pidfs: convert to path_from_stashed() helper

Moving pidfds from the anonymous inode infrastructure to a separate tiny
in-kernel filesystem similar to sockfs, pipefs, and anon_inodefs causes
selinux denials and thus various userspace components that make heavy
use of pidfds to fail as pidfds used anon_inode_getfile() which aren't
subject to any LSM hooks. But dentry_open() is and that would cause
regressions.

The failures that are seen are selinux denials. But the core failure is
dbus-broker. That cascades into other services failing that depend on
dbus-broker. For example, when dbus-broker fails to start polkit and all
the others won't be able to work because they depend on dbus-broker.

The reason for dbus-broker failing is because it doesn't handle failures
for SO_PEERPIDFD correctly. Last kernel release we introduced
SO_PEERPIDFD (and SCM_PIDFD). SO_PEERPIDFD allows dbus-broker and polkit
and others to receive a pidfd for the peer of an AF_UNIX socket. This is
the first time in the history of Linux that we can safely authenticate
clients in a race-free manner.

dbus-broker immediately made use of this but messed up the error
checking. It only allowed EINVAL as a valid failure for SO_PEERPIDFD.
That's obviously problematic not just because of LSM denials but because
of seccomp denials that would prevent SO_PEERPIDFD from working; or any
other new error code from there.

So this is catching a flawed implementation in dbus-broker as well. It
has to fallback to the old pid-based authentication when SO_PEERPIDFD
doesn't work no matter the reasons otherwise it'll always risk such
failures. So overall that LSM denial should not have caused dbus-broker
to fail. It can never assume that a feature released one kernel ago like
SO_PEERPIDFD can be assumed to be available.

So, the next fix separate from the selinux policy update is to try and
fix dbus-broker at [3]. That should make it into Fedora as well. In
addition the selinux reference policy should also be updated. See [4]
for that. If Selinux is in enforcing mode in userspace and it encounters
anything that it doesn't know about it will deny it by default. And the
policy is entirely in userspace including declaring new types for stuff
like nsfs or pidfs to allow it.

For now we continue to raise S_PRIVATE on the inode if it's a pidfs
inode which means things behave exactly like before.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2265630
Link: https://github.com/fedora-selinux/selinux-policy/pull/2050
Link: https://github.com/bus1/dbus-broker/pull/343 [3]
Link: https://github.com/SELinuxProject/refpolicy/pull/762 [4]
Reported-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240222190334.GA412503@dev-arch.thelio-3990X
Link: https://lore.kernel.org/r/20240218-neufahrzeuge-brauhaus-fb0eb6459771@braunerSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/internal.h

@@ -312,4 +312,5 @@ struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap);
 void mnt_idmap_put(struct mnt_idmap *idmap);
 int path_from_stashed(struct dentry **stashed, unsigned long ino,
 		      struct vfsmount *mnt, const struct file_operations *fops,
-		      void *data, struct path *path);
+		      const struct inode_operations *iops, void *data,
+		      struct path *path);

fs/libfs.c

@@ -23,6 +23,7 @@
 #include <linux/fsnotify.h>
 #include <linux/unicode.h>
 #include <linux/fscrypt.h>
+#include <linux/pidfs.h>
 
 #include <linux/uaccess.h>
 
@@ -1990,6 +1991,7 @@ static inline struct dentry *get_stashed_dentry(struct dentry *stashed)
 static struct dentry *stash_dentry(struct dentry **stashed, unsigned long ino,
 				   struct super_block *sb,
 				   const struct file_operations *fops,
+				   const struct inode_operations *iops,
 				   void *data)
 {
 	struct dentry *dentry;
@@ -2007,7 +2009,12 @@ static struct dentry *stash_dentry(struct dentry **stashed, unsigned long ino,
 
 	inode->i_ino = ino;
 	inode->i_flags |= S_IMMUTABLE;
+	if (is_pidfs_sb(sb))
+		inode->i_flags |= S_PRIVATE;
 	inode->i_mode = S_IFREG | S_IRUGO;
+	if (iops)
+		inode->i_op = iops;
+	if (fops)
 		inode->i_fop = fops;
 	inode->i_private = data;
 	simple_inode_init_ts(inode);
@@ -2030,6 +2037,7 @@ static struct dentry *stash_dentry(struct dentry **stashed, unsigned long ino,
  * @stashed:    where to retrieve or stash dentry
  * @ino:        inode number to use
  * @mnt:        mnt of the filesystems to use
+ * @iops:       inode operations to use
  * @fops:       file operations to use
  * @data:       data to store in inode->i_private
  * @path:       path to create
@@ -2048,7 +2056,8 @@ static struct dentry *stash_dentry(struct dentry **stashed, unsigned long ino,
  */
 int path_from_stashed(struct dentry **stashed, unsigned long ino,
 		      struct vfsmount *mnt, const struct file_operations *fops,
-		      void *data, struct path *path)
+		      const struct inode_operations *iops, void *data,
+		      struct path *path)
 {
 	struct dentry *dentry;
 	int ret = 0;
@@ -2057,7 +2066,7 @@ int path_from_stashed(struct dentry **stashed, unsigned long ino,
 	if (dentry)
 		goto out_path;
 
-	dentry = stash_dentry(stashed, ino, mnt->mnt_sb, fops, data);
+	dentry = stash_dentry(stashed, ino, mnt->mnt_sb, fops, iops, data);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 	ret = 1;

fs/nsfs.c

@@ -67,7 +67,7 @@ int ns_get_path_cb(struct path *path, ns_get_path_helper_t *ns_get_cb,
 		if (!ns)
 			return -ENOENT;
 		ret = path_from_stashed(&ns->stashed, ns->inum, nsfs_mnt,
-					&ns_file_operations, ns, path);
+					&ns_file_operations, NULL, ns, path);
 		if (ret <= 0 && ret != -EAGAIN)
 			ns->ops->put(ns);
 	} while (ret == -EAGAIN);
@@ -122,8 +122,9 @@ int open_related_ns(struct ns_common *ns,
 			return PTR_ERR(relative);
 		}
 
-		err = path_from_stashed(&relative->stashed, relative->inum, nsfs_mnt,
-					&ns_file_operations, relative, &path);
+		err = path_from_stashed(&relative->stashed, relative->inum,
+					nsfs_mnt, &ns_file_operations, NULL,
+					relative, &path);
 		if (err <= 0 && err != -EAGAIN)
 			relative->ops->put(relative);
 	} while (err == -EAGAIN);

fs/pidfs.c

@@ -14,6 +14,8 @@
 #include <linux/seq_file.h>
 #include <uapi/linux/pidfd.h>
 
+#include "internal.h"
+
 static int pidfd_release(struct inode *inode, struct file *file)
 {
 #ifndef CONFIG_FS_PID
@@ -186,9 +188,21 @@ static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
 			     d_inode(dentry)->i_ino);
 }
 
+static void pidfs_prune_dentry(struct dentry *dentry)
+{
+	struct inode *inode;
+
+	inode = d_inode(dentry);
+	if (inode) {
+		struct pid *pid = inode->i_private;
+		WRITE_ONCE(pid->stashed, NULL);
+	}
+}
+
 static const struct dentry_operations pidfs_dentry_operations = {
 	.d_delete	= always_delete_dentry,
 	.d_dname	= pidfs_dname,
+	.d_prune	= pidfs_prune_dentry,
 };
 
 static int pidfs_init_fs_context(struct fs_context *fc)
@@ -213,34 +227,28 @@ static struct file_system_type pidfs_type = {
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 {
 
-	struct inode *inode;
 	struct file *pidfd_file;
+	struct path path;
+	int ret;
 
-	inode = iget_locked(pidfs_sb, pid->ino);
-	if (!inode)
-		return ERR_PTR(-ENOMEM);
-
-	if (inode->i_state & I_NEW) {
+	do {
 		/*
 		 * Inode numbering for pidfs start at RESERVED_PIDS + 1.
 		 * This avoids collisions with the root inode which is 1
 		 * for pseudo filesystems.
 		 */
-		inode->i_ino = pid->ino;
-		inode->i_mode = S_IFREG | S_IRUGO;
-		inode->i_op = &pidfs_inode_operations;
-		inode->i_fop = &pidfs_file_operations;
-		inode->i_flags |= S_IMMUTABLE;
-		inode->i_private = get_pid(pid);
-		simple_inode_init_ts(inode);
-		unlock_new_inode(inode);
-	}
-
-	pidfd_file = alloc_file_pseudo(inode, pidfs_mnt, "", flags,
-				       &pidfs_file_operations);
-	if (IS_ERR(pidfd_file))
-		iput(inode);
+		ret = path_from_stashed(&pid->stashed, pid->ino, pidfs_mnt,
+					&pidfs_file_operations,
+					&pidfs_inode_operations, get_pid(pid),
+					&path);
+		if (ret <= 0 && ret != -EAGAIN)
+			put_pid(pid);
+	} while (ret == -EAGAIN);
+	if (ret < 0)
+		return ERR_PTR(ret);
 
+	pidfd_file = dentry_open(&path, flags, current_cred());
+	path_put(&path);
 	return pidfd_file;
 }
 
@@ -253,6 +261,11 @@ void __init pidfs_init(void)
 	pidfs_sb = pidfs_mnt->mnt_sb;
 }
 
+bool is_pidfs_sb(const struct super_block *sb)
+{
+	return sb == pidfs_mnt->mnt_sb;
+}
+
 #else /* !CONFIG_FS_PID */
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
@@ -269,4 +282,8 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 }
 
 void __init pidfs_init(void) { }
+bool is_pidfs_sb(const struct super_block *sb)
+{
+	return false;
+}
 #endif

include/linux/pid.h

@@ -56,6 +56,7 @@ struct pid
 	unsigned int level;
 	spinlock_t lock;
 #ifdef CONFIG_FS_PID
+	struct dentry *stashed;
 	unsigned long ino;
 #endif
 	/* lists of tasks that use this pid */

include/linux/pidfs.h

@@ -4,5 +4,6 @@
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags);
 void __init pidfs_init(void);
+bool is_pidfs_sb(const struct super_block *sb);
 
 #endif /* _LINUX_PID_FS_H */

kernel/pid.c

@@ -281,6 +281,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 	if (!(ns->pid_allocated & PIDNS_ADDING))
 		goto out_unlock;
 #ifdef CONFIG_FS_PID
+	pid->stashed = NULL;
 	pid->ino = ++pidfs_ino;
 #endif
 	for ( ; upid >= pid->numbers; --upid) {