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

[Richard Henderson <richard.henderson@linaro.org>]

On 6/20/22 05:52, Peter Maydell wrote:
> 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 ?

I have no idea.  The whole lookup process is both undocumented and twistedly complex.  I'm 
willing to add an extra masking operation here, if it seems necessary?


r~