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

Kyle Huey
2022-08-03T15:12:53Z

On Wed, Aug 3, 2022 at 2:03 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * 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?

Yes, it should be. The sigreturn case could hit it.

> 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?

There is no security model for this register. The CPU does write
whatever is given to WRPKRU (or XRSTOR) into the PKRU register. The
pkeys(7) man page notes:

Protection keys have the potential to add a layer of security and
reliability to applications. But they have not been primarily designed
as a security feature. For instance, WRPKRU is a completely
unprivileged instruction, so pkeys are useless in any case that an
attacker controls the PKRU register or can execute arbitrary
instructions.

And the ERIM paper
(https://www.usenix.org/system/files/sec19-vahldiek-oberwagner_0.pdf)
explicitly contemplates the need to protect against the less
privileged code containing WRPKRU and XRSTOR instructions (though they
do seem to have missed the implicit XRSTOR in sigreturn).

> Thanks,
>
> Ingo

- Kyle