linux-hardening - Re: [PATCH v2] tpm: vtpm_proxy: Double-check to avoid buffer overflow

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Date:   Tue, 18 Jan 2022 15:39:31 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-integrity@...r.kernel.org,
        Stefan Berger <stefanb@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] tpm: vtpm_proxy: Double-check to avoid buffer overflow

On Tue, Jan 18, 2022 at 08:32:43PM +0100, Jann Horn wrote:
> On Tue, Jan 18, 2022 at 7:37 PM Kees Cook <keescook@...omium.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?

I agree, this looks bogus, or at least the commit message neeeds alot
more explaining.

static int vtpm_proxy_tpm_op_send(struct tpm_chip *chip, u8 *buf, size_t count)

        if (count > sizeof(proxy_dev->buffer))
            [...]
        proxy_dev->req_len = count;

Not clear how req_len can be larger than sizeof(buffer)?

Jason