Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

[Russell King - ARM Linux]

On Fri, May 12, 2017 at 10:30:44PM +0200, Peter Zijlstra wrote:
> On Fri, May 12, 2017 at 09:21:06PM +0100, Russell King - ARM Linux wrote:
> > On Fri, May 12, 2017 at 12:30:02PM -0700, Kees Cook wrote:
> > > I'm clearly not explaining things well enough. I shouldn't say
> > > "corruption", I should say "malicious manipulation". The methodology
> > > of attacks against the stack are quite different from the other kinds
> > > of attacks like use-after-free, heap overflow, etc. Being able to
> > > exhaust the kernel stack (either due to deep recursion or unbounded
> > > alloca())
> > 
> > I really hope we don't have alloca() use in the kernel.  Do you have
> > evidence to support that assertion?
> > 
> > IMHO alloca() (or similar) should not be present in any kernel code
> > because we have a limited stack - we have kmalloc() etc for that kind
> > of thing.
> 
> On stack variable length arrays get implemented by the compiler doing
> alloca(), and we sadly have a few of those around.

I hope their size is appropriately limited, but something tells me it
would be foolish to assume that.

> But yes, fully agreed on the desirability of alloca() and things.

Hmm, I wonder if -fno-builtin-alloca would prevent those... it looks
like it certainly would prevent an explicit alloca() call.

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