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

From: Jason Gunthorpe <jgg@mellanox.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Hector Marco-Gisbert <hecmargi@upv.es>, X86 ML <x86@kernel.org>,
	Will Deacon <will.deacon@arm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs
Date: Fri, 26 Apr 2019 15:02:43 +0000	[thread overview]
Message-ID: <20190426150237.GD2303@mellanox.com> (raw)
In-Reply-To: <20190425200725.GC58719@gmail.com>

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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