[release-branch.go1.14-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/+/901064
diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index c31ec51..b967621 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -928,7 +928,7 @@
// 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)
