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

[Stephen Brennan]

Al Viro <viro at zeniv.linux.org.uk> writes:

> 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.

You're right, this was a major oversight on my part. The main motivation
of this patch is to reduce contention on the /proc dentry, which occurs
directly after d_revalidate() returns -ECHILD the first time in
lookup_fast(). To drop into ref mode, we call unlazy_child(), while
nd->path still refers to /proc and dentry refers to /proc/PID. Grabbing
a reference to /proc is the heart of the contention issue.

But directly after a successful d_revalidate() in lookup_fast(), we
return and go to step_into(), which assigns the /proc/PID dentry to
nd->path. After this point, any unlazy operation will not try to grab
the /proc dentry, resulting in significantly less contention.

So it would already be a significant improvement if we kept this change
to pid_revalidate(), and simply added checks to bail out of each of the
other procfs methods if we're in LOOKUP_RCU. Would that be an acceptable
change for you?