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>

1 files changed, 16 insertions, 34 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index aef6d55b7de6..e0afd326b688 100644
--- a/fs/proc/base.c
+++ b/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)