math/big: fix shift for recursive division

The previous s value could cause a crash
for certain inputs.

Will check in tests and documentation improvements later.

Thanks to the Go Ethereum team and the OSS-Fuzz project for reporting this.
Thanks to Rémy Oudompheng and Robert Griesemer for their help
developing and validating the fix.

Fixes CVE-2020-28362

Change-Id: Ibbf455c4436bcdb07c84a34fa6551fb3422356d3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/899974
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/269657
Trust: Katie Hockman <[email protected]>
Trust: Roland Shoemaker <[email protected]>
Run-TryBot: Katie Hockman <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: katiehockman

src/math/big/nat.go

@@ -929,7 +929,7 @@ func (z nat) divRecursiveStep(u, v nat, depth int, tmp *nat, temps []*nat) {
 
 	// Now u < (v<<B), compute lower bits in the same way.
 	// Choose shift = B-1 again.
-	s := B
+	s := B - 1
 	qhat := *temps[depth]
 	qhat.clear()
 	qhat.divRecursiveStep(u[s:].norm(), v[s:], depth+1, tmp, temps)