Re: [Xen-devel] [PATCH 3/4] AMD/microcode: Avoid use-after-free for the microcode buffer

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/27/2013 09:57 AM, Andrew Cooper wrote:
> It is possible to free the mc_old buffer and the store it for use on in the
I think you don't need *the* store here. And no *on*.

> case of resume.
>
> This keeps the old semantics of being able to return an error even after a
> successful microcode application.
>
> Coverity-ID 1146953
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Keir Fraser <keir@xxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> ---
>   xen/arch/x86/microcode_amd.c |   17 +++++++++--------
>   1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/xen/arch/x86/microcode_amd.c b/xen/arch/x86/microcode_amd.c
> index a3ceef8..8ea4e63 100644
> --- a/xen/arch/x86/microcode_amd.c
> +++ b/xen/arch/x86/microcode_amd.c
> @@ -275,7 +275,7 @@ static int cpu_request_microcode(int cpu, const void *buf, 
> size_t bufsize)
>       struct microcode_amd *mc_amd, *mc_old;
>       size_t offset = bufsize;
>       size_t last_offset, applied_offset = 0;
> -    int error = 0;
> +    int error = 0, save_error = 1;
>       struct ucode_cpu_info *uci = &per_cpu(ucode_cpu_info, cpu);
>   
>       /* We should bind the task to the CPU */
> @@ -338,19 +338,20 @@ static int cpu_request_microcode(int cpu, const void 
> *buf, size_t bufsize)
>        */
>       if ( applied_offset )
>       {
> -        int ret = get_ucode_from_buffer_amd(mc_amd, buf, bufsize,
> -                                            &applied_offset);
> -        if ( ret == 0 )
> -            xfree(mc_old);
> -        else
> -            error = ret;
> +        save_error = get_ucode_from_buffer_amd(
> +            mc_amd, buf, bufsize, &applied_offset);
> +
> +        if ( save_error )
> +            error = save_error
>       }
>   
> -    if ( !applied_offset || error )
> +    if ( save_error )
>       {
>           xfree(mc_amd);
>           uci->mc.mc_amd = mc_old;
>       }
> +    else
> +        xfree(mc_old);
Won't this free mc_old (which is where previous microcode lived) even if 
the new buffer didn't have any valid microcode (i.e. applied_offset is 
zero)?
-boris


>   
>     out:
>       svm_host_osvw_init();

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