Re: Repost: Missing security_mmap_file() in remap_file_pages syscall

From: Stephen Smalley
Date: Mon Mar 25 2019 - 16:02:45 EST


On 3/25/19 1:33 PM, TongZhang wrote:
Dear Kernel Developers,

Weâd like to bring this up for a discussion again.

Several months ago we posted an email discussing a case where remap_file_pages() has no security_mmap_file() check.
At that time we were told that do_mmap_pgoff() will base the new VMA permission on the old one.
But somehow we still think the check is needed, for the reason that the advisory could first do a
mmap() which can pass SELinux check then remap using a completely different file or region of file,
which could possibly pose a risk.

I don't see an issue there. The incoming prot value from userspace is required to be zero (otherwise remap_file_pages returns -EINVAL), so the prot passed to do_mmap_pgoff() is entirely computed based on the existing vma flags. The file is likewise obtained from the existing vma. Any flags supplied by the caller other than MAP_NONBLOCK are ignored. MAP_SHARED is always set in the flags passed to do_mmap_pgoff(), and the existing vma was required to have VM_SHARED set or remap_file_pages() would have returned -EINVAL. If it spans more than one vma, their files and flags must match. It appears that all of the inputs relevant to selinux_mmap_file() are necessarily the same as they would have been when creating the original vma. The same appears to also be true for other security modules IIUC.


Thanks,
- Tong

The original post is pasted below:

8<âââââââââââââââââââââââââââââ
[1.] One line summary of the problem:

Possible missing security_mmap_file() in remap_file_pages

[2.] Full description of the problem/report:

We noticed remap_file_pages syscall uses do_mmap_pgoff without LSM check: security_mmap_file().

This system call passed user controllable parameters to do_mmap_pgoff().

We think that this LSM check should be added in order to be consistent with other cases,
for example:
in system call mmap_pgoff(), shmat(), they all have security_mmap_file() check before calling
do_mmap_pgoff().

[3.] Keywords: LSM check
[4.] Kernel information
[4.1] Kernel Version: 4.14.61

8<âââââââââââââââââââââââââââââ