Re: [PATCH] x86/fpu: Allow PKRU to be (once again) written by ptrace.

Ingo Molnar
2022-08-03T09:03:58Z

* Kyle Huey <me@kylehuey.com> wrote:

> From: Kyle Huey <me@kylehuey.com>
>
> 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.
>
> Signed-off-by: Kyle Huey <me@kylehuey.com>
> Cc: kvm@vger.kernel.org # For edge case behavior of KVM_SET_XSAVE
> Cc: stable@vger.kernel.org # 5.14+
> Fixes: e84ba47e313dbc097bf859bb6e4f9219883d5f78
> ---
> arch/x86/kernel/fpu/core.c | 11 +----------
> arch/x86/kernel/fpu/regset.c | 2 +-
> arch/x86/kernel/fpu/signal.c | 2 +-
> arch/x86/kernel/fpu/xstate.c | 26 +++++++++++++++++++++-----
> arch/x86/kernel/fpu/xstate.h | 4 ++--
> 5 files changed, 26 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 0531d6a06df5..dfb79e2ee81f 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -406,16 +406,7 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
> if (ustate->xsave.header.xfeatures & ~xcr0)
> return -EINVAL;
>
> - ret = copy_uabi_from_kernel_to_xstate(kstate, ustate);
> - if (ret)
> - return ret;
> -
> - /* Retrieve PKRU if not in init state */
> - if (kstate->regs.xsave.header.xfeatures & XFEATURE_MASK_PKRU) {
> - xpkru = get_xsave_addr(&kstate->regs.xsave, XFEATURE_PKRU);
> - *vpkru = xpkru->pkru;
> - }
> - return 0;
> + return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru);
> }
> EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);
> #endif /* CONFIG_KVM */
> diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
> index 75ffaef8c299..6d056b68f4ed 100644
> --- a/arch/x86/kernel/fpu/regset.c
> +++ b/arch/x86/kernel/fpu/regset.c
> @@ -167,7 +167,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
> }
>
> fpu_force_restore(fpu);
> - ret = copy_uabi_from_kernel_to_xstate(fpu->fpstate, kbuf ?: tmpbuf);
> + ret = copy_uabi_from_kernel_to_xstate(fpu->fpstate, kbuf ?: tmpbuf, &target->thread.pkru);
>
> out:
> vfree(tmpbuf);
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 91d4b6de58ab..558076dbde5b 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -396,7 +396,7 @@ static bool __fpu_restore_sig(void __user *buf, void __user *buf_fx,
>
> fpregs = &fpu->fpstate->regs;
> if (use_xsave() && !fx_only) {
> - if (copy_sigframe_from_user_to_xstate(fpu->fpstate, buf_fx))
> + if (copy_sigframe_from_user_to_xstate(tsk, buf_fx))
> return false;
> } else {
> if (__copy_from_user(&fpregs->fxsave, buf_fx,
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index c8340156bfd2..1eea7af4afd9 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -1197,7 +1197,7 @@ static int copy_from_buffer(void *dst, unsigned int offset, unsigned int size,
>
>
> static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
> - const void __user *ubuf)
> + const void __user *ubuf, u32 *pkru)
> {
> struct xregs_state *xsave = &fpstate->regs.xsave;
> unsigned int offset, size;
> @@ -1235,6 +1235,22 @@ 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};
> +
> + copy_from_buffer(&xpkru, xstate_offsets[i],
> + sizeof(xpkru), kbuf, ubuf);

Shouldn't the failure case of copy_from_buffer() be handled?

Also, what's the security model for this register, do we trust all input
values user-space provides for the PKRU field in the XSTATE? I realize that
WRPKRU already gives user-space write access to the register - but does the
CPU write it all into the XSTATE, with no restrictions on content
whatsoever?

Thanks,

Ingo