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

From: Igor Druzhinin
Subject: [PATCH] xen-mapcache: avoid a race on memory map while using MAP_FIXED
Date: Tue, 20 Apr 2021 04:35:02 +0100

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>
 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");

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