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

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    46cf053e Linux 5.5-rc3
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15cf54e1e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ed9d672709340e35
dashboard link: https://syzkaller.appspot.com/bug?extid=f11cda116c57db68c227
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10563ec1e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f11cda116c57db68c227@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: use-after-free in fb_mode_is_equal+0x297/0x300  
drivers/video/fbdev/core/modedb.c:924
Read of size 4 at addr ffff8880946fcf9c by task syz-executor.3/9638

CPU: 0 PID: 9638 Comm: syz-executor.3 Not tainted 5.5.0-rc3-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_load4_noabort+0x14/0x20 mm/kasan/generic_report.c:134
 fb_mode_is_equal+0x297/0x300 drivers/video/fbdev/core/modedb.c:924
 fbcon_mode_deleted+0x12c/0x190 drivers/video/fbdev/core/fbcon.c:3060
 fb_set_var+0xab9/0xdd0 drivers/video/fbdev/core/fbmem.c:971
 do_fb_ioctl+0x390/0x7d0 drivers/video/fbdev/core/fbmem.c:1104
 fb_ioctl+0xe6/0x130 drivers/video/fbdev/core/fbmem.c:1180
 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: 0033:0x45a919
Code: ad b6 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 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ff6d156bc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a919
RDX: 0000000020000000 RSI: 0000000000004601 RDI: 0000000000000013
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ff6d156c6d4
R13: 00000000004c310d R14: 00000000004d8498 R15: 00000000ffffffff

Allocated by task 9451:
 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
 kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3551
 kmalloc include/linux/slab.h:556 [inline]
 fb_add_videomode drivers/video/fbdev/core/modedb.c:1073 [inline]
 fb_add_videomode+0x2fb/0x610 drivers/video/fbdev/core/modedb.c:1057
 fb_set_var+0x5ef/0xdd0 drivers/video/fbdev/core/fbmem.c:1041
 do_fb_ioctl+0x390/0x7d0 drivers/video/fbdev/core/fbmem.c:1104
 fb_ioctl+0xe6/0x130 drivers/video/fbdev/core/fbmem.c:1180
 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 9620:
 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
 fb_delete_videomode+0x3fa/0x540 drivers/video/fbdev/core/modedb.c:1104
 fb_set_var+0xac8/0xdd0 drivers/video/fbdev/core/fbmem.c:974
 do_fb_ioctl+0x390/0x7d0 drivers/video/fbdev/core/fbmem.c:1104
 fb_ioctl+0xe6/0x130 drivers/video/fbdev/core/fbmem.c:1180
 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 ffff8880946fcf80
 which belongs to the cache kmalloc-96 of size 96
The buggy address is located 28 bytes inside of
 96-byte region [ffff8880946fcf80, ffff8880946fcfe0)
The buggy address belongs to the page:
page:ffffea000251bf00 refcount:1 mapcount:0 mapping:ffff8880aa400540  
index:0x0
raw: 00fffe0000000200 ffffea00023b2ac8 ffffea000203fc08 ffff8880aa400540
raw: 0000000000000000 ffff8880946fc000 0000000100000020 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880946fce80: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
 ffff8880946fcf00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
> ffff8880946fcf80: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                            ^
 ffff8880946fd000: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8880946fd080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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