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

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Date:   Thu, 15 Mar 2018 22:27:11 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
cc:     linux-kernel@...r.kernel.org,
        Segher Boessenkool <segher@...nel.crashing.org>,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] rslib: Remove VLAs by setting upper bound on nroots

On Sat, 10 Mar 2018, Kees Cook 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).
> 
> [1] https://lkml.org/lkml/2018/3/7/621
> [2] https://lkml.org/lkml/2018/3/9/838
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>