scudo: Fix quarantine allocation when MTE enabled.

Quarantines have always been broken when MTE is enabled because the
quarantine batch allocator fails to reset tags that may have been
left behind by a user allocation.

This was only noticed when running the Scudo unit tests with Scudo
as the system allocator because quarantines are turned off by
default on Android and the test binary turns them on by defining
__scudo_default_options, which affects the system allocator as well.

Differential Revision: https://reviews.llvm.org/D92881

Author: pcc

compiler-rt/lib/scudo/standalone/combined.h

@@ -98,6 +98,12 @@ class Allocator {
       Header.State = Chunk::State::Allocated;
       Chunk::storeHeader(Allocator.Cookie, Ptr, &Header);
 
+      // Reset tag to 0 as this chunk may have been previously used for a tagged
+      // user allocation.
+      if (UNLIKELY(Allocator.useMemoryTagging()))
+        storeTags(reinterpret_cast<uptr>(Ptr),
+                  reinterpret_cast<uptr>(Ptr) + sizeof(QuarantineBatch));
+
       return Ptr;
     }