Re: [PATCH v2] xen-mapcache: Avoid entry->lock overflow

["Durrant, Paul" <xadimgnik@gmail.com>]

On 24/01/2022 10:44, Ross Lagerwall wrote:
> In some cases, a particular mapcache entry may be mapped 256 times
> causing the lock field to wrap to 0. For example, this may happen when
> using emulated NVME and the guest submits a large scatter-gather write.
> At this point, the entry map be remapped causing QEMU to write the wrong
> data or crash (since remap is not atomic).
> 
> Avoid this overflow by increasing the lock field to a uint32_t and also
> detect it and abort rather than continuing regardless.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

Reviewed-by: Paul Durrant <paul@xen.org>

> ---
> Changes in v2: Change type to uint32_t since there is a hole there
> anyway. The struct size remains at 48 bytes on x86_64.
> 
>   hw/i386/xen/xen-mapcache.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index bd47c3d672..f2ef977963 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
>       hwaddr paddr_index;
>       uint8_t *vaddr_base;
>       unsigned long *valid_mapping;
> -    uint8_t lock;
> +    uint32_t lock;
>   #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
>       uint8_t flags;
>       hwaddr size;
> @@ -355,6 +355,12 @@ tryagain:
>       if (lock) {
>           MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
>           entry->lock++;
> +        if (entry->lock == 0) {
> +            fprintf(stderr,
> +                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
> +                    entry->paddr_index, entry->vaddr_base);
> +            abort();
> +        }
>           reventry->dma = dma;
>           reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
>           reventry->paddr_index = mapcache->last_entry->paddr_index;