Re: [RFC PATCH 2/2] selinux: Propagate RCU walk status from 'security_inode_follow_link()'

[Stephen Smalley <sds@tycho.nsa.gov>]

On 11/20/19 8:13 AM, Will Deacon wrote:
> Hi Stephen,
> 
> Thanks for the quick review.
> 
> On Tue, Nov 19, 2019 at 01:46:37PM -0500, Stephen Smalley wrote:
>> On 11/19/19 1:40 PM, Will Deacon wrote:
>>> 'selinux_inode_follow_link()' can be called as part of an RCU path walk,
>>> and is passed a 'bool rcu' parameter to indicate whether or not it is
>>> being called from within an RCU read-side critical section.
>>>
>>> Unfortunately, this knowledge is not propagated further and, instead,
>>> 'avc_has_perm()' unconditionally passes a flags argument of '0' to both
>>> 'avc_has_perm_noaudit()' and 'avc_audit()' which may block.
>>>
>>> Introduce 'avc_has_perm_flags()' which can be used safely from within an
>>> RCU read-side critical section.
>>
>> Please see e46e01eebbbcf2ff6d28ee7cae9f117e9d1572c8 ("selinux: stop passing
>> MAY_NOT_BLOCK to the AVC upon follow_link").
> 
> Ha, not sure how I missed that -- my patch is almost a direct revert,
> including the name 'avs_has_perm_flags()'! My only concern is that the
> commit message for e46e01eebbbc asserts that the only use of MAY_NOT_BLOCK
> is in slow_avc_audit(), but AVC_NONBLOCKING is used more widely than that.
> 
> For example:
> 
> 	selinux_inode_follow_link()
> 	  -> avc_has_perm()
> 	    -> avc_has_perm_noaudit()
> 	      -> avc_denied()
> 	        -> avc_update_node()
> 
> where we return early if AVC_NONBLOCKING is set, except flags are always
> zero on this path.

That was introduced by 3a28cff3bd4bf43f02be0c4e7933aebf3dc8197e 
("selinux: avoid silent denials in permissive mode under RCU walk") and 
is only needed if we have to pass MAY_NOT_BLOCK to slow_avc_audit(), 
which is only presently needed in the selinux_inode_permission() case 
AFAICT.  Both AVC_NONBLOCKING and MAY_NOT_BLOCK are misnomers wrt the 
AVC since it should never block regardless; the issue IIUC was rather 
the inability to safely collect the dentry name in an audit message 
during RCU walk per commit 0dc1ba24f7fff659725eecbba2c9ad679a0954cd (" 
SELINUX: Make selinux cache VFS RCU walks safe").

I'm not 100% certain about this; it is possible that the test in 
slow_avc_audit() is wrong and we ought to be doing this for any of 
LSM_AUDIT_DATA_PATH, _DENTRY, or _INODE (these were split out of 
LSM_AUDIT_DATA_FS).  In that case, we should revert my earlier commit 
for follow_link and fix the test inside of slow_avc_audit() instead.

I cc'd some additional folks who may have insight.  Al, tell us if we 
got it wrong please!