oss-security - MOV{H,L}PS instructions can fail on Genoa (Zen 4)

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Date: Thu, 21 Sep 2023 07:21:21 -0700
From: Tavis Ormandy <taviso@...il.com>
To: oss-security@...ts.openwall.com
Subject: MOV{H,L}PS instructions can fail on Genoa (Zen 4)

Hey, when fuzzing Genoa (AMD Zen 4) I noticed that sometimes the
MOV{H,L}PS instructions don't seem to work? I asked AMD if they consider
this a vulnerability, and they didn't.. so I'll just document it here
for reference...

Quick background, these instructions load two 32-bit packed singles from the
source operand into the low (movlps) or high (movhps) 64-bits of a vector
register.

Consider this minimal example:

section .data
    a: dq 0x1111111111111111
    b: dq 0x2222222222222222

section .text
    movhps  xmm0, [rel a]
    movlps  xmm0, [rel b]

The result should be xmm0 has the value 0x11111111111111112222222222222222.

Genoa added support for AVX512, which gives you a bunch more vector
registers, so now you can do:

    movhps  xmm28, [rel b]

However, I've found that non-deterministically, when using any register
above xmm15, previous (pipelined?) operations on other registers fail.

Here is an example:

section .data
    data: dd 0x11111111, 0x22222222, 0x33333333, 0x44444444
    zero: dd 0,0,0,0

section .text
    vmovdqu  xmm0, [rel data]
    vmovlps  xmm1, xmm0, [rel zero]
    vmovhps  xmm17, xmm0, [rel zero]

I think the expected result would be:

xmm0  = 0x44444444333333332222222211111111
xmm1  = 0x44444444333333330000000000000000
xmm17 = 0x00000000000000002222222211111111

However, on genoa we non-deterministically get xmm1=0.

I don't know the cause or where the bug is, any feedback welcome. I've
attached a testcase (I ported it to C from a raw fuzzer generated
testcase, hopefully it compiles consistently!).

I can reproduce it with pure intrinsics too (no asm), but the output is
not consistent across gcc versions. The attached version does use some
inline asm.

I think it should produce no output at all, but on Genoa it does sometimes
produce output for me.

Compile with:

$ gcc -mavx512vl -o movhps movhps.c

Tavis.

-- 
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View attachment "movhps.c" of type "text/plain" (1293 bytes)

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