+0x4bd/0x990 fs/read_write.c:1059
>  do_pwritev fs/read_write.c:1153 [inline]
>  __do_sys_pwritev fs/read_write.c:1199 [inline]
>  __se_sys_pwritev fs/read_write.c:1194 [inline]
>  __x64_sys_pwritev+0x19f/0x2a0 fs/read_write.c:1194
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f

I missed that ppp_send_frame() can allocate a new skb and free the old
one. Will fix it later.

>
> The buggy address belongs to the object at ffff88811262cc00
>  which belongs to the cache skbuff_head_cache of size 240
> The buggy address is located 228 bytes inside of
>  freed 240-byte region [ffff88811262cc00, ffff88811262ccf0)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x11262c
> head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0x17ff00000000040(head|node=0|zone=2|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 017ff00000000040 ffff8881616f28c0 ffffea00044a4780 dead000000000002
> raw: 0000000000000000 0000000080150015 00000000f5000000 0000000000000000
> head: 017ff00000000040 ffff8881616f28c0 ffffea00044a4780 dead000000000002
> head: 0000000000000000 0000000080150015 00000000f5000000 0000000000000000
> head: 017ff00000000001 ffffea0004498b01 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 1, tgid 1 (swapper/0), ts 20587824573, free_ts 0
>  set_page_owner include/linux/page_owner.h:32 [inline]
>  post_alloc_hook+0x228/0x280 mm/page_alloc.c:1884
>  prep_new_page mm/page_alloc.c:1892 [inline]
>  get_page_from_freelist+0x24dc/0x2580 mm/page_alloc.c:3945
>  __alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5240
>  alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2486
>  alloc_slab_page mm/slub.c:3075 [inline]
>  allocate_slab+0x86/0x3a0 mm/slub.c:3248
>  new_slab mm/slub.c:3302 [inline]
>  ___slab_alloc+0xd82/0x1760 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]
>  kmem_cache_alloc_node_noprof+0x4b5/0x6f0 mm/slub.c:5315
>  __alloc_skb+0x1d0/0x7d0 net/core/skbuff.c:702
>  alloc_skb include/linux/skbuff.h:1383 [inline]
>  nlmsg_new include/net/netlink.h:1055 [inline]
>  rtmsg_ifinfo_build_skb+0x84/0x260 net/core/rtnetlink.c:4407
>  rtmsg_ifinfo_event net/core/rtnetlink.c:4449 [inline]
>  rtmsg_ifinfo+0x8c/0x1a0 net/core/rtnetlink.c:4458
>  register_netdevice+0x175f/0x1ae0 net/core/dev.c:11458
>  register_netdev+0x40/0x60 net/core/dev.c:11522
>  nr_proto_init+0x14a/0x720 net/netrom/af_netrom.c:1424
>  do_one_initcall+0x250/0x840 init/main.c:1378
>  do_initcall_level+0x104/0x190 init/main.c:1440
> page_owner free stack trace missing
>
> Memory state around the buggy address:
>  ffff88811262cb80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff88811262cc00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88811262cc80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc
>                                                        ^
>  ffff88811262cd00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff88811262cd80: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
>   Tested-by: syzbot@...kaller.appspotmail.com
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at syzkaller@...glegroups.com.

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