From 8331f25e96d6120bb0ec212bd03abcae53282769 Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Wed, 15 Sep 2021 09:56:09 -0700 Subject: [PATCH] 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 Trust: Ian Lance Taylor Run-TryBot: Keith Randall TryBot-Result: Go Bot Reviewed-by: Ian Lance Taylor --- src/reflect/all_test.go | 20 ++++++++++++++++++++ src/reflect/value.go | 17 +++++++++++++++++ src/runtime/mbitmap.go | 9 +++++++++ 3 files changed, 46 insertions(+) diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go index 427855b02e..8642d60f8b 100644 --- a/src/reflect/all_test.go +++ b/src/reflect/all_test.go @@ -7697,3 +7697,23 @@ func TestSetIter(t *testing.T) { t.Errorf("pointer incorrect: got %d want %d", got, b) } } + +//go:notinheap +type nih struct{ x int } + +var global_nih = nih{x: 7} + +func TestNotInHeapDeref(t *testing.T) { + // See issue 48399. + v := ValueOf((*nih)(nil)) + v.Elem() + shouldPanic("reflect: call of reflect.Value.Field on zero Value", func() { v.Elem().Field(0) }) + + v = ValueOf(&global_nih) + if got := v.Elem().Field(0).Int(); got != 7 { + t.Fatalf("got %d, want 7", got) + } + + v = ValueOf((*nih)(unsafe.Pointer(new(int)))) + shouldPanic("reflect: reflect.Value.Elem on an invalid notinheap pointer", func() { v.Elem() }) +} 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. diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go index daf1fcfbc0..3330ddd62e 100644 --- a/src/runtime/mbitmap.go +++ b/src/runtime/mbitmap.go @@ -417,6 +417,15 @@ func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex ui return } +// verifyNotInHeapPtr reports whether converting the not-in-heap pointer into a unsafe.Pointer is ok. +//go:linkname reflect_verifyNotInHeapPtr reflect.verifyNotInHeapPtr +func reflect_verifyNotInHeapPtr(p uintptr) bool { + // Conversion to a pointer is ok as long as findObject above does not call badPointer. + // Since we're already promised that p doesn't point into the heap, just disallow heap + // pointers and the special clobbered pointer. + return spanOf(p) == nil && p != clobberdeadPtr +} + // next returns the heapBits describing the next pointer-sized word in memory. // That is, if h describes address p, h.next() describes p+ptrSize. // Note that next does not modify h. The caller must record the result. -- 2.50.0