fix setuid sometimes wouldn't

check_unsafe_exec() also notes whether the fs_struct is being
shared by more threads than will get killed by the exec, and if so
sets LSM_UNSAFE_SHARE to make bprm_set_creds() careful about euid.
But /proc/<pid>/cwd and /proc/<pid>/root lookups make transient
use of get_fs_struct(), which also raises that sharing count.

This might occasionally cause a setuid program not to change euid,
in the same way as happened with files->count (check_unsafe_exec
also looks at sighand->count, but /proc doesn't raise that one).

We'd prefer exec not to unshare fs_struct: so fix this in procfs,
replacing get_fs_struct() by get_fs_path(), which does path_get
while still holding task_lock, instead of raising fs->count.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: stable@kernel.org
___

 fs/proc/base.c |   50 +++++++++++++++--------------------------------
 1 file changed, 16 insertions(+), 34 deletions(-)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -146,15 +146,22 @@ static unsigned int pid_entry_count_dirs(const struct pid_entry *entries,
 	return count;
 }
 
-static struct fs_struct *get_fs_struct(struct task_struct *task)
+static int get_fs_path(struct task_struct *task, struct path *path, bool root)
 {
 	struct fs_struct *fs;
+	int result = -ENOENT;
+
 	task_lock(task);
 	fs = task->fs;
-	if(fs)
-		atomic_inc(&fs->count);
+	if (fs) {
+		read_lock(&fs->lock);
+		*path = root ? fs->root : fs->pwd;
+		path_get(path);
+		read_unlock(&fs->lock);
+		result = 0;
+	}
 	task_unlock(task);
-	return fs;
+	return result;
 }
 
 static int get_nr_threads(struct task_struct *tsk)
@@ -172,42 +179,24 @@ static int get_nr_threads(struct task_struct *tsk)
 static int proc_cwd_link(struct inode *inode, struct path *path)
 {
 	struct task_struct *task = get_proc_task(inode);
-	struct fs_struct *fs = NULL;
 	int result = -ENOENT;
 
 	if (task) {
-		fs = get_fs_struct(task);
+		result = get_fs_path(task, path, 0);
 		put_task_struct(task);
 	}
-	if (fs) {
-		read_lock(&fs->lock);
-		*path = fs->pwd;
-		path_get(&fs->pwd);
-		read_unlock(&fs->lock);
-		result = 0;
-		put_fs_struct(fs);
-	}
 	return result;
 }
 
 static int proc_root_link(struct inode *inode, struct path *path)
 {
 	struct task_struct *task = get_proc_task(inode);
-	struct fs_struct *fs = NULL;
 	int result = -ENOENT;
 
 	if (task) {
-		fs = get_fs_struct(task);
+		result = get_fs_path(task, path, 1);
 		put_task_struct(task);
 	}
-	if (fs) {
-		read_lock(&fs->lock);
-		*path = fs->root;
-		path_get(&fs->root);
-		read_unlock(&fs->lock);
-		result = 0;
-		put_fs_struct(fs);
-	}
 	return result;
 }
 
@@ -596,7 +585,6 @@ static int mounts_open_common(struct inode *inode, struct file *file,
 	struct task_struct *task = get_proc_task(inode);
 	struct nsproxy *nsp;
 	struct mnt_namespace *ns = NULL;
-	struct fs_struct *fs = NULL;
 	struct path root;
 	struct proc_mounts *p;
 	int ret = -EINVAL;
@@ -610,22 +598,16 @@ static int mounts_open_common(struct inode *inode, struct file *file,
 				get_mnt_ns(ns);
 		}
 		rcu_read_unlock();
-		if (ns)
-			fs = get_fs_struct(task);
+		if (ns && get_fs_path(task, &root, 1) == 0)
+			ret = 0;
 		put_task_struct(task);
 	}
 
 	if (!ns)
 		goto err;
-	if (!fs)
+	if (ret)
 		goto err_put_ns;
 
-	read_lock(&fs->lock);
-	root = fs->root;
-	path_get(&root);
-	read_unlock(&fs->lock);
-	put_fs_struct(fs);
-
 	ret = -ENOMEM;
 	p = kmalloc(sizeof(struct proc_mounts), GFP_KERNEL);
 	if (!p)