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Message-ID: <CALCETrWp+7+9j1vMgv5hxvtq3akZZZVdaVDfOLoFwhX+kL=Ntw@mail.gmail.com>
Date: Tue, 21 Jun 2016 09:59:49 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Jann Horn <jannh@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>, Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>,
Brian Gerst <brgerst@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Linus Torvalds <torvalds@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Jann Horn <jann@...jh.net>, Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 06/13] fork: Add generic vmalloced stack support
On Tue, Jun 21, 2016 at 12:30 AM, Jann Horn <jannh@...gle.com> wrote:
> On Tue, Jun 21, 2016 at 1:43 AM, Andy Lutomirski <luto@...nel.org> wrote:
>> If CONFIG_VMAP_STACK is selected, kernel stacks are allocated with
>> vmalloc_node.
> [...]
>> static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>> int node)
>> {
>> +#ifdef CONFIG_VMAP_STACK
>> + struct thread_info *ti = __vmalloc_node_range(
>> + THREAD_SIZE, THREAD_SIZE, VMALLOC_START, VMALLOC_END,
>> + THREADINFO_GFP | __GFP_HIGHMEM, PAGE_KERNEL,
>> + 0, node, __builtin_return_address(0));
>> +
>
> After spender gave some hints on IRC about the guard pages not working
> reliably, I decided to have a closer look at this. As far as I can
> tell, the idea is that __vmalloc_node_range() automatically adds guard
> pages unless the VM_NO_GUARD flag is specified. However, those guard
> pages are *behind* allocations, not in front of them, while a stack
> guard primarily needs to be in front of the allocation. This wouldn't
> matter if all allocations in the vmalloc area had guard pages behind
> them, but if someone first does some data allocation with VM_NO_GUARD
> and then a stack allocation directly behind that, there won't be a
> guard between the data allocation and the stack allocation.
I'm tempted to explicitly disallow VM_NO_GUARD in the vmalloc range.
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