On Wed, Aug 12, 2026 at 10:04 PM Cristian Rodriguez <[email protected]> wrote: > > c > > On Fri, Apr 19, 2024 at 10:08 AM Mikulas Patocka <[email protected]> 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.
This isn't limited to vector registers. If you don't want to leave anything behind, you should use -fzero-call-used-regs in functions where keys are accessed. -- H.J.
