memcpy is leaking secret data through ZMM vector registers

[Cristian Rodriguez]

c

On Fri, Apr 19, 2024 at 10:08 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> Hi
>
> As a part of LVM2, we are developing the libdevmapper library. The library
> may be used to load cryptographic keys to the kernel, so it avoids leaking
> the data to kernel memory and to the swap partition.
>
> After the use of cryptographic data, the libdevmapper library clears them
> with memset and frees them afterwards. It executes __asm__ volatile("" :::
> "memory") to thwart some compiler optimization regarding writing to
> to-be-freed memory.
>
> We have a test "dmsecuretest.sh" that loads cryptographic keys into the
> kernel, dumps a core, the core file is analyzed and if it contains the
> key, the test fails.
>
> This test fails on AMD Zen 4 - the reason for the failure is that the
> "memcpy" function uses ZMM registers for data copying. When memcpy exits,
> the encryption key is present in the ZMM registers and the key remains
> there even after both source and destination buffers of memcpy were
> cleared.

Isn't this what  -fzero-call-used-regs thing is all about? can-t you
just apply the same technique in the memcpy implementation and zero
them out on return?
vector registers are volatile after all.