From 0b25e97d03b520d8507342d3c7df9b98ebcbe2c0 Mon Sep 17 00:00:00 2001 From: Alberto Donizetti Date: Tue, 21 Nov 2017 14:16:04 +0100 Subject: [release-branch.go1.9] math/big: protect against aliasing in nat.divLarge MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit In nat.divLarge (having signature (z nat).divLarge(u, uIn, v nat)), we check whether z aliases uIn or v, but aliasing is currently not checked for the u parameter. Unfortunately, z and u aliasing each other can in some cases cause errors in the computation. The q return parameter (which will hold the result's quotient), is unconditionally initialized as q = z.make(m + 1) When cap(z) ≥ m+1, z.make() will reuse z's backing array, causing q and z to share the same backing array. If then z aliases u, setting q during the quotient computation will then corrupt u, which at that point already holds computation state. To fix this, we add an alias(z, u) check at the beginning of the function, taking care of aliasing the same way we already do for uIn and v. Fixes #22830 Change-Id: I3ab81120d5af6db7772a062bb1dfc011de91f7ad Reviewed-on: https://go-review.googlesource.com/78995 Run-TryBot: Alberto Donizetti Run-TryBot: Robert Griesemer TryBot-Result: Gobot Gobot Reviewed-by: Robert Griesemer Reviewed-on: https://go-review.googlesource.com/88322 Run-TryBot: Andrew Bonventre --- src/math/big/int_test.go | 20 ++++++++++++++++++++ src/math/big/nat.go | 4 ++-- 2 files changed, 22 insertions(+), 2 deletions(-) diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go index 42e810b3b8..46e2ff1203 100644 --- a/src/math/big/int_test.go +++ b/src/math/big/int_test.go @@ -1536,6 +1536,26 @@ func TestSqrt(t *testing.T) { } } +// We can't test this together with the other Exp tests above because +// it requires a different receiver setup. +func TestIssue22830(t *testing.T) { + one := new(Int).SetInt64(1) + base, _ := new(Int).SetString("84555555300000000000", 10) + mod, _ := new(Int).SetString("66666670001111111111", 10) + want, _ := new(Int).SetString("17888885298888888889", 10) + + var tests = []int64{ + 0, 1, -1, + } + + for _, n := range tests { + m := NewInt(n) + if got := m.Exp(base, one, mod); got.Cmp(want) != 0 { + t.Errorf("(%v).Exp(%s, 1, %s) = %s, want %s", n, base, mod, got, want) + } + } +} + func BenchmarkSqrt(b *testing.B) { n, _ := new(Int).SetString("1"+strings.Repeat("0", 1001), 10) b.ResetTimer() diff --git a/src/math/big/nat.go b/src/math/big/nat.go index 889eacb90f..f2cdbdb909 100644 --- a/src/math/big/nat.go +++ b/src/math/big/nat.go @@ -566,8 +566,8 @@ func (z nat) divLarge(u, uIn, v nat) (q, r nat) { // determine if z can be reused // TODO(gri) should find a better solution - this if statement // is very costly (see e.g. time pidigits -s -n 10000) - if alias(z, uIn) || alias(z, v) { - z = nil // z is an alias for uIn or v - cannot reuse + if alias(z, u) || alias(z, uIn) || alias(z, v) { + z = nil // z is an alias for u or uIn or v - cannot reuse } q = z.make(m + 1) -- cgit v1.2.3-54-g00ecf