From: Jason A. Donenfeld <Jason@zx2c4.com>
Date: Sun, 28 Feb 2021 11:24:02 +0000 (+0100)
Subject: runtime: see whether gp==nil before checking preemption state
X-Git-Tag: go1.17beta1~769
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=98dd205fa4f125c8ba8099c3d84d95189666068e;p=gostls13.git

runtime: see whether gp==nil before checking preemption state

Recent we changed from using gFromTLS to using gFromSP, which apparently
sometimes returns nil. This causes crashes when dereferenced. Fix that
by not checking for preemption in the case that gFromSP returns nil.

Fixes #44679.

Change-Id: I0199ebe7cd113379c5fa35c27932d913df79092a
Reviewed-on: https://go-review.googlesource.com/c/go/+/297390
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
---

diff --git a/src/runtime/os_windows.go b/src/runtime/os_windows.go
index 707c9054b5..705b42c61c 100644
--- a/src/runtime/os_windows.go
+++ b/src/runtime/os_windows.go
@@ -1380,7 +1380,7 @@ func preemptM(mp *m) {
 
 	// Does it want a preemption and is it safe to preempt?
 	gp := gFromSP(mp, c.sp())
-	if wantAsyncPreempt(gp) {
+	if gp != nil && wantAsyncPreempt(gp) {
 		if ok, newpc := isAsyncSafePoint(gp, c.ip(), c.sp(), c.lr()); ok {
 			// Inject call to asyncPreempt
 			targetPC := funcPC(asyncPreempt)