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

From: B K Karthik
Date: Sat Jul 25 2020 - 08:58:10 EST

add a barrier to smk_set_cipso() to check for overflow

==================================================================
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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: B K Karthik <bkkarthik@xxxxxxxxxxxx>
---
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;
--
2.20.1

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