Re: [PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs

[Jason Gunthorpe]

On Thu, Apr 25, 2019 at 10:07:25PM +0200, Ingo Molnar wrote:

> > But yes, your above diff for "has NX" is roughly correct. I'll walk
> > through each piece I'm thinking about. Here is the current state:
> > 
> >                CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 |
> >   ELF:              |            |                  |                |
> >   missing GNU_STACK | exec-all   | exec-all         | exec-all       |
> >   GNU_STACK == RWX  | exec-all   | exec-all         | exec-all       |
> >   GNU_STACK == RW   | exec-none  | exec-none        | exec-none      |
> > 
> > *this column has no architecture effect: NX markings are ignored by
> > hardware, but may have behavioral effects when "wants X" collides with
> > "cannot be X" constraints in memory permission flags, as in [1].
> 
> So [1] appears to be device driver mapping a BAR that isn't intended to 
> be excutable:
> 
>    https://lore.kernel.org/netdev/20190418055759.GA3155@xxxxxxxxxxxx/
> 
> and the question is, do we reject this at the device driver mmap() level 
> already, right?

No, we wanted to reject it at the driver mmap() level, but if an
executable is marked with GNU_STACK=RWX then the core mm code always
calls the driver with VM_EXEC (even though the mmap isn't a stack) and
the driver becomes incompatible with userspace using GNU_STACK=RWX (ie
some Fortran programs, apparently)

> I suspect the best behavior is to reject as early as possible, so I agree 
> with your change here - even though !NX systems tend to become less and 
> less relevant these days.

I suggested the idea of adding a flag in either the struct file or the
file_operations flag that says mmap is never to be executable for this
file with the idea that most/all cdev users would set it.

Does that seem reasonable?

Jason