Re: KASAN: use-after-free Read in n_tty_receive_buf_common

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    9455d25f Merge tag 'ntb-5.5' of git://github.com/jonmason/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=174b042ae00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7a3b8f5088d4043a
dashboard link: https://syzkaller.appspot.com/bug?extid=59997e8d5cbdc486e6f6
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14c641dae00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=110d477ae00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+59997e8d5cbdc486e6f6@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: use-after-free in n_tty_check_throttle drivers/tty/n_tty.c:266  
[inline]
BUG: KASAN: use-after-free in n_tty_receive_buf_common+0x270f/0x2b70  
drivers/tty/n_tty.c:1761
Read of size 1 at addr ffff88809f900b01 by task syz-executor920/9030

CPU: 1 PID: 9030 Comm: syz-executor920 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x197/0x210 lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
 __kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
 kasan_report+0x12/0x20 mm/kasan/common.c:639
 __asan_report_load1_noabort+0x14/0x20 mm/kasan/generic_report.c:132
 n_tty_check_throttle drivers/tty/n_tty.c:266 [inline]
 n_tty_receive_buf_common+0x270f/0x2b70 drivers/tty/n_tty.c:1761
 n_tty_receive_buf2+0x34/0x40 drivers/tty/n_tty.c:1777
 tty_ldisc_receive_buf+0xad/0x1c0 drivers/tty/tty_buffer.c:461
 paste_selection+0x1ff/0x460 drivers/tty/vt/selection.c:372
 tioclinux+0x133/0x480 drivers/tty/vt/vt.c:3044
 vt_ioctl+0x1a41/0x26d0 drivers/tty/vt/vt_ioctl.c:364
 tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
 vfs_ioctl fs/ioctl.c:47 [inline]
 file_ioctl fs/ioctl.c:545 [inline]
 do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x441309
Code: e8 ac e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 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 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fffea28a108 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000441309
RDX: 0000000020000000 RSI: 000000000000541c RDI: 0000000000000003
RBP: 0000000000012d00 R08: 000000000000000d R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000402080
R13: 0000000000402110 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 9027:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 __kasan_kmalloc mm/kasan/common.c:513 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:486
 kasan_kmalloc+0x9/0x10 mm/kasan/common.c:527
 __do_kmalloc mm/slab.c:3656 [inline]
 __kmalloc+0x163/0x770 mm/slab.c:3665
 kmalloc_array include/linux/slab.h:598 [inline]
 set_selection_kernel+0x872/0x13b0 drivers/tty/vt/selection.c:305
 set_selection_user+0x95/0xd9 drivers/tty/vt/selection.c:177
 tioclinux+0x11c/0x480 drivers/tty/vt/vt.c:3039
 vt_ioctl+0x1a41/0x26d0 drivers/tty/vt/vt_ioctl.c:364
 tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
 vfs_ioctl fs/ioctl.c:47 [inline]
 file_ioctl fs/ioctl.c:545 [inline]
 do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 9028:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 kasan_set_free_info mm/kasan/common.c:335 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:474
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:483
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x10a/0x2c0 mm/slab.c:3757
 set_selection_kernel+0x88f/0x13b0 drivers/tty/vt/selection.c:312
 set_selection_user+0x95/0xd9 drivers/tty/vt/selection.c:177
 tioclinux+0x11c/0x480 drivers/tty/vt/vt.c:3039
 vt_ioctl+0x1a41/0x26d0 drivers/tty/vt/vt_ioctl.c:364
 tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
 vfs_ioctl fs/ioctl.c:47 [inline]
 file_ioctl fs/ioctl.c:545 [inline]
 do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
 do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88809f900b00
 which belongs to the cache kmalloc-96 of size 96
The buggy address is located 1 bytes inside of
 96-byte region [ffff88809f900b00, ffff88809f900b60)
The buggy address belongs to the page:
page:ffffea00027e4000 refcount:1 mapcount:0 mapping:ffff8880aa400540  
index:0x0
raw: 00fffe0000000200 ffffea0002871bc8 ffffea00027dc148 ffff8880aa400540
raw: 0000000000000000 ffff88809f900000 0000000100000020 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88809f900a00: 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc
 ffff88809f900a80: 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc
> ffff88809f900b00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                   ^
 ffff88809f900b80: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
 ffff88809f900c00: 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc
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