linux-kernel - Re: crypto: Kernel memory overwrite attempt detected to spans multiple pages

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Date:   Wed, 10 Apr 2019 21:37:41 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Eric Biggers <ebiggers@...nel.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>
Subject: Re: crypto: Kernel memory overwrite attempt detected to spans
 multiple pages

On Wed, 2019-04-10 at 16:11 -0700, Eric Biggers wrote:

> You've explained *what* it does again, but not *why*.  *Why* do you
> want
> hardened usercopy to detect copies across page boundaries, when there
> is no
> actual buffer overflow?

When some subsystem in the kernel allocates multiple
pages without _GFP_COMP, there is no way afterwards
to detect exactly how many pages it allocated.

In other words, there is no way to see how large the
buffer is, nor whether the copy operation in question
would overflow it.

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