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Re: why Glibc does use alloca?

On 05/16/14 10:09, OndÅej BÃlka wrote:
On Fri, May 16, 2014 at 09:37:31AM -0600, Jeff Law wrote:
On 05/16/14 07:53, OndÅej BÃlka wrote:

All you need is reliable way to get stack boundaries. I proposed to add
these some time ago. It would make alloca failures reliable without
need of gcc support.

A better alternative than ten line boilerplate alloca with fallback to malloc
would be using malloca, that handles these automatically. With stack
bounds it could allocate fairly safely, without these it must resort to
heuristic like size is less than 65536.
Better yet, stop using alloca.

Seriously.  I don't have hard data, but there's little doubt in my
mind that if we were to look at CVEs for glibc, the majority are due
to unbound allocas.

Experience clearly shows that we consistently get it wrong with
alloca.  IMHO, it should simply be banned from glibc (and
everywhere, but let's start small).

Yes, I know it's orders of magnitude faster than malloc.  Yes I know
we get automagic cleanup.  But if using it consistently results in
incorrect code that attackers can exploit, then the performance
aspects really should take a back seat.

Problem is that security advice is useless when nobody follows it.
One could enforce such policy by poisoning alloca.

There is additional dull knife risk. If alloca is not available then
users that want performance will replace it with 64k arrays and malloc
fallback which makes overflow more likely.
I'd hope patch review would catch this kind of sillyness.


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