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fs/namespace.c: fix mountpoint reference counter race 

A race condition between threads updating mountpoint reference counter
affects longterm releases 4.4.220, 4.9.220, 4.14.177 and 4.19.118.

The mountpoint reference counter corruption may occur when:
* one thread increments m_count member of struct mountpoint
  [under namespace_sem, but not holding mount_lock]
    pivot_root()
* another thread simultaneously decrements the same m_count
  [under mount_lock, but not holding namespace_sem]
    put_mountpoint()
      unhash_mnt()
        umount_mnt()
          mntput_no_expire()

To fix this race condition, grab mount_lock before updating m_count in
pivot_root().

Reference: CVE-2020-12114
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I2b2cfdbf6529c9d72d54738db6169dd421eb1f51
This commit is contained in:
Piotr Krysiuk 2020-04-27 11:34:12 +01:00 committed by L R
parent e49b967f0b
commit b318271a8b
1 changed files with 1 additions and 1 deletions
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2
fs/namespace.c
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@ -2738,8 +2738,8 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
/* make certain new is below the root */
if (!is_path_reachable(new_mnt, new.dentry, &root))
goto out4;
root_mp->m_count++; /* pin it so it won't go away */
br_write_lock(&vfsmount_lock);
root_mp->m_count++; /* pin it so it won't go away */
detach_mnt(new_mnt, &parent_path);
detach_mnt(root_mnt, &root_parent);
/* mount old root on put_old */