On Fri, Nov 22, 2019 at 11:27:37AM -0500, Stephen Smalley wrote:
> On 11/22/19 11:11 AM, Al Viro wrote:
> > On Thu, Nov 21, 2019 at 09:52:45AM -0500, Stephen Smalley 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@xxxxxxxxxxxxx>
> > > ---
> > > 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))
> >
> > IDGI, to be honest. Why do we bother with slow path if MAY_NOT_BLOCK has
> > been given? If we'd run into "there's something to report" case, we
> > are not on the fastpath anymore. IOW, why not have
> > audited = avc_audit_required(requested, avd, result, 0, &denied);
> > if (likely(!audited))
> > return 0;
> > if (flags & MAY_NOT_BLOCK)
> > return -ECHILD;
> > return slow_avc_audit(state, ssid, tsid, tclass,
> > requested, audited, denied, result,
> > a, flags);
> > in avc_audit() and be done with that?
>
> That works for me; we would also need to do the same in
> selinux_inode_permission(). We can then stop passing flags down to
> slow_avc_audit() entirely.
I'm new to looking at this code, but that would certainly have helped me to
understand it when I was reading it a couple of weeks back! So, for what
it's worth, you can count me in favour.
Cheers,
Will
