Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too

From: Paul Moore <paul@paul-moore.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: selinux@vger.kernel.org, will@kernel.org,
	viro@zeniv.linux.org.uk, neilb@suse.de,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] selinux: fall back to ref-walk upon LSM_AUDIT_DATA_DENTRY too
Date: Thu, 21 Nov 2019 19:30:51 -0500	[thread overview]
Message-ID: <CAHC9VhRURZMtEDagtSKEuuOLEJen=4PQZig3iGNomzXC1HTNSA@mail.gmail.com> (raw)
In-Reply-To: <CAHC9VhTAq7CgcRRcvZCYis7ELAo+bo2q8pCUXfHUP9YAcUhwsQ@mail.gmail.com>

On Thu, Nov 21, 2019 at 7:12 PM Paul Moore <paul@paul-moore.com> wrote:
> On Thu, Nov 21, 2019 at 9:52 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> > passed down the rcu flag to the SELinux AVC, but failed to adjust the
> > test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
> > Previously, we only returned -ECHILD if generating an audit record with
> > LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
> > Return -ECHILD on either LSM_AUDIT_DATA_INODE or LSM_AUDIT_DATA_DENTRY.
> > LSM_AUDIT_DATA_INODE only requires this handling due to the fact
> > that dump_common_audit_data() calls d_find_alias() and collects the
> > dname from the result if any.
> > Other cases that might require similar treatment in the future are
> > LSM_AUDIT_DATA_PATH and LSM_AUDIT_DATA_FILE if any hook that takes
> > a path or file is called under RCU-walk.
> >
> > Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
> > Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> > ---
> >  security/selinux/avc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/security/selinux/avc.c b/security/selinux/avc.c
> > index 74c43ebe34bb..f1fa1072230c 100644
> > --- a/security/selinux/avc.c
> > +++ b/security/selinux/avc.c
> > @@ -779,7 +779,8 @@ noinline int slow_avc_audit(struct selinux_state *state,
> >          * during retry. However this is logically just as if the operation
> >          * happened a little later.
> >          */
> > -       if ((a->type == LSM_AUDIT_DATA_INODE) &&
> > +       if ((a->type == LSM_AUDIT_DATA_INODE ||
> > +            a->type == LSM_AUDIT_DATA_DENTRY) &&
> >             (flags & MAY_NOT_BLOCK))
> >                 return -ECHILD;

With LSM_AUDIT_DATA_INODE we eventually end up calling d_find_alias()
in dump_common_audit_data() which could block, which is bad, that I
understand.  However, looking at LSM_AUDIT_DATA_DENTRY I'm less clear
on why that is bad?  It makes a few audit_log*() calls and one call to
d_backing_inode() which is non-blocking and trivial.

What am I missing?

> Added the LSM list as I'm beginning to wonder if we should push this
> logic down into common_lsm_audit(), this problem around blocking
> shouldn't be SELinux specific.
>
> For the LSM folks just joining, the full patchset can be found here:
> * https://lore.kernel.org/selinux/20191121145245.8637-1-sds@tycho.nsa.gov/T/#t

--
paul moore
www.paul-moore.com