Re: [PATCH] xen-mapcache: avoid a race on memory map while using MAP_FIXED

[Paul Durrant <xadimgnik@gmail.com>]

On 20/04/2021 04:35, Igor Druzhinin wrote:
> When we're replacing the existing mapping there is possibility of a race
> on memory map with other threads doing mmap operations - the address being
> unmapped/re-mapped could be occupied by another thread in between.
> 
> Linux mmap man page recommends keeping the existing mappings in place to
> reserve the place and instead utilize the fact that the next mmap operation
> with MAP_FIXED flag passed will implicitly destroy the existing mappings
> behind the chosen address. This behavior is guaranteed by POSIX / BSD and
> therefore is portable.
> 
> Note that it wouldn't make the replacement atomic for parallel accesses to
> the replaced region - those might still fail with SIGBUS due to
> xenforeignmemory_map not being atomic. So we're still not expecting those.
> 
> Tested-by: Anthony PERARD <anthony.perard@citrix.com>
> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>

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

> ---
>   hw/i386/xen/xen-mapcache.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index 5b120ed..e82b7dc 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -171,7 +171,20 @@ static void xen_remap_bucket(MapCacheEntry *entry,
>           if (!(entry->flags & XEN_MAPCACHE_ENTRY_DUMMY)) {
>               ram_block_notify_remove(entry->vaddr_base, entry->size);
>           }
> -        if (munmap(entry->vaddr_base, entry->size) != 0) {
> +
> +        /*
> +         * If an entry is being replaced by another mapping and we're using
> +         * MAP_FIXED flag for it - there is possibility of a race for vaddr
> +         * address with another thread doing an mmap call itself
> +         * (see man 2 mmap). To avoid that we skip explicit unmapping here
> +         * and allow the kernel to destroy the previous mappings by replacing
> +         * them in mmap call later.
> +         *
> +         * Non-identical replacements are not allowed therefore.
> +         */
> +        assert(!vaddr || (entry->vaddr_base == vaddr && entry->size == size));
> +
> +        if (!vaddr && munmap(entry->vaddr_base, entry->size) != 0) {
>               perror("unmap fails");
>               exit(-1);
>           }
>