Re: [PATCH 04/13] security/selinux: check for LOOKUP_RCU in _follow_link.

[Al Viro]

On Fri, Mar 20, 2015 at 03:39:30PM +1100, NeilBrown wrote:
> 	rcu_read_unlock();
> 	security_compute_av(ssid, tsid, tclass, avd);
> 	rcu_read_lock();
> 
> (yes: unlock, and then lock).
>
> so avc_has_perm_noaudit needs to bail out of RCU-walk if node turns out to be
> NULL.

NFI, but since
	a) the guts of security_compute_av() are under rwlock (shared),
I rather doubt that it could e.g. block
	b) avc_has_perm_noaudit() is called from selinux_inode_permission(),
which is called inside RCU-walk - it's hit on selinux setups in every
successful inode_permission()
I'd say that it's no worse than it already was.  AFAICS, it's a slowpath and
we don't want to hold rcu_read_lock() over it to avoid stalls, but if the
caller of avc_has_perm_noaudit() used to want rcu_read_lock(), well, we'll
just risks stalls
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