KASAN: use-after-free Write in vcs_read

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    605cbf3d Add linux-next specific files for 20200820
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=157e75ce900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a61d44f28687f508
dashboard link: https://syzkaller.appspot.com/bug?extid=ad1f53726c3bd11180cb
compiler:       gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ad1f53726c3bd11180cb@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: use-after-free in vcs_read_buf drivers/tty/vt/vc_screen.c:357 [inline]
BUG: KASAN: use-after-free in vcs_read+0xaa7/0xb40 drivers/tty/vt/vc_screen.c:449
Write of size 2 at addr ffff8880a8014000 by task syz-executor.5/16936

CPU: 1 PID: 16936 Comm: syz-executor.5 Not tainted 5.9.0-rc1-next-20200820-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+0x18f/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 vcs_read_buf drivers/tty/vt/vc_screen.c:357 [inline]
 vcs_read+0xaa7/0xb40 drivers/tty/vt/vc_screen.c:449
 vfs_read+0x1df/0x5a0 fs/read_write.c:479
 ksys_read+0x12d/0x250 fs/read_write.c:607
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d4d9
Code: 5d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 2b b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fa15c60dc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000000025ec0 RCX: 000000000045d4d9
RDX: 0000000000002020 RSI: 0000000020000340 RDI: 0000000000000003
RBP: 000000000118cf80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007ffd27ec646f R14: 00007fa15c60e9c0 R15: 000000000118cf4c

Allocated by task 5:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
 slab_post_alloc_hook mm/slab.h:517 [inline]
 slab_alloc_node mm/slab.c:3254 [inline]
 kmem_cache_alloc_node+0x136/0x3f0 mm/slab.c:3574
 __alloc_skb+0x71/0x550 net/core/skbuff.c:198
 alloc_skb include/linux/skbuff.h:1085 [inline]
 nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:501 [inline]
 nsim_dev_trap_report drivers/net/netdevsim/dev.c:558 [inline]
 nsim_dev_trap_report_work+0x2b2/0xbe0 drivers/net/netdevsim/dev.c:599
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Freed by task 5:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
 kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
 __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:422
 __cache_free mm/slab.c:3418 [inline]
 kmem_cache_free.part.0+0x67/0x1f0 mm/slab.c:3693
 kfree_skbmem+0xef/0x1b0 net/core/skbuff.c:622
 __kfree_skb net/core/skbuff.c:679 [inline]
 consume_skb net/core/skbuff.c:837 [inline]
 consume_skb+0xcf/0x160 net/core/skbuff.c:831
 nsim_dev_trap_report drivers/net/netdevsim/dev.c:574 [inline]
 nsim_dev_trap_report_work+0x889/0xbe0 drivers/net/netdevsim/dev.c:599
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

The buggy address belongs to the object at ffff8880a8014000
 which belongs to the cache skbuff_head_cache of size 224
The buggy address is located 0 bytes inside of
 224-byte region [ffff8880a8014000, ffff8880a80140e0)
The buggy address belongs to the page:
page:00000000a263cd02 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0xa8014
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea000277a2c8 ffffea0002551888 ffff8880a9050d00
raw: 0000000000000000 ffff8880a8014000 000000010000000c 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880a8013f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8880a8013f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8880a8014000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff8880a8014080: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
 ffff8880a8014100: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
==================================================================


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