linux-kernel - Re: [PATCH v4] powerpc: Do not make the entire heap executable

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Message-ID: <20160810133621.GA30167@redhat.com>
Date:	Wed, 10 Aug 2016 15:36:21 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Florian Weimer <fweimer@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] powerpc: Do not make the entire heap executable

On 08/10, Denys Vlasenko wrote:
>
> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
> brk area* with executable rights for all binaries, even --secure-plt ones.
>
> Stop doing that.

Can't really review this patch, but at least the change in mm/mmap.c looks
technically correct to me... One nit below, feel free to ignore.

> @@ -2668,7 +2668,7 @@ static int do_brk(unsigned long addr, unsigned long request)
>  	if (!len)
>  		return 0;
>
> -	flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> +	flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;

OK. But note that we have

	mlock_future_check(mm->def_flags);

a few lines below and after this change this _looks_ wrong because
VM_LOCKED can come from the new "flags" argument passed to do_brk().
Nobody does this right now, still this looks wrong/confusing.

I'd suggest to add another change

	-	mlock_future_check(mm->def_flags);
	+	mlock_future_check(flags);

or add a sanity check at the start to deny VM_LOCKED and perhaps
something else...

The same for vm_brk_flags() which after your change does

	do_brk_flags(flags);
	populate = (mm->def_flags & VM_LOCKED);

again, this is just a nit, I do not think it will be ever called
with VM_LOCKED in "flags".

Oleg.