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Re: [Xen-devel] [PATCH] console: avoid buffer overflow in guest_console_write()
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 29 Nov 2019 10:22:52 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ilja Van Sprundel <ivansprundel@xxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>
- Delivery-date: Fri, 29 Nov 2019 10:22:59 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 29/11/2019 10:13, Jan Beulich wrote:
> The switch of guest_console_write()'s second parameter from plain to
> unsigned int has caused the function's main loop header to no longer
> guard the min_t() use within the function against effectively negative
> values, due to the casts hidden inside the macro. Replace by a plain
> min(), converting one of the arguments suitably without involving any
> cast.
>
> Fixes: ea601ec9995b ("xen/console: Rework HYPERCALL_console_io interface")
> Reported-by: Ilja Van Sprundel <ivansprundel@xxxxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/drivers/char/console.c
> +++ b/xen/drivers/char/console.c
> @@ -538,7 +538,7 @@ static long guest_console_write(XEN_GUES
> __HYPERVISOR_console_io, "iih",
> CONSOLEIO_write, count, buffer);
>
> - kcount = min_t(int, count, sizeof(kbuf)-1);
> + kcount = min(count + sizeof(char[0]), sizeof(kbuf) - 1);
Is sizeof(array[0]) always 0, or is this just a GCC-ism ? Godbolt
suggests is 0 on all compiler we support.
Either way, isn't the more common idiom + 0ul ? Personally, I feel that
is clearer to follow.
That said, given the severity and urgency of this
extremely-lucky-its-not-an-XSA, Reviewed-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx>, but ideally using the +0ul form.
~Andrew
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