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
