runtime: handle sysReserve returning a pointer below the arena

In mheap.sysAlloc, if an allocation at arena_used would exceed
arena_end (but wouldn't yet push us past arena_start+_MaxArean32), it
trie to extend the arena reservation by another 256 MB. It extends the
arena by calling sysReserve, which, on 32-bit, calls mmap without
MAP_FIXED, which means the address is just a hint and the kernel can
put the mapping wherever it wants. In particular, mmap may choose an
address below arena_start (the kernel also chose arena_start, so there
could be lots of space below it). Currently, we don't detect this case
and, if it happens, mheap.sysAlloc will corrupt arena_end and
arena_used then return the low pointer to mheap.grow, which will crash
when it attempts to index in to h_spans with an underflowed index.

Fix this by checking not only that that p+p_size isn't too high, but
that p isn't too low.

Fixes #13143.

Change-Id: I8d0f42bd1484460282a83c6f1a6f8f0df7fb2048
Reviewed-on: https://go-review.googlesource.com/16927
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: aclements

src/runtime/malloc.go

@@ -404,7 +404,7 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
 			if p == h.arena_end {
 				h.arena_end = new_end
 				h.arena_reserved = reserved
-			} else if p+p_size <= h.arena_start+_MaxArena32 {
+			} else if h.arena_start <= p && p+p_size <= h.arena_start+_MaxArena32 {
 				// Keep everything page-aligned.
 				// Our pages are bigger than hardware pages.
 				h.arena_end = p + p_size