autofs: dont trigger mount if it cant succeed

If a mount namespace contains autofs mounts, and they are propagation
private, and there is no namespace specific automount daemon to handle
possible automounting then attempted path resolution will loop until
MAXSYMLINKS is reached before failing causing quite a bit of noise in
the log.

Add a check for this in autofs ->d_automount() so that the VFS can
immediately return an error in this case. Since the mount is propagation
private an EPERM return seems most appropriate.

Suggested by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Ian Kent <raven@themaw.net>
Link: https://patch.msgid.link/20251118024631.10854-2-raven@themaw.net
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/autofs/autofs_i.h

@@ -16,6 +16,7 @@
 #include <linux/wait.h>
 #include <linux/sched.h>
 #include <linux/sched/signal.h>
+#include <uapi/linux/mount.h>
 #include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/uaccess.h>
@@ -27,6 +28,9 @@
 #include <linux/magic.h>
 #include <linux/fs_context.h>
 #include <linux/fs_parser.h>
+#include "../mount.h"
+#include <linux/ns_common.h>
+
 
 /* This is the range of ioctl() numbers we claim as ours */
 #define AUTOFS_IOC_FIRST     AUTOFS_IOC_READY
@@ -114,6 +118,7 @@ struct autofs_sb_info {
 	int pipefd;
 	struct file *pipe;
 	struct pid *oz_pgrp;
+	u64 mnt_ns_id;
 	int version;
 	int sub_version;
 	int min_proto;

fs/autofs/dev-ioctl.c

@@ -381,6 +381,7 @@ static int autofs_dev_ioctl_setpipefd(struct file *fp,
 		swap(sbi->oz_pgrp, new_pid);
 		sbi->pipefd = pipefd;
 		sbi->pipe = pipe;
+		sbi->mnt_ns_id = to_ns_common(current->nsproxy->mnt_ns)->ns_id;
 		sbi->flags &= ~AUTOFS_SBI_CATATONIC;
 	}
 out:

fs/autofs/inode.c

@@ -251,6 +251,7 @@ static struct autofs_sb_info *autofs_alloc_sbi(void)
 	sbi->min_proto = AUTOFS_MIN_PROTO_VERSION;
 	sbi->max_proto = AUTOFS_MAX_PROTO_VERSION;
 	sbi->pipefd = -1;
+	sbi->mnt_ns_id = to_ns_common(current->nsproxy->mnt_ns)->ns_id;
 
 	set_autofs_type_indirect(&sbi->type);
 	mutex_init(&sbi->wq_mutex);

fs/autofs/root.c

@@ -341,6 +341,14 @@ static struct vfsmount *autofs_d_automount(struct path *path)
 	if (autofs_oz_mode(sbi))
 		return NULL;
 
+	/* Refuse to trigger mount if current namespace is not the owner
+	 * and the mount is propagation private.
+	 */
+	if (sbi->mnt_ns_id != to_ns_common(current->nsproxy->mnt_ns)->ns_id) {
+		if (vfsmount_to_propagation_flags(path->mnt) & MS_PRIVATE)
+			return ERR_PTR(-EPERM);
+	}
+
 	/*
 	 * If an expire request is pending everyone must wait.
 	 * If the expire fails we're still mounted so continue

fs/namespace.c

@@ -5150,6 +5150,12 @@ static u64 mnt_to_propagation_flags(struct mount *m)
 	return propagation;
 }
 
+u64 vfsmount_to_propagation_flags(struct vfsmount *mnt)
+{
+	return mnt_to_propagation_flags(real_mount(mnt));
+}
+EXPORT_SYMBOL_GPL(vfsmount_to_propagation_flags);
+
 static void statmount_sb_basic(struct kstatmount *s)
 {
 	struct super_block *sb = s->mnt->mnt_sb;

include/linux/fs.h

@@ -3269,6 +3269,7 @@ extern struct file * open_exec(const char *);
 /* fs/dcache.c -- generic fs support functions */
 extern bool is_subdir(struct dentry *, struct dentry *);
 extern bool path_is_under(const struct path *, const struct path *);
+u64 vfsmount_to_propagation_flags(struct vfsmount *mnt);
 
 extern char *file_path(struct file *, char *, int);