linux-kernel - VLAs and security

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Message-ID: <1535875700.17858.3.camel@med.uni-goettingen.de>
Date:   Sun, 2 Sep 2018 08:08:20 +0000
From:   "Uecker, Martin" <Martin.Uecker@....uni-goettingen.de>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: VLAs and security

I do not agree that VLAs are generally bad for security.
I think the opposite is true. A VLA with the right size
allows the compiler to automatically perform or insert
meaningful bounds checks, while a fixed upper bound does not.

For example:

char buf[N];
buf[n] = 1;

Here, a compiler / analysis tool can for  n < N  using
static analysis or insert a run-time check.

Replacing this with

char buf[MAX_SIZE]

hides the information about the true upper bound
from automatic tools.

Limiting the stack usage can also be achieved in
the following way:

assert(N <= MAX_SIZE)
char buf[N];

Of course, having predictable stack usage might be more 
important in the kernel and might be a good argument
to still prefer the constant bound.

But loosing the tighter bounds is clearly a disadvantage
with respect to security that one should keep it mind.

Best,
Martin