linux-hardening - Re: [PATCH][next] xen: privcmd: Replace zero-length array with flex-array member and use __counted

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Date: Thu, 16 Nov 2023 15:18:00 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Juergen Gross <jgross@...e.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] xen: privcmd: Replace zero-length array with
 flex-array member and use __counted_by



On 11/16/23 15:08, Kees Cook wrote:
> 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@...nel.org>
> 
> Looks right to me. I can see the allocation:

Yep, I always check for that; in particular, the 'counter' assignment. :)

Do you want me to mention it in the changelog text?

> 
>          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@...omium.org>

Thanks!
--
Gustavo