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From:   Jann Horn <jannh@google.com>
Date:   Tue, 18 Jan 2022 20:32:43 +0100
Message-ID: <CAG48ez00FFW-n_Pi=+ya1xY5QuB3q2mPr8++scVe3h3ROeF_mg@mail.gmail.com>
Subject: Re: [PATCH v2] tpm: vtpm_proxy: Double-check to avoid buffer overflow
To:     Kees Cook <keescook@chromium.org>
Cc:     Peter Huewe <peterhuewe@gmx.de>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        linux-integrity@vger.kernel.org,
        Stefan Berger <stefanb@linux.ibm.com>,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Tue, Jan 18, 2022 at 7:37 PM Kees Cook <keescook@chromium.org> wrote:
> When building with -Warray-bounds, this warning was emitted:
>
> In function 'memset',
>     inlined from 'vtpm_proxy_fops_read' at drivers/char/tpm/tpm_vtpm_proxy.c:102:2:
> ./include/linux/fortify-string.h:43:33: warning: '__builtin_memset' pointer overflow between offset 164 and size [2147483648, 4294967295]
> [-Warray-bounds]
>    43 | #define __underlying_memset     __builtin_memset
>       |                                 ^

Can you explain what that compiler warning actually means, and which
compiler it is from? Is this from a 32-bit or a 64-bit architecture?

It sounds like the compiler (GCC?) is hallucinating a codepath on
which "len" is guaranteed to be >=2147483648, right? Why is it doing
that? Is this some kinda side effect from the fortify code?