[release-branch.go1.15-security] 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 <bracewell@google.com>
Reviewed-by: Filippo Valsorda <valsorda@google.com>
(cherry picked from commit 28015462c2a83239543dc2bef651e9a5f234b633)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/901065

diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index 6a3989bf9d82bf1c9ec3b57aedead466002bfb8e..8c43de69d33ff8e841c000d15808a2e7e2b160da 100644 (file)
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -928,7 +928,7 @@ 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)