[Android 6.1] KASAN: null-ptr-deref Write in n_tty_flush_buffer

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: db710ea87c32 BACKPORT: mm: avoid unconditional one-tick sl..
git tree: android14-6.1
console+strace: https://syzkaller.appspot.com/x/log.txt?x=13b2f00c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=62149ca91101e547
dashboard link: https://syzkaller.appspot.com/bug?extid=6b46de683272d9ba4716
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17b2f00c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16f42c82580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/34e0e938b0cc/disk-db710ea8.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/aa7b00cb205a/vmlinux-db710ea8.xz
kernel image: https://storage.googleapis.com/syzbot-assets/acf5e1aff905/bzImage-db710ea8.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+6b46de...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in reset_buffer_flags drivers/tty/n_tty.c:332 [inline]
BUG: KASAN: null-ptr-deref in n_tty_flush_buffer+0x6c/0x220 drivers/tty/n_tty.c:371
Write of size 24 at addr 0000000000000000 by task kworker/0:3/303

CPU: 0 PID: 303 Comm: kworker/0:3 Not tainted 6.1.138-syzkaller-00046-gdb710ea87c32 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Workqueue: events vc_SAK
Call Trace:
<TASK>
__dump_stack+0x21/0x24 lib/dump_stack.c:88
dump_stack_lvl+0xee/0x150 lib/dump_stack.c:106
print_report+0x3d/0x60 mm/kasan/report.c:430
kasan_report+0x122/0x150 mm/kasan/report.c:531
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x280/0x290 mm/kasan/generic.c:189
memset+0x23/0x40 mm/kasan/shadow.c:44
reset_buffer_flags drivers/tty/n_tty.c:332 [inline]
n_tty_flush_buffer+0x6c/0x220 drivers/tty/n_tty.c:371
tty_buffer_flush+0x31d/0x3e0 drivers/tty/tty_buffer.c:242
tty_ldisc_flush+0x6c/0xc0 drivers/tty/tty_ldisc.c:389
__do_SAK+0x132/0x640 drivers/tty/tty_io.c:3011
vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:995
process_one_work+0x71f/0xc40 kernel/workqueue.c:2299
worker_thread+0xa29/0x11f0 kernel/workqueue.c:2446
kthread+0x281/0x320 kernel/kthread.c:386
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>
==================================================================
BUG: unable to handle page fault for address: 0000000000002260
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 303 Comm: kworker/0:3 Tainted: G B 6.1.138-syzkaller-00046-gdb710ea87c32 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Workqueue: events vc_SAK
RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
RSP: 0018:ffffc90000fa7b50 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 0000000000000010 RCX: 0000000000000010
RDX: 0000000000000010 RSI: 0000000000000000 RDI: 0000000000002260
RBP: ffffc90000fa7b70 R08: 0000000000000000 R09: 0000000000002260
R10: fffffbfff0f2cafd R11: 1ffffffff0f2cafc R12: 000000000000005d
R13: 0000000000002260 R14: 0000000000000000 R15: 0000000000002260
FS: 0000000000000000(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000002260 CR3: 000000011cf43000 CR4: 00000000003506b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
reset_buffer_flags drivers/tty/n_tty.c:334 [inline]
n_tty_flush_buffer+0x7b/0x220 drivers/tty/n_tty.c:371
tty_buffer_flush+0x31d/0x3e0 drivers/tty/tty_buffer.c:242
tty_ldisc_flush+0x6c/0xc0 drivers/tty/tty_ldisc.c:389
__do_SAK+0x132/0x640 drivers/tty/tty_io.c:3011
vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:995
process_one_work+0x71f/0xc40 kernel/workqueue.c:2299
worker_thread+0xa29/0x11f0 kernel/workqueue.c:2446
kthread+0x281/0x320 kernel/kthread.c:386
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>
Modules linked in:
CR2: 0000000000002260
---[ end trace 0000000000000000 ]---
RIP: 0010:memset_erms+0x9/0x10 arch/x86/lib/memset_64.S:64
Code: c1 e9 03 40 0f b6 f6 48 b8 01 01 01 01 01 01 01 01 48 0f af c6 f3 48 ab 89 d1 f3 aa 4c 89 c8 c3 90 49 89 f9 40 88 f0 48 89 d1 <f3> aa 4c 89 c8 c3 90 49 89 fa 40 0f b6 ce 48 b8 01 01 01 01 01 01
RSP: 0018:ffffc90000fa7b50 EFLAGS: 00010202
RAX: 0000000000000000 RBX: 0000000000000010 RCX: 0000000000000010
RDX: 0000000000000010 RSI: 0000000000000000 RDI: 0000000000002260
RBP: ffffc90000fa7b70 R08: 0000000000000000 R09: 0000000000002260
R10: fffffbfff0f2cafd R11: 1ffffffff0f2cafc R12: 000000000000005d
R13: 0000000000002260 R14: 0000000000000000 R15: 0000000000002260
FS: 0000000000000000(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000002260 CR3: 000000011cf43000 CR4: 00000000003506b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: c1 e9 03 shr $0x3,%ecx
3: 40 0f b6 f6 movzbl %sil,%esi
7: 48 b8 01 01 01 01 01 movabs $0x101010101010101,%rax
e: 01 01 01
11: 48 0f af c6 imul %rsi,%rax
15: f3 48 ab rep stos %rax,%es:(%rdi)
18: 89 d1 mov %edx,%ecx
1a: f3 aa rep stos %al,%es:(%rdi)
1c: 4c 89 c8 mov %r9,%rax
1f: c3 ret
20: 90 nop
21: 49 89 f9 mov %rdi,%r9
24: 40 88 f0 mov %sil,%al
27: 48 89 d1 mov %rdx,%rcx
* 2a: f3 aa rep stos %al,%es:(%rdi) <-- trapping instruction
2c: 4c 89 c8 mov %r9,%rax
2f: c3 ret
30: 90 nop
31: 49 89 fa mov %rdi,%r10
34: 40 0f b6 ce movzbl %sil,%ecx
38: 48 rex.W
39: b8 01 01 01 01 mov $0x1010101,%eax
3e: 01 01 add %eax,(%rcx)


---
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