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[release-branch.go1.22] crypto/internal/fips140/nistec: make p256NegCond constant time on ppc64le

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Remove the branching instruction from p256NegCond which made it variable
time. The technique used matches that used in p256MovCond.

Fixes #71383
Fixes #71422
Fixes CVE-2025-22866

Change-Id: Ibc2a46814d856cbbdaf6cc0c5a415ed5d42ca793
Reviewed-on: https://go-review.googlesource.com/c/go/+/643735
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 6fc23a3cff5e38ff72923fee50f51254dcdc6e93)
Reviewed-on: https://go-review.googlesource.com/c/go/+/645535
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Bypass: Carlos Amedee <carlos@golang.org>
This commit is contained in:
Roland Shoemaker 2025-01-21 16:03:14 -08:00 committed by Carlos Amedee
parent c3c6a50095
commit 0cc45e7ca6
1 changed files with 17 additions and 3 deletions
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20
src/crypto/internal/nistec/p256_asm_ppc64le.s
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@ -124,14 +124,23 @@ GLOBL p256mul<>(SB), 8, $160
#define PH V31
#define CAR1 V6
#define SEL V8
#define ZER V9
// func p256NegCond(val *p256Point, cond int)
TEXT ·p256NegCond(SB), NOSPLIT, $0-16
MOVD val+0(FP), P1ptr
MOVD $16, R16
MOVD cond+8(FP), R6
CMP $0, R6
BC 12, 2, LR // just return if cond == 0
// Copy cond into SEL (cond is R1 + 8 (cond offset) + 32)
MOVD $40, R17
LXVDSX (R1)(R17), SEL
// Zeroize ZER
VSPLTISB $0, ZER
// SEL controls whether to return the original value (Y1H/Y1L)
// or the negated value (T1H/T1L).
VCMPEQUD SEL, ZER, SEL
MOVD $p256mul<>+0x00(SB), CPOOL
@ -148,6 +157,9 @@ TEXT ·p256NegCond(SB), NOSPLIT, $0-16
VSUBUQM PL, Y1L, T1L // subtract part2 giving result
VSUBEUQM PH, Y1H, CAR1, T1H // subtract part1 using carry from part2
VSEL T1H, Y1H, SEL, T1H
VSEL T1L, Y1L, SEL, T1L
XXPERMDI T1H, T1H, $2, T1H
XXPERMDI T1L, T1L, $2, T1L
@ -164,6 +176,8 @@ TEXT ·p256NegCond(SB), NOSPLIT, $0-16
#undef PL
#undef PH
#undef CAR1
#undef SEL
#undef ZER
#define P3ptr R3
#define P1ptr R4