[PATCH] smack: fix slab-out-of-bounds by checking for overflow

[Casey Schaufler]

On 7/25/2020 5:58 AM, B K Karthik wrote:
> add a barrier to smk_set_cipso() to check for overflow

Thank you for your patch. Dan Carpenter <dan.carpenter at oracle.com>
has already submitted an identical patch.

>
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in vsscanf+0x2666/0x2ef0 lib/vsprintf.c:3321
> Read of size 1 at addr ffff888097d682b8 by task syz-executor980/6804
>
> CPU: 0 PID: 6804 Comm: syz-executor980 Not tainted 5.8.0-rc5-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+0x1f0/0x31e lib/dump_stack.c:118
>  print_address_description+0x66/0x5a0 mm/kasan/report.c:383
>  __kasan_report mm/kasan/report.c:513 [inline]
>  kasan_report+0x132/0x1d0 mm/kasan/report.c:530
>  vsscanf+0x2666/0x2ef0 lib/vsprintf.c:3321
>  sscanf+0x6c/0x90 lib/vsprintf.c:3527
>  smk_set_cipso+0x374/0x6c0 security/smack/smackfs.c:908
>  vfs_write+0x2dd/0xc70 fs/read_write.c:576
>  ksys_write+0x11b/0x220 fs/read_write.c:631
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x4402d9
> Code: Bad RIP value.
> RSP: 002b:00007ffe89010db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 00000000004402d9
> RDX: 0000000000000037 RSI: 0000000020000040 RDI: 0000000000000003
> RBP: 00000000006ca018 R08: 0000000000000014 R09: 00000000004002c8
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401ae0
> R13: 0000000000401b70 R14: 0000000000000000 R15: 0000000000000000
>
> Allocated by task 6804:
>  save_stack mm/kasan/common.c:48 [inline]
>  set_track mm/kasan/common.c:56 [inline]
>  __kasan_kmalloc+0x103/0x140 mm/kasan/common.c:494
>  __do_kmalloc mm/slab.c:3656 [inline]
>  __kmalloc_track_caller+0x249/0x320 mm/slab.c:3671
>  memdup_user_nul+0x26/0xf0 mm/util.c:259
>  smk_set_cipso+0xff/0x6c0 security/smack/smackfs.c:859
>  vfs_write+0x2dd/0xc70 fs/read_write.c:576
>  ksys_write+0x11b/0x220 fs/read_write.c:631
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Freed by task 4906:
>  save_stack mm/kasan/common.c:48 [inline]
>  set_track mm/kasan/common.c:56 [inline]
>  kasan_set_free_info mm/kasan/common.c:316 [inline]
>  __kasan_slab_free+0x114/0x170 mm/kasan/common.c:455
>  __cache_free mm/slab.c:3426 [inline]
>  kfree+0x10a/0x220 mm/slab.c:3757
>  tomoyo_path_number_perm+0x525/0x690 security/tomoyo/file.c:736
>  tomoyo_path_mknod+0x128/0x150 security/tomoyo/tomoyo.c:240
>  security_path_mknod+0xdc/0x160 security/security.c:1077
>  may_o_create fs/namei.c:2919 [inline]
>  lookup_open fs/namei.c:3060 [inline]
>  open_last_lookups fs/namei.c:3169 [inline]
>  path_openat+0xbe8/0x37f0 fs/namei.c:3357
>  do_filp_open+0x191/0x3a0 fs/namei.c:3387
>  do_sys_openat2+0x463/0x770 fs/open.c:1179
>  do_sys_open fs/open.c:1195 [inline]
>  ksys_open include/linux/syscalls.h:1388 [inline]
>  __do_sys_open fs/open.c:1201 [inline]
>  __se_sys_open fs/open.c:1199 [inline]
>  __x64_sys_open+0x1af/0x1e0 fs/open.c:1199
>  do_syscall_64+0x73/0xe0 arch/x86/entry/common.c:384
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The buggy address belongs to the object at ffff888097d68280
>  which belongs to the cache kmalloc-64 of size 64
> The buggy address is located 56 bytes inside of
>  64-byte region [ffff888097d68280, ffff888097d682c0)
> The buggy address belongs to the page:
> page:ffffea00025f5a00 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888097d68c80
> flags: 0xfffe0000000200(slab)
> raw: 00fffe0000000200 ffffea000288fe08 ffffea00026f38c8 ffff8880aa400380
> raw: ffff888097d68c80 ffff888097d68000 000000010000001e 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
>  ffff888097d68180: 00 00 00 00 01 fc fc fc fc fc fc fc fc fc fc fc
>  ffff888097d68200: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>> ffff888097d68280: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
>                                         ^
>  ffff888097d68300: 00 00 00 00 01 fc fc fc fc fc fc fc fc fc fc fc
>  ffff888097d68380: 00 00 00 00 01 fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
> Reported-and-testedby: syzbot+a22c6092d003d6fe1122 at syzkaller.appspotmail.com
> Signed-off-by: B K Karthik <bkkarthik at pesu.pes.edu>
> ---
>  security/smack/smackfs.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
> index 58d3f43cc8bb..17809310d046 100644
> --- a/security/smack/smackfs.c
> +++ b/security/smack/smackfs.c
> @@ -905,6 +905,11 @@ static ssize_t smk_set_cipso(struct file *file, const char __user *buf,
>  
>  	for (i = 0; i < catlen; i++) {
>  		rule += SMK_DIGITLEN;
> +		if (rule > data + count) {
> +			rc = -EOVERFLOW;
> +			goto out;
> +		}
> +
>  		ret = sscanf(rule, "%u", &cat);
>  		if (ret != 1 || cat > SMACK_CIPSO_MAXCATNUM)
>  			goto out;