[PATCH v4] proc: Allow pid_revalidate() during LOOKUP_RCU

[Al Viro]

On Mon, Jan 04, 2021 at 03:21:22PM -0800, Stephen Brennan wrote:
> The pid_revalidate() function drops from RCU into REF lookup mode. When
> many threads are resolving paths within /proc in parallel, this can
> result in heavy spinlock contention on d_lockref as each thread tries to
> grab a reference to the /proc dentry (and drop it shortly thereafter).
> 
> Investigation indicates that it is not necessary to drop RCU in
> pid_revalidate(), as no RCU data is modified and the function never
> sleeps. So, remove the LOOKUP_RCU check.

Umm...  I'm rather worried about the side effect you are removing here -
you are suddenly exposing a bunch of methods in there to RCU mode.
E.g. is proc_pid_permission() safe with MAY_NOT_BLOCK in the mask?
generic_permission() call in there is fine, but has_pid_permission()
doesn't even see the mask.  Is that thing safe in RCU mode?  AFAICS,
this
static int selinux_ptrace_access_check(struct task_struct *child,
                                     unsigned int mode)
{
        u32 sid = current_sid();
        u32 csid = task_sid(child);

        if (mode & PTRACE_MODE_READ)
                return avc_has_perm(&selinux_state,
                                    sid, csid, SECCLASS_FILE, FILE__READ, NULL);

        return avc_has_perm(&selinux_state,
                            sid, csid, SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
}
is reachable and IIRC avc_has_perm() should *NOT* be called in RCU mode.
If nothing else, audit handling needs care...

And LSM-related stuff is only a part of possible issues here.  It does need
a careful code audit - you are taking a bunch of methods into the conditions
they'd never been tested in.  ->permission(), ->get_link(), ->d_revalidate(),
->d_hash() and ->d_compare() instances for objects that subtree.  The last
two are not there in case of anything in /proc/<pid>, but the first 3 very
much are.