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runtime: make it harder to find collisions in the 64-bit fallback hash

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Currently the first argument to mix() can be set by an attacker, as it
is just the input bytes xored by some constants. That lets an attacker
set the value being multipled by to 0. That can lead to lots of
collisions. To fix, xor the first argument with the process-wide seed,
so the magic collision-generating value isn't a constant known to the
attacker.  (Maybe there's a timing attack that could figure out the
process-wide seed, but that's a much harder attack.)

Fixes #66841

Change-Id: I33e073c78355d1cee08660de52074e6ccc38b426
Reviewed-on: https://go-review.googlesource.com/c/go/+/579115
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
khr@golang.org 2024-04-15 17:52:20 -07:00 committed by Keith Randall
parent 2b82a4f488
commit ef2f3391da
2 changed files with 33 additions and 7 deletions
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14
src/runtime/hash64.go
View File

@ -50,32 +50,32 @@ func memhashFallback(p unsafe.Pointer, seed, s uintptr) uintptr {
seed1 := seed
seed2 := seed
for ; l > 48; l -= 48 {
seed = mix(r8(p)^m2, r8(add(p, 8))^seed)
seed1 = mix(r8(add(p, 16))^m3, r8(add(p, 24))^seed1)
seed2 = mix(r8(add(p, 32))^m4, r8(add(p, 40))^seed2)
seed = mix(r8(p)^hashkey[1]^m2, r8(add(p, 8))^seed)
seed1 = mix(r8(add(p, 16))^hashkey[2]^m3, r8(add(p, 24))^seed1)
seed2 = mix(r8(add(p, 32))^hashkey[3]^m4, r8(add(p, 40))^seed2)
p = add(p, 48)
}
seed ^= seed1 ^ seed2
}
for ; l > 16; l -= 16 {
seed = mix(r8(p)^m2, r8(add(p, 8))^seed)
seed = mix(r8(p)^hashkey[1]^m2, r8(add(p, 8))^seed)
p = add(p, 16)
}
a = r8(add(p, l-16))
b = r8(add(p, l-8))
}
return mix(m5^s, mix(a^m2, b^seed))
return mix(m5^s, mix(a^hashkey[1]^m2, b^seed))
}
func memhash32Fallback(p unsafe.Pointer, seed uintptr) uintptr {
a := r4(p)
return mix(m5^4, mix(a^m2, a^seed^hashkey[0]^m1))
return mix(m5^4, mix(a^hashkey[1]^m2, a^seed^hashkey[0]^m1))
}
func memhash64Fallback(p unsafe.Pointer, seed uintptr) uintptr {
a := r8(p)
return mix(m5^8, mix(a^m2, a^seed^hashkey[0]^m1))
return mix(m5^8, mix(a^hashkey[1]^m2, a^seed^hashkey[0]^m1))
}
func mix(a, b uintptr) uintptr {

26
src/runtime/hash_test.go
View File

@ -5,10 +5,13 @@
package runtime_test
import (
"encoding/binary"
"fmt"
"internal/race"
"internal/testenv"
"math"
"math/rand"
"os"
. "runtime"
"slices"
"strings"
@ -625,6 +628,29 @@ func TestSmhasherSeed(t *testing.T) {
h.check(t)
}
func TestIssue66841(t *testing.T) {
testenv.MustHaveExec(t)
if *UseAeshash && os.Getenv("TEST_ISSUE_66841") == "" {
// We want to test the backup hash, so if we're running on a machine
// that uses aeshash, exec ourselves while turning aes off.
cmd := testenv.CleanCmdEnv(testenv.Command(t, os.Args[0], "-test.run=^TestIssue66841$"))
cmd.Env = append(cmd.Env, "GODEBUG=cpu.aes=off", "TEST_ISSUE_66841=1")
out, err := cmd.CombinedOutput()
if err != nil {
t.Errorf("%s", string(out))
}
// Fall through. Might as well run this test when aeshash is on also.
}
h := newHashSet()
var b [16]byte
binary.LittleEndian.PutUint64(b[:8], 0xe7037ed1a0b428db) // runtime.m2
for i := 0; i < 1000; i++ {
binary.LittleEndian.PutUint64(b[8:], uint64(i))
h.addB(b[:])
}
h.check(t)
}
// size of the hash output (32 or 64 bits)
const hashSize = 32 + int(^uintptr(0)>>63<<5)