On Fri, Aug 26, 2016 at 4:32 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
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.
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. :)
> 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