linux-hardening - Re: [PATCH v3] usercopy: Check valid lifetime via stack depth

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Date:   Fri, 25 Feb 2022 16:01:57 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux-mm@...ck.org,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        David Laight <David.Laight@...LAB.COM>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] usercopy: Check valid lifetime via stack depth

On Fri, 25 Feb 2022 09:33:45 -0800 Kees Cook <keescook@...omium.org> wrote:

> Under CONFIG_HARDENED_USERCOPY=y, when exact stack frame boundary checking
> is not available (i.e. everything except x86 with FRAME_POINTER), check
> a stack object as being at least "current depth valid", in the sense
> that any object within the stack region but not between start-of-stack
> and current_stack_pointer should be considered unavailable (i.e. its
> lifetime is from a call no longer present on the stack).
> 
> Introduce ARCH_HAS_CURRENT_STACK_POINTER to track which architectures
> have actually implemented the common global register alias.
> 
> Additionally report usercopy bounds checking failures with an offset
> from current_stack_pointer, which may assist with diagnosing failures.
> 
> The LKDTM USERCOPY_STACK_FRAME_TO and USERCOPY_STACK_FRAME_FROM tests
> (once slightly adjusted in a separate patch) will pass again with
> this fixed.

Again, what does this actually do?

> Reported-by: Muhammad Usama Anjum <usama.anjum@...labora.com>

A link to that report would shed some light.  But actually describing
the user-visible impact right there in the changelog is preferable.

It sounds like a selftest is newly failing, which makes it a
userspace-visible regression, perhaps?

If so, do we have a Fixes: and is a cc:stable warranted?