Re: [PATCH 0/6] SUNRPC: Address remaining cache_check_rcu() UAF in cache content files

[yangerkun]

Hello  Chuck,

在 2026/5/8 0:12, Chuck Lever 写道:
> Hello Erkun -
>
> On Thu, May 7, 2026, at 11:09 AM, yangerkun wrote:
> > Hi,
> >
> > 在 2026/5/1 22:51, Chuck Lever 写道:
> > > Misbah Anjum reported a use-after-free in cache_check_rcu()
> > > reached through e_show() while sosreport was reading
> > > /proc/fs/nfsd/exports on ppc64le.  Two fixes for that report
> > > landed in v7.0:
> > >
> > >     48db892356d6 ("NFSD: Defer sub-object cleanup in export put callbacks")
> > >     e7fcf179b82d ("NFSD: Hold net reference for the lifetime of /proc/fs/nfs/exports fd")
> >
> > Back to the problem fixed by this patches, I'm a little confused why
> > this UAF can be trigged.
> >
> > Before this patches, svc_export_put show as follow:
> >
> >    368 static void svc_export_put(struct kref *ref)
> >    369 {
> >    370         struct svc_export *exp = container_of(ref, struct
> > svc_export, h.ref);
> >    371
> >    372         path_put(&exp->ex_path);
> >    373         auth_domain_put(exp->ex_client);
> >    374         call_rcu(&exp->ex_rcu, svc_export_release);
> >    375 }
> >
> > The auth_domain_put function releases ->name using call_rcu, and
> > path_put may release the dentry also via call_rcu. All of this seems to
> > prevent e_show from causing a UAF. Could you point out which line in
> > d_path triggers the issue?
>
> The dentry, the mount, and the auth_domain ->name buffer all
> end up RCU-freed (dentry_free() and delayed_free_vfsmnt in
> fs/, svcauth_unix_domain_release_rcu() in svcauth_unix.c).
> The eventual kfree isn't the problem.
>
> The problem is the synchronous teardown inside path_put(),
> which runs before svc_export_put() ever reaches its own
> call_rcu():
>
>    path_put(&exp->ex_path)
>      -> dput(dentry)
>         -> __dentry_kill()              [if last ref]
>            -> __d_drop()                /* unhashes */
>            -> dentry_unlink_inode()     /* d_inode = NULL */
>            -> d_op->d_release() if set
>            -> drops parent d_lockref    /* may cascade up */
>            -> dentry_free()             /* call_rcu deferred */
>      -> mntput(mnt)                     /* deferred via task_work */
>
> The dentry pointer itself is RCU-safe, so prepend_path()'s walk
> of d_parent and d_name doesn't read freed memory.  But by the
> time the reader gets there, __d_clear_type_and_inode() has
> already stored NULL into d_inode, __d_drop() has broken the
> hash linkage, and the parent's d_lockref has been decremented
> -- which can in turn fire __dentry_kill() on the parent, and
> on up the tree.  An e_show() that's still inside its cache RCU
> read section walks into that half-dismantled state through
> seq_path(), and that's the NULL deref Misbah reported.

Thank you for your detailed explanation! Yes, e_show might be called 
when the state is partially dismantled, but after carefully reviewing 
the code with dput up to __dentry_kill, I still cannot find anything 
that could cause this issue. Additionally, the comments for prepend_path 
indicate that they have already taken into account that the dentry can 
be removed concurrently. I have also run some tests on my arm64 QEMU, 
but I couldn't reproduce the problem either. Could you please help me 
identify the specific line or pointer in the dentry that triggers this 
use-after-free or null pointer issue?

Maybe I am not be very familiar with the code, which caused me to fail 
to identify the real root cause. I'm so sorry for that.


265 char *d_path(const struct path *path, char *buf, int buflen)
266 {
267         DECLARE_BUFFER(b, buf, buflen);
268         struct path root;
269
270         /*
271          * We have various synthetic filesystems that never get 
mounted.  On
272          * these filesystems dentries are never used for lookup 
purposes, and
273          * thus don't need to be hashed.  They also don't need a 
name until a
274          * user wants to identify the object in /proc/pid/fd/.  The 
little hack
275          * below allows us to generate a name for these objects on 
demand:
276          *
277          * Some pseudo inodes are mountable.  When they are mounted
278          * path->dentry == path->mnt->mnt_root.  In that case don't 
call d_dname
279          * and instead have d_path return the mounted path.
280          */
281         if (path->dentry->d_op && path->dentry->d_op->d_dname &&
282             (!IS_ROOT(path->dentry) || path->dentry != 
path->mnt->mnt_root))
283                 return path->dentry->d_op->d_dname(path->dentry, 
buf, buflen);
284
285         rcu_read_lock();
286         get_fs_root_rcu(current->fs, &root);
287         if (unlikely(d_unlinked(path->dentry)))
288                 prepend(&b, " (deleted)", 11);
289         else
290                 prepend_char(&b, 0);
291         prepend_path(path, &root, &b);
292         rcu_read_unlock();
293
294         return extract_string(&b);
295 }


> The earlier fix (2530766492ec, "nfsd: fix UAF when access
> ex_uuid or ex_stats") moved the kfree of ex_uuid and ex_stats
> into svc_export_release() so those are RCU-safe now.
> path_put() and auth_domain_put() couldn't go in there because
> both may sleep, and call_rcu callbacks run in softirq context.
> This series uses queue_rcu_work() instead: it defers past the
> grace period AND runs the callback in process context, so the
> sleeping puts move into the deferred path and the window
> closes.

Yeah, I can get this! Thanks again for your detail explanation!

Thanks,
Erkun.

>