linux-kernel - Re: [syzbot] [net?] KASAN: null-ptr-deref Write in rcuref

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Message-ID: <689095a5.050a0220.1fc43d.0009.GAE@google.com>
Date: Mon, 04 Aug 2025 04:12:37 -0700
From: syzbot <syzbot+27d7cfbc93457e472e00@...kaller.appspotmail.com>
To: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org, 
	kuba@...nel.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	pabeni@...hat.com, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] KASAN: null-ptr-deref Write in rcuref_put (4)

syzbot has found a reproducer for the following issue on:

HEAD commit:    5c5a10f0be96 Add linux-next specific files for 20250804
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=14f23aa2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f4ccbd076877954b
dashboard link: https://syzkaller.appspot.com/bug?extid=27d7cfbc93457e472e00
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1628faa2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12490434580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/cb2134de7be8/disk-5c5a10f0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fe8a36605e0c/vmlinux-5c5a10f0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/12df22603d55/bzImage-5c5a10f0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+27d7cfbc93457e472e00@...kaller.appspotmail.com

BUG: unable to handle page fault for address: ffffffffffffffdb
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD df3b067 P4D df3b067 PUD df3d067 PMD 0 
Oops: Oops: 0002 [#1] SMP KASAN PTI
CPU: 1 UID: 0 PID: 6346 Comm: syz.0.336 Not tainted 6.16.0-next-20250804-syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:arch_atomic_add_return arch/x86/include/asm/atomic.h:85 [inline]
RIP: 0010:raw_atomic_sub_return_release include/linux/atomic/atomic-arch-fallback.h:846 [inline]
RIP: 0010:atomic_sub_return_release include/linux/atomic/atomic-instrumented.h:327 [inline]
RIP: 0010:__rcuref_put include/linux/rcuref.h:109 [inline]
RIP: 0010:rcuref_put+0x172/0x210 include/linux/rcuref.h:173
Code: c7 c7 80 72 94 8c be 68 00 00 00 48 c7 c2 c0 72 94 8c e8 21 f7 35 f8 48 89 df be 04 00 00 00 e8 84 90 bb f8 41 be ff ff ff ff <f0> 44 0f c1 33 41 8d 76 ff bf ff ff ff ff e8 ab 27 58 f8 41 ff ce
RSP: 0018:ffffc9000431f960 EFLAGS: 00010256
RAX: ffffffff89678f01 RBX: ffffffffffffffdb RCX: ffffffff89678fdc
RDX: 0000000000000001 RSI: 0000000000000004 RDI: ffffffffffffffdb
RBP: ffffc9000431f9f8 R08: ffffffffffffffde R09: 1ffffffffffffffb
R10: dffffc0000000000 R11: fffffbfffffffffc R12: dffffc0000000000
R13: dffffc0000000000 R14: 00000000ffffffff R15: 1ffff92000863f2c
FS:  0000555583637500(0000) GS:ffff888125d17000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffdb CR3: 000000002465a000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 dst_release+0x24/0x1b0 net/core/dst.c:167
 ip_rt_put include/net/route.h:285 [inline]
 pptp_xmit+0x14b/0x1a90 drivers/net/ppp/pptp.c:267
 __ppp_channel_push+0xf2/0x1c0 drivers/net/ppp/ppp_generic.c:2166
 ppp_channel_push+0x123/0x660 drivers/net/ppp/ppp_generic.c:2198
 ppp_write+0x2b0/0x400 drivers/net/ppp/ppp_generic.c:544
 vfs_write+0x27b/0xb30 fs/read_write.c:684
 ksys_write+0x145/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fda7098eb69
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:00007fffb2fc7658 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fda70bb5fa0 RCX: 00007fda7098eb69
RDX: 0000000000000013 RSI: 00002000000002c0 RDI: 0000000000000004
RBP: 00007fda70a11df1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fda70bb5fa0 R14: 00007fda70bb5fa0 R15: 0000000000000003
 </TASK>
Modules linked in:
CR2: ffffffffffffffdb
---[ end trace 0000000000000000 ]---
RIP: 0010:arch_atomic_add_return arch/x86/include/asm/atomic.h:85 [inline]
RIP: 0010:raw_atomic_sub_return_release include/linux/atomic/atomic-arch-fallback.h:846 [inline]
RIP: 0010:atomic_sub_return_release include/linux/atomic/atomic-instrumented.h:327 [inline]
RIP: 0010:__rcuref_put include/linux/rcuref.h:109 [inline]
RIP: 0010:rcuref_put+0x172/0x210 include/linux/rcuref.h:173
Code: c7 c7 80 72 94 8c be 68 00 00 00 48 c7 c2 c0 72 94 8c e8 21 f7 35 f8 48 89 df be 04 00 00 00 e8 84 90 bb f8 41 be ff ff ff ff <f0> 44 0f c1 33 41 8d 76 ff bf ff ff ff ff e8 ab 27 58 f8 41 ff ce
RSP: 0018:ffffc9000431f960 EFLAGS: 00010256
RAX: ffffffff89678f01 RBX: ffffffffffffffdb RCX: ffffffff89678fdc
RDX: 0000000000000001 RSI: 0000000000000004 RDI: ffffffffffffffdb
RBP: ffffc9000431f9f8 R08: ffffffffffffffde R09: 1ffffffffffffffb
R10: dffffc0000000000 R11: fffffbfffffffffc R12: dffffc0000000000
R13: dffffc0000000000 R14: 00000000ffffffff R15: 1ffff92000863f2c
FS:  0000555583637500(0000) GS:ffff888125d17000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffdb CR3: 000000002465a000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	c7 c7 80 72 94 8c    	mov    $0x8c947280,%edi
   6:	be 68 00 00 00       	mov    $0x68,%esi
   b:	48 c7 c2 c0 72 94 8c 	mov    $0xffffffff8c9472c0,%rdx
  12:	e8 21 f7 35 f8       	call   0xf835f738
  17:	48 89 df             	mov    %rbx,%rdi
  1a:	be 04 00 00 00       	mov    $0x4,%esi
  1f:	e8 84 90 bb f8       	call   0xf8bb90a8
  24:	41 be ff ff ff ff    	mov    $0xffffffff,%r14d
* 2a:	f0 44 0f c1 33       	lock xadd %r14d,(%rbx) <-- trapping instruction
  2f:	41 8d 76 ff          	lea    -0x1(%r14),%esi
  33:	bf ff ff ff ff       	mov    $0xffffffff,%edi
  38:	e8 ab 27 58 f8       	call   0xf85827e8
  3d:	41 ff ce             	dec    %r14d


---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.