Re: [syzbot] general protection fault in legacy_parse_param

[Casey Schaufler <casey@schaufler-ca.com>]

On 8/30/2021 7:25 AM, Casey Schaufler wrote:
> On 8/30/2021 5:23 AM, Christian Brauner wrote:
>> On Fri, Aug 27, 2021 at 07:11:18PM -0700, syzbot wrote:
>>> syzbot has bisected this issue to:
>>>
>>> commit 54261af473be4c5481f6196064445d2945f2bdab
>>> Author: KP Singh <kpsingh@google.com>
>>> Date:   Thu Apr 30 15:52:40 2020 +0000
>>>
>>>     security: Fix the default value of fs_context_parse_param hook
>>>
>>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=160c5d75300000
>>> start commit:   77dd11439b86 Merge tag 'drm-fixes-2021-08-27' of git://ano..
>>> git tree:       upstream
>>> final oops:     https://syzkaller.appspot.com/x/report.txt?x=150c5d75300000
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=110c5d75300000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=2fd902af77ff1e56
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=d1e3b1d92d25abf97943
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=126d084d300000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16216eb1300000
>>>
>>> Reported-by: syzbot+d1e3b1d92d25abf97943@syzkaller.appspotmail.com
>>> Fixes: 54261af473be ("security: Fix the default value of fs_context_parse_param hook")
>>>
>>> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>> So ok, this seems somewhat clear now. When smack and 
>> CONFIG_BPF_LSM=y
>> is selected the bpf LSM will register NOP handlers including
>>
>> bpf_lsm_fs_context_fs_param()
>>
>> for the
>>
>> fs_context_fs_param
>>
>> LSM hook. The bpf LSM runs last, i.e. after smack according to:
>>
>> CONFIG_LSM="landlock,lockdown,yama,safesetid,integrity,tomoyo,smack,bpf"
>>
>> in the appended config. The smack hook runs and sets
>>
>> param->string = NULL
>>
>> then the bpf NOP handler runs returning -ENOPARM indicating to the vfs
>> parameter parser that this is not a security module option so it should
>> proceed processing the parameter subsequently causing the crash because
>> param->string is not allowed to be NULL (Which the vfs parameter parser
>> verifies early in fsconfig().).
> The security_fs_context_parse_param() function is incorrectly
> implemented using the call_int_hook() macro. It should return
> zero if any of the modules return zero. It does not follow the
> usual failure model of LSM hooks. It could be argued that the
> code was fine before the addition of the BPF hook, but it was
> going to fail as soon as any two security modules provided
> mount options.
>
> Regardless, I will have a patch later today. Thank you for
> tracking this down.

Here's my proposed patch. I'll tidy it up with a proper
commit message if it looks alright to y'all. I've tested
with Smack and with and without BPF.


 security/security.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/security/security.c b/security/security.c
index 09533cbb7221..3cf0faaf1c5b 100644
--- a/security/security.c
+++ b/security/security.c
@@ -885,7 +885,19 @@ int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
 
 int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
-	return call_int_hook(fs_context_parse_param, -ENOPARAM, fc, param);
+	struct security_hook_list *hp;
+	int trc;
+	int rc = -ENOPARAM;
+
+	hlist_for_each_entry(hp, &security_hook_heads.fs_context_parse_param,
+			     list) {
+		trc = hp->hook.fs_context_parse_param(fc, param);
+		if (trc == 0)
+			rc = 0;
+		else if (trc != -ENOPARAM)
+			return trc;
+	}
+	return rc;
 }
 
 int security_sb_alloc(struct super_block *sb)

>
>> If you take the appended syzkaller config and additionally select
>> kprobes you can observe this by registering bpf kretprobes for:
>> security_fs_context_parse_param()
>> smack_fs_context_parse_param()
>> bpf_lsm_fs_context_parse_param()
>> in different terminal windows and then running the syzkaller provided
>> reproducer:
>>
>> root@f2-vm:~# bpftrace -e 'kretprobe:smack_fs_context_parse_param { printf("returned: %d\n", retval); }'
>> Attaching 1 probe...
>> returned: 0
>>
>> root@f2-vm:~# bpftrace -e 'kretprobe:bpf_lsm_fs_context_parse_param { printf("returned: %d\n", retval); }'
>> Attaching 1 probe...
>> returned: -519
>>
>> root@f2-vm:~# bpftrace -e 'kretprobe:security_fs_context_parse_param { printf("returned: %d\n", retval); }'
>> Attaching 1 probe...
>> returned: -519
>>
>> ^^^^^
>> This will ultimately tell the vfs to move on causing the crash because
>> param->string is null at that point.
>>
>> Unless I missed something why that can't happen.
>>
>> Christian