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

[Andrew Morton]

On Fri, 25 Feb 2022 09:33:45 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxx>

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?