[RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

[Stephen Smalley]

On 11/22/19 9:49 AM, Paul Moore wrote:
> On Fri, Nov 22, 2019 at 8:37 AM Stephen Smalley <sds at tycho.nsa.gov> wrote:
>> On 11/21/19 7:30 PM, Paul Moore wrote:
>>> On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul at paul-moore.com> wrote:
>>>> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds at tycho.nsa.gov> wrote:
>>>>> commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>>> passed down the rcu flag to the SELinux AVC, but failed to adjust the
>>>>> test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
>>>>> Previously, we only returned -ECHILD if generating an audit record with
>>>>> LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
>>>>> Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
>>>>> LSM_AUDIT_DATA_INODE only requires this handling due to the fact
>>>>> that dump_common_audit_data() calls d_find_alias() and collects the
>>>>> dname from the result if any.
>>>>> Other cases that might require similar treatment in the future are
>>>>> LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
>>>>> a path or file is called under RCU-walk.
>>>>>
>>>>> Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
>>>>> Signed-off-by: Stephen Smalley <sds at tycho.nsa.gov>
>>>>> ---
>>>>>    security/selinux/avc.c | 3 ++-
>>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>>>>> index 74c43ebe34bb..f1fa1072230c 100644
>>>>> --- a/security/selinux/avc.c
>>>>> +++ b/security/selinux/avc.c
>>>>> @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
>>>>>            * during retry. However this is logically just as if the operation
>>>>>            * happened a little later.
>>>>>            */
>>>>> -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
>>>>> +       if ((a->type == LSM_AUDIT_DATA_INODE ||
>>>>> +            a->type == LSM_AUDIT_DATA_DENTRY) &&
>>>>>               (flags & MAY_NOT_BLOCK))
>>>>>                   return -ECHILD;
>>>
>>> With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
>>> in dump_common_audit_data() which could block, which is bad, that I
>>> understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
>>> on why that is bad?  It makes a few audit_log*() calls and one call to
>>> d_backing_inode() which is non-blocking and trivial.
>>>
>>> What am I missing?
>>
>> For those who haven't, you may wish to also read the earlier thread here
>> that led to this one:
>> https://lore.kernel.org/selinux/20191119184057.14961-1-will@kernel.org/T/#t
>>
>> AFAIK, neither the LSM_AUDIT_DATA_INODE nor the LSM_AUDIT_DATA_DENTRY
>> case truly block (d_find_alias does not block AFAICT, nor should
>> audit_log* as long as we use audit_log_start with GFP_ATOMIC or
>> GFP_NOWAIT).
> 
> Yes, the audit_log*() functions should be safe, if not I would
> consider that a bug; I thought d_find_alias() might block, but it's
> very likely I'm wrong in that regard.

No, it doesn't appear to block.  However, it does take d_lock and 
increment d_lockref.count, which IIUC aren't permitted during rcu-walk.

>> My impression from the comment in slow_avc_audit() is that
>> the issue is not really about blocking but rather about the inability to
>> safely dereference the dentry->d_name during RCU walk, which is
>> something that can occur under LSM_AUDIT_DATA_INODE or _DENTRY (or _PATH
>> or _FILE, but neither of the latter two are currently used from the two
>> hooks that are called during RCU walk, inode_permission and
>> inode_follow_link).  Originally _PATH, _DENTRY, and _INODE were all
>> under a single _FS type and the original test in slow_avc_audit() was
>> against LSM_AUDIT_DATA_FS before the split.
> 
> Thanks, I think that is the part I was missing.  I was focused too
> much on the VFS stuff that I didn't pay enough attention to
> slow_avc_audit().
> 
> If that is the case, the comment and code in dentry_cmp() would seem
> to indicate that it would be safe to fetch the dentry name string as
> long as we use READ_ONCE().  The length field in the qstr might be
> off, but the audit_log_untrustedstring() function doesn't use the
> qstr's length information.  I suppose if we don't mind the extra
> spinlock we could use take_dentry_name_snapshot(); that should be safe
> and we are already in the "slow" path.  I didn't check the _PATH or
> _FILE cases.
> 
> Once again, let me know if I'm missing something.

We can't take any spinlocks on the dentry during rcu-walk IIUC; that 
would defeat the purpose. In looking for a parallel with filesystem 
implementations, I noted that fs/namei.c:get_link() doesn't even pass 
the dentry to the filesystem get_link() method in the rcu-walk case, 
only doing so under ref-walk.  So they won't permit the filesystem 
implementations to ever dereference the dentry for get_link() under 
rcu-walk.  Not sure why it gets passed to security_inode_follow_link() 
then, or if it is ever safe for a security module to dereference its fields.

I was hoping to get fsdevel folks to comment since I feel like we're 
guessing about exactly what guarantees we have in this area.

> 
> As an aside, if we somehow can guarantee (e.g. via a name_snapshot)
> that qstr length information is valid, we might want to consider
> moving from audit_log_unstrustedstring() to
> audit_log_n_untrustedstring() to save us a call to strlen().
> 
>>>> Added the LSM list as I'm beginning to wonder if we should push this
>>>> logic down into common_lsm_audit(), this problem around blocking
>>>> shouldn't be SELinux specific.
>>
>> That would require passing down the MAY_NOT_BLOCK flag or a rcu bool to
>> common_lsm_audit() just so that it could immediately return if that is
>> set and a->type is _INODE or _DENTRY (or _PATH or _FILE).  And the
>> individual security module still needs to have its own handling of
>> MAY_NOT_BLOCK/rcu for its own processing, so it won't free the security
>> module authors from thinking about it.
> 
> Looking at the current SELinux code, all we do is bail out early with
> -ECHILD.  If we didn't have that check it looks like the only impact
> would be some extra assignments into a struct living on the stack and
> a call into common_lsm_audit().  That doesn't seem terrible for a slow
> path, especially if it pushes this code into a LSM common function.

Not terrible, just not sure if it ends up being a worthwhile change.  If 
the LSM maintainers would like it that way, I can do that.

> 
>>>> For the LSM folks just joining, the full patchset can be found here:
>>>> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t
>