linux-kernel - Re: [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds

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Message-ID: <202108301259.1FC43498@keescook>
Date:   Mon, 30 Aug 2021 13:12:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Keith Packard <keithp@...thp.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        clang-built-linux@...glegroups.com,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds

On Mon, Aug 30, 2021 at 11:44:54AM -0700, Nathan Chancellor wrote:
> On Fri, Aug 27, 2021 at 09:30:10AM -0700, Kees Cook wrote:
> > v3:
> > - fix typo in treewide conversion (u8 should have been __u8)
> > - improve changelog for DECLARE_FLEX_ARRAY patch
> > - add acks/reviews
> > v2: https://lore.kernel.org/lkml/20210826050458.1540622-1-keescook@chromium.org/
> > v1: https://lore.kernel.org/lkml/20210818081118.1667663-1-keescook@chromium.org/
> > 
> > Hi,
> > 
> > In support of the improved buffer overflow detection for memcpy(),
> > this enables -Warray-bounds and -Wzero-length-bounds globally. Mostly
> > it involves some struct member tricks with the new DECLARE_FLEX_ARRAY()
> > macro. Everything else is just replacing stacked 0-element arrays
> > with actual unions in two related treewide patches. There is one set of
> > special cases that were fixed separately[1] and are needed as well.
> > 
> > I'm expecting to carry this series with the memcpy() series in my
> > "overflow" tree. Reviews appreciated! :)
> 
> Hi Kees,
> 
> I ran this series through my local build tests and uncovered two
> warnings in the same file that appear to be unhandled as of
> next-20210830. This is from ARCH=powerpc pseries_defconfig with
> clang-14, I did not try earlier versions of clang.

Thanks for double-checking!

> 
> arch/powerpc/kernel/signal_32.c:780:2: error: array index 3 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
>         unsafe_put_sigset_t(&frame->uc.uc_sigmask, oldset, failed);
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> [...]
> arch/powerpc/kernel/signal_32.c:1044:3: error: array index 2 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
>                 unsafe_put_sigset_t(&old_ctx->uc_sigmask, &current->blocked, failed);
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This smells like some kind of casting issue. uc_sigmask has only a single
unsigned long element but unsafe_put_compat_sigset() seems to be doing
stuff with [3], etc. Is it expecting u8? I will keep looking...

-- 
Kees Cook