linux-kernel - Re: [PATCH v2] rslib: Remove VLAs by setting upper bound on nroots

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Message-ID: <CAGXu5jLAqki8oE7AF9BEwLxKzpTOOPsT1imjdSAG_U44uGXJdQ@mail.gmail.com>
Date:   Fri, 16 Mar 2018 23:25:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2] rslib: Remove VLAs by setting upper bound on nroots

On Fri, Mar 16, 2018 at 3:59 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Thu, 15 Mar 2018 15:59:19 -0700 Kees Cook <keescook@...omium.org> wrote:
>
>> Avoid stack VLAs[1] by always allocating the upper bound of stack space
>> needed. The existing users of rslib appear to max out at 24 roots[2],
>> so use that as the upper bound until we have a reason to change it.
>>
>> Alternative considered: make init_rs() a true caller-instance and
>> pre-allocate the workspaces. This would possibly need locking and
>> a refactoring of the returned structure.
>>
>> Using kmalloc in this path doesn't look great, especially since at
>> least one caller (pstore) is sensitive to allocations during rslib
>> usage (it expects to run it during an Oops, for example).
>
> Oh.
>
> Could we allocate the storage during init_rs(), attach it to `struct
> rs_control'?

No, because they're modified during decode, and struct rs_control is
shared between users. :(

Doing those changes is possible, but it requires a rather extensive
analysis of callers, etc.

Hence, the 24 ultimately.

-Kees

-- 
Kees Cook
Pixel Security