Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

[Matthew Wilcox <willy@infradead.org>]

On Thu, Dec 03, 2020 at 04:02:12PM -0800, Stephen Brennan wrote:
> -void pid_update_inode(struct task_struct *task, struct inode *inode)
> +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
> +			       unsigned int flags)

I'm really nitpicking here, but this function only _updates_ the inode
if flags says it should.  So I was thinking something like this
(compile tested only).

I'd really appreocate feedback from someone like Casey or Stephen on
what they need for their security modules.

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b362523a9829..771f330bfce7 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1968,6 +1968,25 @@ void pid_update_inode(struct task_struct *task, struct inode *inode)
 	security_task_to_inode(task, inode);
 }
 
+/* See if we can avoid the above call.  Assumes RCU lock held */
+static bool inode_needs_pid_update(struct task_struct *task,
+		const struct inode *inode)
+{
+	kuid_t uid;
+	kgid_t gid;
+
+	if (inode->i_mode & (S_ISUID | S_ISGID))
+		return true;
+	task_dump_owner(task, inode->i_mode, &uid, &gid);
+	if (!uid_eq(uid, inode->i_uid) || !gid_eq(gid, inode->i_gid))
+		return true;
+	/*
+	 * XXX: Do we need to call the security system here to see if
+	 * there's a pending update?
+	 */
+	return false;
+}
+
 /*
  * Rewrite the inode's ownerships here because the owning task may have
  * performed a setuid(), etc.
@@ -1978,8 +1997,15 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 	struct inode *inode;
 	struct task_struct *task;
 
-	if (flags & LOOKUP_RCU)
+	if (flags & LOOKUP_RCU) {
+		inode = d_inode_rcu(dentry);
+		task = pid_task(proc_pid(inode), PIDTYPE_PID);
+		if (!task)
+			return 0;
+		if (!inode_needs_pid_update(task, inode))
+			return 1;
 		return -ECHILD;
+	}
 
 	inode = d_inode(dentry);
 	task = get_proc_task(inode);