WARNING in apparmor_cred_free

[Casey Schaufler]

On 1/11/2019 3:20 PM, Casey Schaufler wrote:
> On 1/11/2019 2:43 PM, Casey Schaufler wrote:
>> On 1/11/2019 2:30 PM, John Johansen wrote:
>>> On 1/11/19 2:11 PM, Casey Schaufler wrote:
>>>> On 1/11/2019 1:43 AM, syzbot wrote:
>>>>> Hello,
>>>>>
>>>>> syzbot found the following crash on:
>>>>>
>>>>> HEAD commit:    b808822a75a3 Add linux-next specific files for 20190111
>>>>> git tree:       linux-next
>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=179c22f7400000
>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=c052ead0aed5001b
>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=69ca07954461f189e808
>>>>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=162d947f400000
>>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=139f6c37400000
>>>>>
>>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>>> Reported-by: syzbot+69ca07954461f189e808 at syzkaller.appspotmail.com
>>>>>
>>>>> ------------[ cut here ]------------
>>>>> AppArmor WARN cred_label: ((!blob)):
>>>>> WARNING: CPU: 0 PID: 0 at security/apparmor/include/cred.h:30 cred_label security/apparmor/include/cred.h:30 [inline]
>>>>> WARNING: CPU: 0 PID: 0 at security/apparmor/include/cred.h:30 apparmor_cred_free+0x12f/0x1a0 security/apparmor/lsm.c:62
>>>>> Kernel panic - not syncing: panic_on_warn set ...
>>>>> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.0.0-rc1-next-20190111 #10
>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>>>>> Call Trace:
>>>>>  <IRQ>
>>>>>  __dump_stack lib/dump_stack.c:77 [inline]
>>>>>  dump_stack+0x1db/0x2d0 lib/dump_stack.c:113
>>>>>  panic+0x2cb/0x65c kernel/panic.c:214
>>>>>  __warn.cold+0x20/0x48 kernel/panic.c:571
>>>>>  report_bug+0x263/0x2b0 lib/bug.c:186
>>>>>  fixup_bug arch/x86/kernel/traps.c:178 [inline]
>>>>>  fixup_bug arch/x86/kernel/traps.c:173 [inline]
>>>>>  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
>>>>>  do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:290
>>>>>  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
>>>>> RIP: 0010:cred_label security/apparmor/include/cred.h:30 [inline]
>>>>> RIP: 0010:apparmor_cred_free+0x12f/0x1a0 security/apparmor/lsm.c:62
>>>>> Code: 7c 88 48 c7 c7 00 d0 7c 88 e8 fd 70 f2 fd 0f 0b eb a9 e8 54 3f 29 fe 48 c7 c6 c0 df 7c 88 48 c7 c7 00 d0 7c 88 e8 e1 70 f2 fd <0f> 0b 48 b8 00 00 00 00 00 fc ff df 80 38 00 75 4a 4c 8b 2c 25 00
>>>>> RSP: 0018:ffff8880ae6079f8 EFLAGS: 00010286
>>>>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
>>>>> RDX: 0000000000000100 RSI: ffffffff81687fa6 RDI: 0000000000000006
>>>>> RBP: ffff8880ae607a18 R08: ffffffff8987dec0 R09: 0000000000000000
>>>>> R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880a86b3100
>>>>> R13: ffff8880a86b3100 R14: ffff8880a86b3188 R15: dffffc0000000000
>>>>>  security_cred_free+0x4b/0xf0 security/security.c:1490
>>>> The obvious thing to do is put a check in security_cred_free
>>>> for a NULL cred->security, in which case the LSM hooks
>>>> wouldn't get called.
>>> Right, but the question is should we? To my thinking we shouldn't
>>> ever have a cred without cred->security, unless the cred was
>>> allocated but a later step in its construction, say allocating
>>> ->security failed.
>> If allocating ->security fails in security_cred_alloc_blank()
>> or security_prepare_creds() you don't have to do anything but
>> fail because the LSM hooks are not called before the allocation.
>>
>>> In which case I'd rather see the cred directly freed and not
>>> call into security_cred_free() as I like being able to detect
>>> corrupt creds.
>> I think we need to look for some bit of code that's setting
>> cred->security to NULL inappropriately.
> If security_cred_alloc_blank() fails for lack of memory
> in cred_alloc_blank() abort_creds() will be called. This
> in turn calls put_cred() and put_cred_rcu(), which will
> call security_cred_free() with ->security set to NULL.
>
> put_cred_rcu() is the only caller of security_cred_free().
> The ->security == NULL check can be in either put_cred_rcu()
> or in security_cred_free(). I suggest the latter as the
> cleanest option.

>From 47134986133c822e1d88860fa2b108f92c97a7ff Mon Sep 17 00:00:00 2001
From: Casey Schaufler <casey at schaufler-ca.com>
Date: Fri, 11 Jan 2019 17:31:50 -0800
Subject: [PATCH 1/2] LSM: Check for NULL cred-security on free

Check that the cred security blob has been set before trying
to clean it up. There is a case during credential initialization
that could result in this.

Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
---
 security/security.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/security/security.c b/security/security.c
index a618e22df5c6..7bffc86d4e87 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1477,6 +1477,13 @@ int security_cred_alloc_blank(struct cred *cred, gfp_t gfp)
 
 void security_cred_free(struct cred *cred)
 {
+	/*
+	 * There is a failure case in prepare_creds() that
+	 * may result in a call here with ->security being NULL.
+	 */
+	if (unlikely(cred->security == NULL))
+		return;
+
 	call_void_hook(cred_free, cred);
 
 	kfree(cred->security);
-- 
2.20.1