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19 Nov 2019 18:59:41 +0000 Received: from moss-pluto.infosec.tycho.ncsc.mil (moss-pluto [192.168.25.131]) by tarius.tycho.ncsc.mil (8.14.4/8.14.4) with ESMTP id xAJIxedE005094; Tue, 19 Nov 2019 13:59:41 -0500 Subject: Re: [RFC PATCH 1/2] selinux: Don't call avc_compute_av() from RCU path walk To: Will Deacon , selinux@vger.kernel.org Cc: linux-kernel@vger.kernel.org References: <20191119184057.14961-1-will@kernel.org> <20191119184057.14961-2-will@kernel.org> From: Stephen Smalley Message-ID: <5e51f9a5-ba76-a42d-fc2b-9255f8544859@tycho.nsa.gov> Date: Tue, 19 Nov 2019 13:59:40 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20191119184057.14961-2-will@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 'avc_denied()' already handles this by simply returning success and > postponing the auditing until we're called again on the slowpath, so > follow the same approach here and return early if the node lookup fails > on the RCU walk path. > > Signed-off-by: Will Deacon > --- > security/selinux/avc.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/security/selinux/avc.c b/security/selinux/avc.c > index ecd3829996aa..9c183c899e92 100644 > --- a/security/selinux/avc.c > +++ b/security/selinux/avc.c > @@ -1159,16 +1159,19 @@ inline int avc_has_perm_noaudit(struct selinux_state *state, > rcu_read_lock(); > > node = avc_lookup(state->avc, ssid, tsid, tclass); > - if (unlikely(!node)) > + if (unlikely(!node)) { > + if (flags & AVC_NONBLOCKING) > + goto out; > node = avc_compute_av(state, ssid, tsid, tclass, avd, &xp_node); > - else > + } else { > memcpy(avd, &node->ae.avd, sizeof(*avd)); > + } > > denied = requested & ~(avd->allowed); > if (unlikely(denied)) > rc = avc_denied(state, ssid, tsid, tclass, requested, 0, 0, > flags, avd); > - > +out: > rcu_read_unlock(); > return rc; > } >