[release-branch.go1.16] misc/wasm, cmd/link: do not let command line args overwrite global data

On Wasm, wasm_exec.js puts command line arguments at the beginning
of the linear memory (following the "zero page"). Currently there
is no limit for this, and a very long command line can overwrite
the program's data section. Prevent this by limiting the command
line to 4096 bytes, and in the linker ensuring the data section
starts at a high enough address (8192).

(Arguably our address assignment on Wasm is a bit confusing. This
is the minimum fix I can come up with.)

Thanks to Ben Lubar for reporting this issue.

Change by Cherry Mui <cherryyz@google.com>.

For #48797
Fixes #48799
Fixes CVE-2021-38297

Change-Id: I0f50fbb2a5b6d0d047e3c134a88988d9133e4ab3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1205933
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/354591
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

Author: mknyszek

misc/wasm/wasm_exec.js

@@ -564,6 +564,13 @@
 				offset += 8;
 			});
 
+			// The linker guarantees global data starts from at least wasmMinDataAddr.
+			// Keep in sync with cmd/link/internal/ld/data.go:wasmMinDataAddr.
+			const wasmMinDataAddr = 4096 + 4096;
+			if (offset >= wasmMinDataAddr) {
+				throw new Error("command line too long");
+			}
+
 			this._inst.exports.run(argc, argv);
 			if (this.exited) {
 				this._resolveExitPromise();

src/cmd/link/internal/ld/data.go

@@ -2330,6 +2330,11 @@ func assignAddress(ctxt *Link, sect *sym.Section, n int, s loader.Sym, va uint64
 	return sect, n, va
 }
 
+// On Wasm, we reserve 4096 bytes for zero page, then 4096 bytes for wasm_exec.js
+// to store command line args. Data sections starts from at least address 8192.
+// Keep in sync with wasm_exec.js.
+const wasmMinDataAddr = 4096 + 4096
+
 // address assigns virtual addresses to all segments and sections and
 // returns all segments in file order.
 func (ctxt *Link) address() []*sym.Segment {
@@ -2339,10 +2344,14 @@ func (ctxt *Link) address() []*sym.Segment {
 	order = append(order, &Segtext)
 	Segtext.Rwx = 05
 	Segtext.Vaddr = va
-	for _, s := range Segtext.Sections {
+	for i, s := range Segtext.Sections {
 		va = uint64(Rnd(int64(va), int64(s.Align)))
 		s.Vaddr = va
 		va += s.Length
+
+		if ctxt.IsWasm() && i == 0 && va < wasmMinDataAddr {
+			va = wasmMinDataAddr
+		}
 	}
 
 	Segtext.Length = va - uint64(*FlagTextAddr)