Kyle!
On Mon, Aug 08 2022 at 07:15, Kyle Huey wrote:
> When management of the PKRU register was moved away from XSTATE, emulation
> of PKRU's existence in XSTATE was added for APIs that read XSTATE, but not
> for APIs that write XSTATE. This can be seen by running gdb and executing
> `p $pkru`, `set $pkru = 42`, and `p $pkru`. On affected kernels (5.14+) the
> write to the PKRU register (which gdb performs through ptrace) is ignored.
>
> There are three relevant APIs: PTRACE_SETREGSET with NT_X86_XSTATE,
> sigreturn, and KVM_SET_XSAVE. KVM_SET_XSAVE has its own special handling to
> make PKRU writes take effect (in fpu_copy_uabi_to_guest_fpstate). Push that
> down into copy_uabi_to_xstate and have PTRACE_SETREGSET with NT_X86_XSTATE
> and sigreturn pass in pointers to the appropriate PKRU value.
>
> This also adds code to initialize the PKRU value to the hardware init value
> (namely 0) if the PKRU bit is not set in the XSTATE header to match XRSTOR.
> This is a change to the current KVM_SET_XSAVE behavior.
You are stating a fact here, but provide 0 justification why this is
correct.
>
> Changelog since v4:
Can you please put the change log past the --- seperator line, so it
gets stripped off when the patch is applied? That spares manual fixups.
>
> Signed-off-by: Kyle Huey <me@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx # For edge case behavior of KVM_SET_XSAVE
> Cc: stable@xxxxxxxxxxxxxxx # 5.14+
> Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")
Can you please use the documented tag ordering?
https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-submission-notes
> @@ -1235,6 +1235,24 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
> for (i = 0; i < XFEATURE_MAX; i++) {
> mask = BIT_ULL(i);
>
> + if (i == XFEATURE_PKRU) {
> + /*
> + * Retrieve PKRU if not in init state, otherwise
> + * initialize it.
> + */
> + if (hdr.xfeatures & mask) {
> + struct pkru_state xpkru = {0};
> +
> + if (copy_from_buffer(&xpkru, xstate_offsets[i],
> + sizeof(xpkru), kbuf, ubuf))
> + return -EFAULT;
> +
> + *pkru = xpkru.pkru;
> + } else {
> + *pkru = 0;
> + }
> + }
That's really horrible and there is no point in copying the stuff from
the buffer twice:
@@ -1246,6 +1246,15 @@ static int copy_uabi_to_xstate(struct fp
}
}
+ /* Update the user protection key storage */
+ *pkru = 0;
+ if (hdr.xfeatures & XFEATURE_MASK_PKRU) {
+ struct pkru_state *xpkru;
+
+ xpkru = get_xsave_addr(xsave, XFEATURE_PKRU);
+ *pkru = xpkru->pkru;
+ }
+
Hmm?
Thanks,
tglx
