linux-kernel - Re: vmalloced stacks on x86

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Message-ID: <CA+55aFyLcyvTqMvGKOQEW_v3UuMLsdcTpO_2B1sCW8aUTz67zw@mail.gmail.com>
Date:	Sun, 26 Oct 2014 11:16:10 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Richard Weinberger <richard.weinberger@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: vmalloced stacks on x86_64?

On Sat, Oct 25, 2014 at 4:16 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> On brief inspection, grsecurity isn't actually vmallocing the stack.
> It seems to be allocating it the normal way and then vmapping it.
> That allows it to modify sg_set_buf to work on stack addresses (sigh).

Perhaps more importantly, the vmalloc space is a limited resource (at
least on 32-bit), and using vmap probably results in less
fragmentation.

I don't think either is really even an option on 32-bit due to the
limited address space. On 64-bit, maybe a virtually remapped stack
would be ok.

                   Linus
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