Re: [PATCH] softmmu: Always initialize xlat in address_space_translate_for_iotlb

[Peter Maydell]

On Wed, 15 Jun 2022 at 17:43, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> The bug is an uninitialized memory read, along the translate_fail
> path, which results in garbage being read from iotlb_to_section,
> which can lead to a crash in io_readx/io_writex.
>
> The bug may be fixed by writing any value with zero
> in ~TARGET_PAGE_MASK, so that the call to iotlb_to_section using
> the xlat'ed address returns io_mem_unassigned, as desired by the
> translate_fail path.
>
> It is most useful to record the original physical page address,
> which will eventually be logged by memory_region_access_valid
> when the access is rejected by unassigned_mem_accepts.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1065
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  softmmu/physmem.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> index 657841eed0..fb0f0709b5 100644
> --- a/softmmu/physmem.c
> +++ b/softmmu/physmem.c
> @@ -681,6 +681,9 @@ address_space_translate_for_iotlb(CPUState *cpu, int 
> asidx, hwaddr addr,
>      AddressSpaceDispatch *d =
>          qatomic_rcu_read(&cpu->cpu_ases[asidx].memory_dispatch);
>
> +    /* Record the original phys page for use by the translate_fail path. */
> +    *xlat = addr;

There's no doc comment for address_space_translate_for_iotlb(),
so there's nothing that says explicitly that addr is obliged
to be page aligned, although it happens that its only caller
does pass a page-aligned address. Were we already implicitly
requiring a page-aligned address here, or does not masking
addr before assigning to *xlat impose a new requirement ?

thanks
-- PMM