[release-branch.go1.8] runtime: don't corrupt arena bounds on low mmap

Cherry-pick of CL 43870.

If mheap.sysAlloc doesn't have room in the heap arena for an
allocation, it will attempt to map more address space with sysReserve.
sysReserve is given a hint, but can return any unused address range.
Currently, mheap.sysAlloc incorrectly assumes the returned region will
never fall between arena_start and arena_used. If it does,
mheap.sysAlloc will blindly accept the new region as the new
arena_used and arena_end, causing these to decrease and make it so any
Go heap above the new arena_used is no longer considered part of the
Go heap. This assumption *used to be* safe because we had all memory
between arena_start and arena_used mapped, but when we switched to an
arena_start of 0 on 32-bit, it became no longer safe.

Most likely, we've only recently seen this bug occur because we
usually start arena_used just above the binary, which is low in the
address space. Hence, the kernel is very unlikely to give us a region
before arena_used.

Since mheap.sysAlloc is a linear allocator, there's not much we can do
to handle this well. Hence, we fix this problem by simply rejecting
the new region if it isn't after arena_end. In this case, we'll take
the fall-back path and mmap a small region at any address just for the
requested memory.

Fixes #20259.

Change-Id: Ib72e8cd621545002d595c7cade1e817cfe3e5b1e
Reviewed-on: https://go-review.googlesource.com/43954
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index da39dac5102923da939df0b637238aa42c7793d0..6f07731a493b659017726e59b63b02eb3c15109b 100644 (file)
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -400,10 +400,12 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
                        if p == 0 {
                                return nil
                        }
+                       // p can be just about anywhere in the address
+                       // space, including before arena_end.
                        if p == h.arena_end {
                                h.arena_end = new_end
                                h.arena_reserved = reserved
-                       } else if h.arena_start <= p && p+p_size-h.arena_start-1 <= _MaxArena32 {
+                       } else if h.arena_end < p && p+p_size-h.arena_start-1 <= _MaxArena32 {
                                // Keep everything page-aligned.
                                // Our pages are bigger than hardware pages.
                                h.arena_end = p + p_size
@@ -413,6 +415,16 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
                                h.arena_used = used
                                h.arena_reserved = reserved
                        } else {
+                               // We got a mapping, but it's not
+                               // linear with our current arena, so
+                               // we can't use it.
+                               //
+                               // TODO: Make it possible to allocate
+                               // from this. We can't decrease
+                               // arena_used, but we could introduce
+                               // a new variable for the current
+                               // allocation position.
+
                                // We haven't added this allocation to
                                // the stats, so subtract it from a
                                // fake stat (but avoid underflow).