Fix races around the access to ->s_options

Put generic_show_options read access to s_options under rcu_read_lock,
split save_mount_options() into "we are setting it the first time"
(uses in foo_fill_super()) and "we are relacing and freeing the old one",
synchronize_rcu() before kfree() in the latter.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

7 files changed, 25 insertions, 13 deletions

diff --git a/drivers/isdn/capi/capifs.c b/drivers/isdn/capi/capifs.c
index b129409925af..8f9f3b5a3e8c 100644
--- a/drivers/isdn/capi/capifs.c
+++ b/drivers/isdn/capi/capifs.c
@@ -75,8 +75,7 @@ static int capifs_remount(struct super_block *s, int *flags, char *data)
                 }
         }
 
-        kfree(s->s_options);
-        s->s_options = new_opt;
+        replace_mount_options(s, new_opt);
 
         config.setuid  = setuid;
         config.setgid  = setgid;
diff --git a/fs/affs/super.c b/fs/affs/super.c
index 5ce695e707fe..63f5183f263b 100644
--- a/fs/affs/super.c
+++ b/fs/affs/super.c
@@ -507,8 +507,7 @@ affs_remount(struct super_block *sb, int *flags, char *data)
                 kfree(new_opts);
                 return -EINVAL;
         }
-        kfree(sb->s_options);
-        sb->s_options = new_opts;
+        replace_mount_options(sb, new_opts);
 
         sbi->s_flags = mount_flags;
         sbi->s_mode  = mode;
diff --git a/fs/afs/super.c b/fs/afs/super.c
index 2753f16dd315..76828e5f8a39 100644
--- a/fs/afs/super.c
+++ b/fs/afs/super.c
@@ -408,17 +408,17 @@ static int afs_get_sb(struct file_system_type *fs_type,
                         deactivate_locked_super(sb);
                         goto error;
                 }
-                sb->s_options = new_opts;
+                save_mount_options(sb, new_opts);
                 sb->s_flags |= MS_ACTIVE;
         } else {
                 _debug("reuse");
-                kfree(new_opts);
                 ASSERTCMP(sb->s_flags, &, MS_ACTIVE);
         }
 
         simple_set_mnt(mnt, sb);
         afs_put_volume(params.volume);
         afs_put_cell(params.cell);
+        kfree(new_opts);
         _leave(" = 0 [%p]", sb);
         return 0;
 
diff --git a/fs/hpfs/super.c b/fs/hpfs/super.c
index fecf402d7b8a..fc77965be841 100644
--- a/fs/hpfs/super.c
+++ b/fs/hpfs/super.c
@@ -423,8 +423,7 @@ static int hpfs_remount_fs(struct super_block *s, int *flags, char *data)
 
         if (!(*flags & MS_RDONLY)) mark_dirty(s);
 
-        kfree(s->s_options);
-        s->s_options = new_opts;
+        replace_mount_options(s, new_opts);
 
         return 0;
 
diff --git a/fs/namespace.c b/fs/namespace.c
index 0d2003fb4377..134d494158d9 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -695,12 +695,16 @@ static inline void mangle(struct seq_file *m, const char *s)
  */
 int generic_show_options(struct seq_file *m, struct vfsmount *mnt)
 {
-        const char *options = mnt->mnt_sb->s_options;
+        const char *options;
+
+        rcu_read_lock();
+        options = rcu_dereference(mnt->mnt_sb->s_options);
 
         if (options != NULL && options[0]) {
                 seq_putc(m, ',');
                 mangle(m, options);
         }
+        rcu_read_unlock();
 
         return 0;
 }
@@ -721,11 +725,22 @@ EXPORT_SYMBOL(generic_show_options);
  */
 void save_mount_options(struct super_block *sb, char *options)
 {
-        kfree(sb->s_options);
-        sb->s_options = kstrdup(options, GFP_KERNEL);
+        BUG_ON(sb->s_options);
+        rcu_assign_pointer(sb->s_options, kstrdup(options, GFP_KERNEL));
 }
 EXPORT_SYMBOL(save_mount_options);
 
+void replace_mount_options(struct super_block *sb, char *options)
+{
+        char *old = sb->s_options;
+        rcu_assign_pointer(sb->s_options, options);
+        if (old) {
+                synchronize_rcu();
+                kfree(old);
+        }
+}
+EXPORT_SYMBOL(replace_mount_options);
+
 #ifdef CONFIG_PROC_FS
 /* iterator */
 static void *m_start(struct seq_file *m, loff_t *pos)
diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c
index d444fe0013a4..1215a4f50cd2 100644
--- a/fs/reiserfs/super.c
+++ b/fs/reiserfs/super.c
@@ -1316,8 +1316,7 @@ static int reiserfs_remount(struct super_block *s, int *mount_flags, char *arg)
         }
 
 out_ok:
-        kfree(s->s_options);
-        s->s_options = new_opts;
+        replace_mount_options(s, new_opts);
         return 0;
 
 out_err:
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ed788426f464..3b534e527e09 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2368,6 +2368,7 @@ extern void file_update_time(struct file *file);
 
 extern int generic_show_options(struct seq_file *m, struct vfsmount *mnt);
 extern void save_mount_options(struct super_block *sb, char *options);
+extern void replace_mount_options(struct super_block *sb, char *options);
 
 static inline ino_t parent_ino(struct dentry *dentry)
 {