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Message-ID: <20190426150237.GD2303@mellanox.com>
Date: Fri, 26 Apr 2019 15:02:43 +0000
From: Jason Gunthorpe <jgg@...lanox.com>
To: Ingo Molnar <mingo@...nel.org>
CC: Kees Cook <keescook@...omium.org>, Andrew Morton
<akpm@...ux-foundation.org>, Hector Marco-Gisbert <hecmargi@....es>, Marc
Gonzalez <marc.w.gonzalez@...e.fr>, Will Deacon <will.deacon@....com>, X86 ML
<x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski
<luto@...capital.net>, Stephen Rothwell <sfr@...b.auug.org.au>, Catalin
Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, Arnd
Bergmann <arnd@...db.de>, Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, LKML
<linux-kernel@...r.kernel.org>, Linus Torvalds
<torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>, Peter
Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern
CPUs
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@mellanox.com/
>
> 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
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