Re: KASAN: slab-out-of-bounds Read in fbcon_get_font

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Tue, Dec 3, 2019 at 11:25 PM syzbot
<syzbot+4455ca3b3291de891abc@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    76bb8b05 Merge tag 'kbuild-v5.5' of git://git.kernel.org/p..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10bfe282e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
> dashboard link: https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+4455ca3b3291de891abc@syzkaller.appspotmail.com
>
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in memcpy include/linux/string.h:380 [inline]
> BUG: KASAN: slab-out-of-bounds in fbcon_get_font+0x2b2/0x5e0
> drivers/video/fbdev/core/fbcon.c:2465
> Read of size 16 at addr ffff888094b0aa10 by task syz-executor414/9999

So fbcon allocates some memory, security/tomoyo goes around and frees
it, fbcon goes boom because the memory is gone. I'm kinda leaning
towards "not an fbcon bug". Adding relevant security folks and mailing
lists.

But from a very quick look in tomoyo it loosk more like "machine on
fire, random corruption all over". No idea what's going on here.
-Daniel


>
> CPU: 0 PID: 9999 Comm: syz-executor414 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:638
>   check_memory_region_inline mm/kasan/generic.c:185 [inline]
>   check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
>   memcpy+0x24/0x50 mm/kasan/common.c:124
>   memcpy include/linux/string.h:380 [inline]
>   fbcon_get_font+0x2b2/0x5e0 drivers/video/fbdev/core/fbcon.c:2465
>   con_font_get drivers/tty/vt/vt.c:4446 [inline]
>   con_font_op+0x20b/0x1250 drivers/tty/vt/vt.c:4605
>   vt_ioctl+0x181a/0x26d0 drivers/tty/vt/vt_ioctl.c:965
>   tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
>   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: 0033:0x4444d9
> Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 7b d8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007fff6f4393b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007fff6f4393c0 RCX: 00000000004444d9
> RDX: 0000000020000440 RSI: 0000000000004b72 RDI: 0000000000000005
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000400da0
> R10: 00007fff6f438f00 R11: 0000000000000246 R12: 00000000004021e0
> R13: 0000000000402270 R14: 0000000000000000 R15: 0000000000000000
>
> Allocated by task 9999:
>   save_stack+0x23/0x90 mm/kasan/common.c:71
>   set_track mm/kasan/common.c:79 [inline]
>   __kasan_kmalloc mm/kasan/common.c:512 [inline]
>   __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:485
>   kasan_kmalloc+0x9/0x10 mm/kasan/common.c:526
>   __do_kmalloc mm/slab.c:3656 [inline]
>   __kmalloc+0x163/0x770 mm/slab.c:3665
>   kmalloc include/linux/slab.h:561 [inline]
>   fbcon_set_font+0x32d/0x860 drivers/video/fbdev/core/fbcon.c:2663
>   con_font_set drivers/tty/vt/vt.c:4538 [inline]
>   con_font_op+0xe18/0x1250 drivers/tty/vt/vt.c:4603
>   vt_ioctl+0xd2e/0x26d0 drivers/tty/vt/vt_ioctl.c:913
>   tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
>   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 9771:
>   save_stack+0x23/0x90 mm/kasan/common.c:71
>   set_track mm/kasan/common.c:79 [inline]
>   kasan_set_free_info mm/kasan/common.c:334 [inline]
>   __kasan_slab_free+0x102/0x150 mm/kasan/common.c:473
>   kasan_slab_free+0xe/0x10 mm/kasan/common.c:482
>   __cache_free mm/slab.c:3426 [inline]
>   kfree+0x10a/0x2c0 mm/slab.c:3757
>   tomoyo_init_log+0x15c1/0x2070 security/tomoyo/audit.c:294
>   tomoyo_supervisor+0x33f/0xef0 security/tomoyo/common.c:2095
>   tomoyo_audit_env_log security/tomoyo/environ.c:36 [inline]
>   tomoyo_env_perm+0x18e/0x210 security/tomoyo/environ.c:63
>   tomoyo_environ security/tomoyo/domain.c:670 [inline]
>   tomoyo_find_next_domain+0x1354/0x1f6c security/tomoyo/domain.c:876
>   tomoyo_bprm_check_security security/tomoyo/tomoyo.c:107 [inline]
>   tomoyo_bprm_check_security+0x124/0x1a0 security/tomoyo/tomoyo.c:97
>   security_bprm_check+0x63/0xb0 security/security.c:784
>   search_binary_handler+0x71/0x570 fs/exec.c:1645
>   exec_binprm fs/exec.c:1701 [inline]
>   __do_execve_file.isra.0+0x1329/0x22b0 fs/exec.c:1821
>   do_execveat_common fs/exec.c:1867 [inline]
>   do_execve fs/exec.c:1884 [inline]
>   __do_sys_execve fs/exec.c:1960 [inline]
>   __se_sys_execve fs/exec.c:1955 [inline]
>   __x64_sys_execve+0x8f/0xc0 fs/exec.c:1955
>   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 ffff888094b0a000
>   which belongs to the cache kmalloc-4k of size 4096
> The buggy address is located 2576 bytes inside of
>   4096-byte region [ffff888094b0a000, ffff888094b0b000)
> The buggy address belongs to the page:
> page:ffffea000252c280 refcount:1 mapcount:0 mapping:ffff8880aa402000
> index:0x0 compound_mapcount: 0
> raw: 00fffe0000010200 ffffea0002a3ae08 ffffea0002a6aa88 ffff8880aa402000
> raw: 0000000000000000 ffff888094b0a000 0000000100000001 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
>   ffff888094b0a900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>   ffff888094b0a980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > ffff888094b0aa00: 00 00 fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>                           ^
>   ffff888094b0aa80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>   ffff888094b0ab00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
>
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this bug report. See:
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> syzbot can test patches for this bug, for details see:
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch