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bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.


From: Demetrios Obenour
Subject: bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.
Date: Sun, 7 Sep 2014 23:17:30 -0400

This is crucial. Otherwise, a security vulnerability could result.

MAX_ALLOCA should not be larger than the page size for the target architecture.

Demetrios Obenour

-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Paul Eggert
Sent: Sunday, September 7, 2014 10:38 PM
To: Stefan Monnier
Cc: address@hidden
Subject: bug#18410: Use SAFE_ALLOCA etc. to avoid unbounded stack allocation.

Stefan Monnier wrote:
> MAX_ALLOCA is chosen small so that we can allocate several/many 
> objects of size MAX_ALLOCA.

That's one reason, but another is that stack-overflow checking often relies on 
guard pages.  If we blindly increase MAX_ALLOCA (or some variant of it, just 
for call-process) Emacs could bypass stack-overflow checking, resulting in 
behavior that could be worse than simply dumping core.

If I understand things correctly, Dmitry's recent stack-overflow changes don't 
affect this, as they don't deal with the guard-page region size.









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