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

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 25, 2022 at 05:46:57PM -0800, Andrew Morton wrote:
> On Fri, 25 Feb 2022 17:35:49 -0800 Kees Cook <keescook@chromium.org> wrote:
> 
> > On Fri, Feb 25, 2022 at 04:01:57PM -0800, Andrew Morton wrote:
> > > On Fri, 25 Feb 2022 09:33:45 -0800 Kees Cook <keescook@chromium.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?
> > 
> > [answers]
> >
> 
> OK, thanks.  I think a new changelog is warranted?

Yup, I've cut/pasted most of that into the new changelog:

    usercopy: Check valid lifetime via stack depth

    One of the things that CONFIG_HARDENED_USERCOPY sanity-checks is whether
    an object that is about to be copied to/from userspace is overlapping
    the stack at all. If it is, it performs a number of inexpensive
    bounds checks. One of the finer-grained checks is whether an object
    crosses stack frames within the stack region. Doing this on x86 with
    CONFIG_FRAME_POINTER was cheap/easy. Doing it with ORC was deemed too
    heavy, and was left out (a while ago), leaving the courser whole-stack
    check.

    The LKDTM tests USERCOPY_STACK_FRAME_TO and USERCOPY_STACK_FRAME_FROM
    try to exercise these cross-frame cases to validate the defense is
    working. They have been failing ever since ORC was added (which was
    expected). While Muhammad was investigating various LKDTM failures[1],
    he asked me for additional details on them, and I realized that when
    exact stack frame boundary checking is not available (i.e.  everything
    except x86 with FRAME_POINTER), it could check if a stack object is 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) pass again with this fixed.

    [1] https://github.com/kernelci/kernelci-project/issues/84

    Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: linux-mm@kvack.org
    Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
    Signed-off-by: Kees Cook <keescook@chromium.org>
    ---
    v1: https://lore.kernel.org/lkml/20220216201449.2087956-1-keescook@chromium.org
    v2: https://lore.kernel.org/lkml/20220224060342.1855457-1-keescook@chromium.org
    v3: https://lore.kernel.org/lkml/20220225173345.3358109-1-keescook@chromium.org
    v4: - improve commit log (akpm)


> What's your preferred path for upstreaming this change?

I figured I would take it via my for-next/hardening tree; I have 2
arch changes ready to go (Acked by maintainers) there too (to add
current_stack_pointer).

Thanks for the review!

-- 
Kees Cook