linux-kernel - Re: [RFC PATCH 1/2] selinux: Don't call avc_compute

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Date:   Wed, 20 Nov 2019 13:12:30 +0000
From:   Will Deacon <will@...nel.org>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     selinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] selinux: Don't call avc_compute_av() from RCU
 path walk

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.

Will