reflect: make Elem panic on bad notinheap pointers

This CL fixes the subtle issue that Elem can promote a
not-in-heap pointer, which could be any bit pattern, into an
unsafe.Pointer, which the garbage collector can see. If that
resulting value is bad, it can crash the GC.

Make sure that we don't introduce bad pointers that way. We can
make Elem() panic, because any such bad pointers are in the Go heap,
and not-in-heap pointers are not allowed to point into the Go heap.

Update #48399

Change-Id: Ieaf35a611b16b4dfb5e907e229ed4a2aed30e18c
Reviewed-on: https://go-review.googlesource.com/c/go/+/350153
Trust: Keith Randall <khr@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/reflect/value.go b/src/reflect/value.go
index abcc346de8..449f3bbb3c 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1169,6 +1169,21 @@ func (v Value) Elem() Value {
 	case Ptr:
 		ptr := v.ptr
 		if v.flag&flagIndir != 0 {
+			if ifaceIndir(v.typ) {
+				// This is a pointer to a not-in-heap object. ptr points to a uintptr
+				// in the heap. That uintptr is the address of a not-in-heap object.
+				// In general, pointers to not-in-heap objects can be total junk.
+				// But Elem() is asking to dereference it, so the user has asserted
+				// that at least it is a valid pointer (not just an integer stored in
+				// a pointer slot). So let's check, to make sure that it isn't a pointer
+				// that the runtime will crash on if it sees it during GC or write barriers.
+				// Since it is a not-in-heap pointer, all pointers to the heap are
+				// forbidden! That makes the test pretty easy.
+				// See issue 48399.
+				if !verifyNotInHeapPtr(*(*uintptr)(ptr)) {
+					panic("reflect: reflect.Value.Elem on an invalid notinheap pointer")
+				}
+			}
 			ptr = *(*unsafe.Pointer)(ptr)
 		}
 		// The returned value's address is v's value.
@@ -3406,6 +3421,8 @@ func typedslicecopy(elemType *rtype, dst, src unsafeheader.Slice) int
 //go:noescape
 func typehash(t *rtype, p unsafe.Pointer, h uintptr) uintptr
 
+func verifyNotInHeapPtr(p uintptr) bool
+
 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.