Re: [PATCH] fuzz: fix sparse memory access in the DMA callback

[Darren Kenny]

On Tuesday, 2021-07-06 at 12:17:35 -04, Alexander Bulekov wrote:
> The code mistakenly relied on address_space_translate to store the
> length remaining until the next memory-region. We care about this
> because when there is RAM or sparse-memory neighboring on an MMIO
> region, we should only write up to the border, to prevent inadvertently
> invoking MMIO handlers within the DMA callback.
>
> However address_space_translate_internal only stores the length until
> the end of the MemoryRegion if memory_region_is_ram(mr). Otherwise
> the *len is left unmodified. This caused some false-positive issues,
> where the fuzzer found a way to perform a nested MMIO write through a
> DMA callback on an [address, length] that started within sparse memory
> and spanned some device MMIO regions.
>
> To fix this, write to sparse memory in small chunks of
> memory_access_size (similar to the underlying address_space_write code),
> which will prevent accidentally hitting MMIO handlers through large
> writes.
>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

> ---
>  tests/qtest/fuzz/generic_fuzz.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c
> index 59be7219de..3e8ce29227 100644
> --- a/tests/qtest/fuzz/generic_fuzz.c
> +++ b/tests/qtest/fuzz/generic_fuzz.c
> @@ -240,10 +240,17 @@ void fuzz_dma_read_cb(size_t addr, size_t len, 
> MemoryRegion *mr)
>                                        addr, &addr1, &l, true,
>                                        MEMTXATTRS_UNSPECIFIED);
>  
> -        if (!(memory_region_is_ram(mr1) ||
> -              memory_region_is_romd(mr1)) && mr1 != sparse_mem_mr) {
> +        /*
> +         *  If mr1 isn't RAM, address_space_translate doesn't update l. Use
> +         *  memory_access_size to identify the number of bytes that it is 
> safe
> +         *  to write without accidentally writing to another MemoryRegion.
> +         */
> +        if (!memory_region_is_ram(mr1)) {
>              l = memory_access_size(mr1, l, addr1);
> -        } else {
> +        }
> +        if (memory_region_is_ram(mr1) ||
> +            memory_region_is_romd(mr1) ||
> +            mr1 == sparse_mem_mr) {
>              /* ROM/RAM case */
>              if (qtest_log_enabled) {
>                  /*
> -- 
> 2.28.0