Repost: Missing security_mmap_file() in remap_file_pages syscall

[TongZhang]

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.

Thanks,
- Tong

The original post is pasted below:

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[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

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