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Date:   Thu, 16 Nov 2023 13:08:35 -0800
From:   Kees Cook <keescook@chromium.org>
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc:     Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH][next] xen: privcmd: Replace zero-length array with
 flex-array member and use __counted_by
Message-ID: <202311161307.9422E1896B@keescook>
References: <ZVZlg3tPMPCRdteh@work>
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On Thu, Nov 16, 2023 at 12:54:59PM -0600, Gustavo A. R. Silva wrote:
> Fake flexible arrays (zero-length and one-element arrays) are deprecated,
> and should be replaced by flexible-array members. So, replace
> zero-length array with a flexible-array member in `struct
> privcmd_kernel_ioreq`.
> 
> Also annotate array `ports` with `__counted_by()` to prepare for the
> coming implementation by GCC and Clang of the `__counted_by` attribute.
> Flexible array members annotated with `__counted_by` can have their
> accesses bounds-checked at run-time via `CONFIG_UBSAN_BOUNDS` (for array
> indexing) and `CONFIG_FORTIFY_SOURCE` (for strcpy/memcpy-family functions).
> 
> This fixes multiple -Warray-bounds warnings:
> drivers/xen/privcmd.c:1239:30: warning: array subscript i is outside array bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> drivers/xen/privcmd.c:1240:30: warning: array subscript i is outside array bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> drivers/xen/privcmd.c:1241:30: warning: array subscript i is outside array bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> drivers/xen/privcmd.c:1245:33: warning: array subscript i is outside array bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> drivers/xen/privcmd.c:1258:67: warning: array subscript i is outside array bounds of 'struct ioreq_port[0]' [-Warray-bounds=]
> 
> This results in no differences in binary output.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Looks right to me. I can see the allocation:

        size = struct_size(kioreq, ports, ioeventfd->vcpus);
        kioreq = kzalloc(size, GFP_KERNEL);
        if (!kioreq)
                return ERR_PTR(-ENOMEM);

        kioreq->dom = ioeventfd->dom;
        kioreq->vcpus = ioeventfd->vcpus;


Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook