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

On Fri, Aug 26, 2016 at 4:32 PM, Kirill A. Shutemov [off-list ref]
wrote:

On Fri, Aug 26, 2016 at 12:30:04PM -0400, robert.foss@collabora.com wrote:

quoted

From: Will Drewry <wad@chromium.org>

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.

I don't believe this patch is necessary anymore (though, thank you Robert
for testing and re-sending!).

The primary offenders wrt to needing to mmap/mprotect a file in /dev/shm
was the older nvidia
driver (binary only iirc) and the Chrome Native Client code.

The reason why half-exec is an "ok" (half) mitigation is because it blocks
simple gadgets and other paths for using loadable libraries or binaries
(via glibc) as it disallows mmap(PROT_EXEC) even though it allows
mprotect(PROT_EXEC).  This stops ld in its tracks since it does the obvious
thing and uses mmap(PROT_EXEC).

I think time has marched on and this patch is now something I can toss in
the dustbin of history. Both Chrome's Native Client and an older nvidia
driver relied on creating-then-unlinking a file in tmpfs, but there is now
a better facility!

NAK.

Agreed - this is old and software that predicated it should be gone.. I
hope. :)

quoted

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.

This is a relatively old patch ( https://lwn.net/Articles/455256/ ) which
predated memfd_create().  memfd_create() is the right solution to this
problem!


Thanks again!
will