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 <20210318171111.706303-6-samitolvanen@google.com> <CAKwvOd=fWs6g2Bf2a_bA58_-uoWtVmNQnvrPxNhio4R5qGjcMQ@mail.gmail.com>
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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Thu, 18 Mar 2021 14:38:12 -0700
Message-ID: <CABCJKufa_-WSSYzHBSjZ+3i0DfvoGBox7Xa0PcE_Kuhf2rd07g@mail.gmail.com>
Subject: Re: [PATCH v2 05/17] workqueue: use WARN_ON_FUNCTION_MISMATCH
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Will Deacon <will@kernel.org>, Jessica Yu <jeyu@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Tejun Heo <tj@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        bpf <bpf@vger.kernel.org>, linux-hardening@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        PCI <linux-pci@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Mar 18, 2021 at 11:50 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Mar 18, 2021 at 10:11 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > With CONFIG_CFI_CLANG, a callback function passed to
> > __queue_delayed_work from a module points to a jump table entry
> > defined in the module instead of the one used in the core kernel,
> > which breaks function address equality in this check:
> >
> >   WARN_ON_ONCE(timer->function != delayed_work_timer_fn);
> >
> > Use WARN_ON_FUNCTION_MISMATCH() instead to disable the warning
> > when CFI and modules are both enabled.
>
> Does __cficanonical help with such comparisons? Or would that be a
> very invasive change, if the concern was to try to keep these checks
> in place for CONFIG_CFI_CLANG?

The last time I checked, Clang ignored the __cficanonical attribute in
header files, which means it would still generate a local jump table
entry in each module for such functions, and the comparison here would
fail. We could avoid the issue by using __cficanonical for the
callback function *and* using __va_function() when we take the
function address in modules, but that feels way too invasive for this
particular use case.

Sami