Bug/inconsistency in hugepage permission checking between mmap and shmget

[Tudor Brindus <me@tbrindus.ca>]

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Hi,

The documentation for both `man 2 mmap` and `man 2 shmget` states that
anonymous hugepage mappings can only be obtained if the caller has
`CAP_IPC_LOCK`, or is part of the `sysctl_hugetlb_shm_group`.

I believe that in practice this is only enforced for `shmget`, and not
`mmap`.
This is true as of current master, and has been true at least since 4.x (I
have
not looked further back).

`mm/mmap.c` contains, in a `MAP_HUGETLB` branch:

  file = hugetlb_file_setup(HUGETLB_ANON_FILE, len,
      VM_NORESERVE,
      HUGETLB_ANONHUGE_INODE,
      (flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);

While `fs/hugetlbfs/inode.c` contains:

  static int can_do_hugetlb_shm(void)
  {
    kgid_t shm_group;
    shm_group = make_kgid(&init_user_ns, sysctl_hugetlb_shm_group);
    return capable(CAP_IPC_LOCK) || in_group_p(shm_group);
  }

  ...

  struct file *hugetlb_file_setup(const char *name, size_t size,
        vm_flags_t acctflag, int creat_flags,
        int page_size_log)
  {
  ...
    if (creat_flags == HUGETLB_SHMFS_INODE && !can_do_hugetlb_shm()) {
      ...
      return ERR_PTR(-EPERM);
    }

i.e., only checks `can_do_hugetlb_shm` when `create_flags ==
HUGETLB_SHMFS_INODE`, whereas the callsite in `mm/mmap.c` passes in
`HUGETLB_ANONHUGE_INODE`.

A simple test program that tries allocating hugepage memory with `mmap` and
`shmget` while not possessing `CAP_IPC_LOCK` and not being in the
`sysctl_hugetlb_shm_group` confirms this behavior.

What's the right course of action here?

- The logic in `hugetlb_file_setup` could be modified to enforce the
  permissions on `mmap` calls. This might break userspace code that's been
  relying on this working, though.

- The restriction could be removed from `shmget`.

- The inconsistency between `mmap` and `shmget` could be accepted as a fact
of
  life, and the documentation fixed to match this reality.

-- Tudor

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