linux-kernel - Re: [PATCH] selinux: KASAN: slab-out-of-bounds in xattr

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Date:   Wed, 30 May 2018 11:31:31 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Sachin Grover <sgrover@...eaurora.org>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov
Subject: Re: [PATCH] selinux: KASAN: slab-out-of-bounds in xattr_getsecurity

On Wed, May 30, 2018 at 11:23 AM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
> On 05/30/2018 11:19 AM, Paul Moore wrote:
>> On Fri, May 25, 2018 at 4:31 AM, Sachin Grover <sgrover@...eaurora.org> wrote:
>>> Call trace:
>>>  [<ffffff9203a8d7a8>] dump_backtrace+0x0/0x428
>>>  [<ffffff9203a8dbf8>] show_stack+0x28/0x38
>>>  [<ffffff920409bfb8>] dump_stack+0xd4/0x124
>>>  [<ffffff9203d187e8>] print_address_description+0x68/0x258
>>>  [<ffffff9203d18c00>] kasan_report.part.2+0x228/0x2f0
>>>  [<ffffff9203d1927c>] kasan_report+0x5c/0x70
>>>  [<ffffff9203d1776c>] check_memory_region+0x12c/0x1c0
>>>  [<ffffff9203d17cdc>] memcpy+0x34/0x68
>>>  [<ffffff9203d75348>] xattr_getsecurity+0xe0/0x160
>>>  [<ffffff9203d75490>] vfs_getxattr+0xc8/0x120
>>>  [<ffffff9203d75d68>] getxattr+0x100/0x2c8
>>>  [<ffffff9203d76fb4>] SyS_fgetxattr+0x64/0xa0
>>>  [<ffffff9203a83f70>] el0_svc_naked+0x24/0x28
>>>
>>> If user get root access and calls security.selinux setxattr() with an
>>> embedded NUL on a file and then if some process performs a getxattr()
>>> on that file with a length greater than the actual length of the string,
>>> it would result in a panic.
>>>
>>> To fix this, add the actual length of the string to the security context
>>> instead of the length passed by the userspace process.
>>>
>>> Signed-off-by: Sachin Grover <sgrover@...eaurora.org>
>>> ---
>>>  security/selinux/ss/services.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Thanks for reporting this and providing a patch.  It's small enough,
>> and passes all the regular tests, so I've merged it into
>> selinux/stable-4.17 (adding the stable metadata) and I'm going to send
>> it up to Linus today.
>>
>> If Linus doesn't pull the fix in time for v4.17 I'll send it up during
>> the upcoming merge window.
>
> NB Such a setxattr() call can only be performed by a process with CAP_MAC_ADMIN that is also allowed mac_admin permission in SELinux policy. Consequently, this is never possible on Android (no process is allowed mac_admin permission, always enforcing) and is only possible in Fedora/RHEL for a few domains (if enforcing).

Yes the risk is small, and if it wasn't such a trivial and
self-contained patch I probably would have just deferred it for the
merge window, but considering everything I think there is value in
getting this in for v4.17.  If Linus decides not to merge this into
v4.17 I think that is okay too.

> Fixes: 9a59daa03df72526d234b91dd3e32ded5aebd3ef ("SELinux: fix sleeping allocation in security_context_to_sid")
>
>>
>>> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
>>> index 66ea81c..d17f5b4 100644
>>> --- a/security/selinux/ss/services.c
>>> +++ b/security/selinux/ss/services.c
>>> @@ -1434,7 +1434,7 @@ static int security_context_to_sid_core(const char *scontext, u32 scontext_len,
>>>                                       scontext_len, &context, def_sid);
>>>         if (rc == -EINVAL && force) {
>>>                 context.str = str;
>>> -               context.len = scontext_len;
>>> +               context.len = strlen(str) + 1;
>>>                 str = NULL;
>>>         } else if (rc)
>>>                 goto out_unlock;
>>> --
>>> 1.9.1

-- 
paul moore
www.paul-moore.com