Re: [PATCH v1] mm, sysctl: Add sysctl for controlling VM_MAYEXEC taint

[Kirill A. Shutemov]

On Fri, Aug 26, 2016 at 12:30:04PM -0400, robert.foss@xxxxxxxxxxxxx wrote:
> From: Will Drewry <wad@xxxxxxxxxxxx>
> 
> This patch proposes a sysctl knob that allows a privileged user to
> disable ~VM_MAYEXEC tainting when mapping in a vma from a MNT_NOEXEC
> mountpoint.  It does not alter the normal behavior resulting from
> attempting to directly mmap(PROT_EXEC) a vma (-EPERM) nor the behavior
> of any other subsystems checking MNT_NOEXEC.

Wouldn't it be equal to remounting all filesystems without noexec from
attacker POV? It's hardly a fence to make additional mprotect(PROT_EXEC)
call, before starting executing code from such filesystems.

If administrator of the system wants this, he can just mount filesystem
without noexec, no new kernel code required. And it's more fine-grained
than this.

So, no, I don't think we should add knob like this. Unless I miss
something.

NAK.

> It is motivated by a common /dev/shm, /tmp usecase. There are few
> facilities for creating a shared memory segment that can be remapped in
> the same process address space with different permissions.

What about using memfd_create(2) for such cases? You'll get a file
descriptor from in-kernel tmpfs (shm_mnt) which is not exposed to
userspace for remount as noexec.

-- 
 Kirill A. Shutemov