From 3e81a71c9f3d23002b1e0dfff902c155d6c8d224 Mon Sep 17 00:00:00 2001 From: Igor Druzhinin Date: Tue, 20 Apr 2021 04:35:02 +0100 Subject: xen-mapcache: avoid a race on memory map while using MAP_FIXED 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 Signed-off-by: Igor Druzhinin Reviewed-by: Paul Durrant Message-Id: <1618889702-13104-1-git-send-email-igor.druzhinin@citrix.com> Signed-off-by: Anthony PERARD --- hw/i386/xen/xen-mapcache.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) (limited to 'hw/i386') 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); } -- cgit v1.1