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

[patchbot@xxxxxxx]

commit 73cf1ac4b6fc9f1678f27a51fd1d4ece0c56c77c
Author:     Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
AuthorDate: Tue Apr 20 04:35:02 2021 +0100
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Mon Jul 5 10:45:20 2021 +0100

    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 <anthony.perard@xxxxxxxxxx>
    Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    Reviewed-by: Paul Durrant <paul@xxxxxxx>
    Message-Id: <1618889702-13104-1-git-send-email-igor.druzhinin@xxxxxxxxxx>
    Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    (cherry picked from commit 3e81a71c9f3d23002b1e0dfff902c155d6c8d224)
---
 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 5b120ed44b..e82b7dcdd2 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);
         }
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#staging