linux-kernel - Re: [PATCH 0/3] x86/dumpstack: Inline copy_from_user

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Date:   Fri, 16 Sep 2022 20:20:00 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, dev@...-flo.net,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-perf-users@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-hardening@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 0/3] x86/dumpstack: Inline copy_from_user_nmi()

On Fri, Sep 16, 2022 at 8:01 AM Kees Cook <keescook@...omium.org> wrote:
>
> Hi,
>
> This fixes a find_vmap_area() deadlock. The main fix is patch 2, repeated here:
>
>     The check_object_size() helper under CONFIG_HARDENED_USERCOPY is
>     designed to skip any checks where the length is known at compile time as
>     a reasonable heuristic to avoid "likely known-good" cases. However, it can
>     only do this when the copy_*_user() helpers are, themselves, inline too.
>
>     Using find_vmap_area() requires taking a spinlock. The check_object_size()
>     helper can call find_vmap_area() when the destination is in vmap memory.
>     If show_regs() is called in interrupt context, it will attempt a call to
>     copy_from_user_nmi(), which may call check_object_size() and then
>     find_vmap_area(). If something in normal context happens to be in the
>     middle of calling find_vmap_area() (with the spinlock held), the interrupt
>     handler will hang forever.
>
>     The copy_from_user_nmi() call is actually being called with a fixed-size
>     length, so check_object_size() should never have been called in the
>     first place. In order for check_object_size() to see that the length is
>     a fixed size, inline copy_from_user_nmi(), as already done with all the
>     other uaccess helpers.
>
>     Reported-by: Yu Zhao <yuzhao@...gle.com>
>     Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com
>     Reported-by: dev@...-flo.net
>
> Patch 1 is a refactor for patch 2, and patch 3 should make sure we avoid
> future deadlocks.

Thanks!

Tested-by: Yu Zhao <yuzhao@...gle.com>

The same test has been holding up well for a few hours now. It used to
throw that warning in less than half an hour.