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[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
This commit is contained in:
Katie Hockman 2020-11-10 15:54:12 -05:00 committed by Katie Hockman
parent ec06b6d6be
commit 84150d0af1

View File

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