1 files changed, 112 insertions, 28 deletions

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 64c2d0814ed6..34699f3b6f88 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -6,7 +6,9 @@
 #include <linux/path.h>
 #include <linux/slab.h>
 #include <linux/fs_struct.h>
+#include <linux/init_task.h>
 #include "internal.h"
+#include "mount.h"
 
 /*
  * Replace the fs->{rootmnt,root} with {mnt,dentry}. Put the old values.
@@ -17,12 +19,10 @@ void set_fs_root(struct fs_struct *fs, const struct path *path)
 	struct path old_root;
 
 	path_get(path);
-	spin_lock(&fs->lock);
-	write_seqcount_begin(&fs->seq);
+	write_seqlock(&fs->seq);
 	old_root = fs->root;
 	fs->root = *path;
-	write_seqcount_end(&fs->seq);
-	spin_unlock(&fs->lock);
+	write_sequnlock(&fs->seq);
 	if (old_root.dentry)
 		path_put(&old_root);
 }
@@ -36,12 +36,10 @@ void set_fs_pwd(struct fs_struct *fs, const struct path *path)
 	struct path old_pwd;
 
 	path_get(path);
-	spin_lock(&fs->lock);
-	write_seqcount_begin(&fs->seq);
+	write_seqlock(&fs->seq);
 	old_pwd = fs->pwd;
 	fs->pwd = *path;
-	write_seqcount_end(&fs->seq);
-	spin_unlock(&fs->lock);
+	write_sequnlock(&fs->seq);
 
 	if (old_pwd.dentry)
 		path_put(&old_pwd);
@@ -63,20 +61,21 @@ void chroot_fs_refs(const struct path *old_root, const struct path *new_root)
 
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, p) {
+		if (p->flags & (PF_KTHREAD | PF_EXITING | PF_DUMPCORE))
+			continue;
+
 		task_lock(p);
-		fs = p->fs;
+		fs = p->real_fs;
 		if (fs) {
 			int hits = 0;
-			spin_lock(&fs->lock);
-			write_seqcount_begin(&fs->seq);
+			write_seqlock(&fs->seq);
 			hits += replace_path(&fs->root, old_root, new_root);
 			hits += replace_path(&fs->pwd, old_root, new_root);
-			write_seqcount_end(&fs->seq);
 			while (hits--) {
 				count++;
 				path_get(new_root);
 			}
-			spin_unlock(&fs->lock);
+			write_sequnlock(&fs->seq);
 		}
 		task_unlock(p);
 	}
@@ -94,15 +93,16 @@ void free_fs_struct(struct fs_struct *fs)
 
 void exit_fs(struct task_struct *tsk)
 {
-	struct fs_struct *fs = tsk->fs;
+	struct fs_struct *fs = tsk->real_fs;
 
 	if (fs) {
 		int kill;
 		task_lock(tsk);
-		spin_lock(&fs->lock);
+		read_seqlock_excl(&fs->seq);
+		tsk->real_fs = NULL;
 		tsk->fs = NULL;
 		kill = !--fs->users;
-		spin_unlock(&fs->lock);
+		read_sequnlock_excl(&fs->seq);
 		task_unlock(tsk);
 		if (kill)
 			free_fs_struct(fs);
@@ -116,23 +116,22 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 	if (fs) {
 		fs->users = 1;
 		fs->in_exec = 0;
-		spin_lock_init(&fs->lock);
-		seqcount_spinlock_init(&fs->seq, &fs->lock);
+		seqlock_init(&fs->seq);
 		fs->umask = old->umask;
 
-		spin_lock(&old->lock);
+		read_seqlock_excl(&old->seq);
 		fs->root = old->root;
 		path_get(&fs->root);
 		fs->pwd = old->pwd;
 		path_get(&fs->pwd);
-		spin_unlock(&old->lock);
+		read_sequnlock_excl(&old->seq);
 	}
 	return fs;
 }
 
 int unshare_fs_struct(void)
 {
-	struct fs_struct *fs = current->fs;
+	struct fs_struct *fs = current->real_fs;
 	struct fs_struct *new_fs = copy_fs_struct(fs);
 	int kill;
 
@@ -140,10 +139,12 @@ int unshare_fs_struct(void)
 		return -ENOMEM;
 
 	task_lock(current);
-	spin_lock(&fs->lock);
+	read_seqlock_excl(&fs->seq);
+	VFS_WARN_ON_ONCE(fs != current->fs);
 	kill = !--fs->users;
 	current->fs = new_fs;
-	spin_unlock(&fs->lock);
+	current->real_fs = new_fs;
+	read_sequnlock_excl(&fs->seq);
 	task_unlock(current);
 
 	if (kill)
@@ -153,16 +154,99 @@ int unshare_fs_struct(void)
 }
 EXPORT_SYMBOL_GPL(unshare_fs_struct);
 
-int current_umask(void)
+/*
+ * PID 1 may choose to stop sharing fs_struct state with us.
+ * Either via unshare(CLONE_FS) or unshare(CLONE_NEWNS). Of
+ * course, PID 1 could have chosen to create arbitrary process
+ * trees that all share fs_struct state via CLONE_FS. This is a
+ * strong statement: We only care about PID 1 aka the thread-group
+ * leader so subthread's fs_struct state doesn't matter.
+ *
+ * PID 1 unsharing fs_struct state is a bug. PID 1 relies on
+ * various kthreads to be able to perform work based on its
+ * fs_struct state. Breaking that contract sucks for both sides.
+ * So just don't bother with extra work for this. No sane init
+ * system should ever do this.
+ *
+ * On older kernels if PID 1 unshared its filesystem state with us the
+ * kernel simply used the stale fs_struct state implicitly pinning
+ * anything that PID 1 had last used. Even if PID 1 might've moved on to
+ * some completely different fs_struct state and might've even unmounted
+ * the old root.
+ *
+ * This has hilarious consequences: Think continuing to dump coredump
+ * state into an implicitly pinned directory somewhere. Calling random
+ * binaries in the old rootfs via usermodehelpers.
+ *
+ * Be aggressive about this: We simply reject operating on stale
+ * fs_struct state by reverting to nullfs. Every kworker that does
+ * lookups after this point will fail. Every usermodehelper call will
+ * fail. Tough luck but let's be kind and emit a warning to userspace.
+ */
+static inline void validate_fs_switch(struct fs_struct *old_fs)
+{
+	might_sleep();
+
+	if (likely(current->pid != 1))
+		return;
+	/* @old_fs may be dangling but for comparison it's fine */
+	if (old_fs != userspace_init_fs)
+		return;
+	pr_warn("VFS: Pid 1 stopped sharing filesystem state\n");
+	set_fs_root(userspace_init_fs, &init_fs.root);
+	set_fs_pwd(userspace_init_fs, &init_fs.root);
+}
+
+struct fs_struct *switch_fs_struct(struct fs_struct *new_fs)
 {
-	return current->fs->umask;
+	struct fs_struct *fs;
+
+	scoped_guard(task_lock, current) {
+		fs = current->fs;
+		VFS_WARN_ON_ONCE(fs != current->real_fs);
+		read_seqlock_excl(&fs->seq);
+		current->fs = new_fs;
+		current->real_fs = new_fs;
+		if (--fs->users)
+			new_fs = NULL;
+		else
+			new_fs = fs;
+		read_sequnlock_excl(&fs->seq);
+	}
+
+	validate_fs_switch(fs);
+	return new_fs;
 }
-EXPORT_SYMBOL(current_umask);
 
 /* to be mentioned only in INIT_TASK */
 struct fs_struct init_fs = {
 	.users		= 1,
-	.lock		= __SPIN_LOCK_UNLOCKED(init_fs.lock),
-	.seq		= SEQCNT_SPINLOCK_ZERO(init_fs.seq, &init_fs.lock),
+	.seq		= __SEQLOCK_UNLOCKED(init_fs.seq),
 	.umask		= 0022,
 };
+
+struct fs_struct *userspace_init_fs __ro_after_init;
+EXPORT_SYMBOL_GPL(userspace_init_fs);
+
+void __init init_userspace_fs(void)
+{
+	struct mount *m;
+	struct path root;
+
+	/* Move PID 1 from nullfs into the initramfs. */
+	m = topmost_overmount(current->nsproxy->mnt_ns->root);
+	root.mnt = &m->mnt;
+	root.dentry = root.mnt->mnt_root;
+
+	VFS_WARN_ON_ONCE(current->pid != 1);
+
+	set_fs_root(current->fs, &root);
+	set_fs_pwd(current->fs, &root);
+
+	/* Hold a reference for the global pointer. */
+	read_seqlock_excl(&current->fs->seq);
+	current->fs->users++;
+	read_sequnlock_excl(&current->fs->seq);
+
+	userspace_init_fs = current->fs;
+}