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

[Robert Foss]

On 2016-08-29 11:25 AM, Will Drewry wrote:
> On Fri, Aug 26, 2016 at 4:32 PM, Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx <mailto:kirill@xxxxxxxxxxxxx>> wrote:
>
>     On Fri, Aug 26, 2016 at 12:30:04PM -0400, robert.foss@xxxxxxxxxxxxx
>     <mailto:robert.foss@xxxxxxxxxxxxx> wrote:
>     > From: Will Drewry <wad@xxxxxxxxxxxx <mailto: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. :)

Splendid, patch dropped!
Thanks Will and Kirill!


Rob.

>     > 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/
> <https://lwn.net/Articles/455256/> ) which predated memfd_create().
>  memfd_create() is the right solution to this problem!
>
>
> Thanks again!
> will