Re: [RFC PATCH 1/2] selinux: Don't call avc_compute_av() from RCU path walk

[Will Deacon]

On Wed, Nov 20, 2019 at 11:07:43AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 20, 2019 at 10:28:31AM -0500, Stephen Smalley wrote:
> > On 11/20/19 8:12 AM, Will Deacon wrote:
> > > Hi Stephen,
> > > 
> > > Thanks for the quick reply.
> > > 
> > > On Tue, Nov 19, 2019 at 01:59:40PM -0500, Stephen Smalley wrote:
> > > > On 11/19/19 1:40 PM, Will Deacon wrote:
> > > > > 'avc_compute_av()' can block, so we carefully exit the RCU read-side
> > > > > critical section before calling it in 'avc_has_perm_noaudit()'.
> > > > > Unfortunately, if we're calling from the VFS layer on the RCU path walk
> > > > > via 'selinux_inode_permission()' then we're still actually in an RCU
> > > > > read-side critical section and must not block.
> > > > 
> > > > avc_compute_av() should never block AFAIK. The blocking concern was with
> > > > slow_avc_audit(), and even that appears dubious to me. That seems to be more
> > > > about misuse of d_find_alias in dump_common_audit_data() than anything.
> > > 
> > > Apologies, I lost track of GFP_ATOMIC when I reading the code and didn't
> > > think it was propagated down to all of the potential allocations and
> > > string functions. Having looked at it again, I can't see where it blocks.
> > > 
> > > Might be worth a comment in avc_compute_av(), because the temporary
> > > dropping of rcu_read_lock() looks really dodgy when we could be running
> > > on the RCU path walk path anyway.
> > 
> > I don't think that's a problem but I'll defer to the fsdevel and rcu folks.
> > The use of RCU within the SELinux AVC long predates the introduction of RCU
> > path walk, and the rcu_read_lock()/unlock() pairs inside the AVC are not
> > related in any way to RCU path walk.  Hopefully they don't break it.  The
> > SELinux security server (i.e. security_compute_av() and the rest of
> > security/selinux/ss/*) internally has its own locking for its data
> > structures, primarily the policy rwlock.  There was also a patch long ago to
> > convert use of that policy rwlock to RCU but it didn't seem justified at the
> > time.  We are interested in revisiting that however.  That would introduce
> > its own set of rcu_read_lock/unlock pairs inside of security_compute_av() as
> > well.
> 
> RCU readers nest, so it should be fine.  (Famous last words...)

Agreed. It was blocking that worried me, and it turns out that doesn't
happen for this code.

Will