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block-backend: Fix potential double blk_delete()

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blk_unref() first decreases the refcount of the BlockBackend and calls
blk_delete() if the refcount reaches zero. Requests can still be in
flight at this point, they are only drained during blk_delete():

At this point, arbitrary callbacks can run. If any callback takes a
temporary BlockBackend reference, it will first increase the refcount to
1 and then decrease it to 0 again, triggering another blk_delete(). This
will cause a use-after-free crash in the outer blk_delete().

Fix it by draining the BlockBackend before decreasing to refcount to 0.
Assert in blk_ref() that it never takes the first refcount (which would
mean that the BlockBackend is already being deleted).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Kevin Wolf 2018-09-07 13:45:54 +02:00
parent fe5258a503
commit 5ca9d21bd1
1 changed files with 8 additions and 1 deletions
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9
block/block-backend.c
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@ -435,6 +435,7 @@ int blk_get_refcnt(BlockBackend *blk)
*/
void blk_ref(BlockBackend *blk)
{
assert(blk->refcnt > 0);
blk->refcnt++;
}
@ -447,7 +448,13 @@ void blk_unref(BlockBackend *blk)
{
if (blk) {
assert(blk->refcnt > 0);
if (!--blk->refcnt) {
if (blk->refcnt > 1) {
blk->refcnt--;
} else {
blk_drain(blk);
/* blk_drain() cannot resurrect blk, nobody held a reference */
assert(blk->refcnt == 1);
blk->refcnt = 0;
blk_delete(blk);
}
}