Re: [PATCH v5 1/2] x86/fpu: Allow PKRU to be (once again) written by ptrace.
From: Kyle Huey
Date: Thu Aug 18 2022 - 00:03:19 EST
On Mon, Aug 8, 2022 at 7:15 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
>
> From: Kyle Huey <me@xxxxxxxxxxxx>
>
> 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.
>
> Changelog since v4:
> - Selftest additionally checks PKRU readbacks through ptrace.
> - Selftest flips all PKRU bits (except the key used for PROT_EXEC).
>
> Changelog since v3:
> - The v3 patch is now part 1 of 2.
> - Adds a selftest in part 2 of 2.
>
> Changelog since v2:
> - Removed now unused variables in fpu_copy_uabi_to_guest_fpstate
>
> Changelog since v1:
> - Handles the error case of copy_to_buffer().
>
> 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()")
> ---
> arch/x86/kernel/fpu/core.c | 13 +------------
> arch/x86/kernel/fpu/regset.c | 2 +-
> arch/x86/kernel/fpu/signal.c | 2 +-
> arch/x86/kernel/fpu/xstate.c | 28 +++++++++++++++++++++++-----
> arch/x86/kernel/fpu/xstate.h | 4 ++--
> 5 files changed, 28 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 3b28c5b25e12..46b935bc87c8 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -391,8 +391,6 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
> {
> struct fpstate *kstate = gfpu->fpstate;
> const union fpregs_state *ustate = buf;
> - struct pkru_state *xpkru;
> - int ret;
>
> if (!cpu_feature_enabled(X86_FEATURE_XSAVE)) {
> if (ustate->xsave.header.xfeatures & ~XFEATURE_MASK_FPSSE)
> @@ -406,16 +404,7 @@ 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..e01d3514ae68 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 +1235,24 @@ 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;
> + }
> + }
> +
> if (hdr.xfeatures & mask) {
> void *dst = __raw_xsave_addr(xsave, i);
>
> @@ -1264,9 +1282,9 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
> * Convert from a ptrace standard-format kernel buffer to kernel XSAVE[S]
> * format and copy to the target thread. Used by ptrace and KVM.
> */
> -int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf)
> +int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf, u32 *pkru)
> {
> - return copy_uabi_to_xstate(fpstate, kbuf, NULL);
> + return copy_uabi_to_xstate(fpstate, kbuf, NULL, pkru);
> }
>
> /*
> @@ -1274,10 +1292,10 @@ int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf)
> * XSAVE[S] format and copy to the target thread. This is called from the
> * sigreturn() and rt_sigreturn() system calls.
> */
> -int copy_sigframe_from_user_to_xstate(struct fpstate *fpstate,
> +int copy_sigframe_from_user_to_xstate(struct task_struct *tsk,
> const void __user *ubuf)
> {
> - return copy_uabi_to_xstate(fpstate, NULL, ubuf);
> + return copy_uabi_to_xstate(tsk->thread.fpu.fpstate, NULL, ubuf, &tsk->thread.pkru);
> }
>
> static bool validate_independent_components(u64 mask)
> diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
> index 5ad47031383b..a4ecb04d8d64 100644
> --- a/arch/x86/kernel/fpu/xstate.h
> +++ b/arch/x86/kernel/fpu/xstate.h
> @@ -46,8 +46,8 @@ extern void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
> u32 pkru_val, enum xstate_copy_mode copy_mode);
> extern void copy_xstate_to_uabi_buf(struct membuf to, struct task_struct *tsk,
> enum xstate_copy_mode mode);
> -extern int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf);
> -extern int copy_sigframe_from_user_to_xstate(struct fpstate *fpstate, const void __user *ubuf);
> +extern int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf, u32 *pkru);
> +extern int copy_sigframe_from_user_to_xstate(struct task_struct *tsk, const void __user *ubuf);
>
>
> extern void fpu__init_cpu_xstate(void);
> --
> 2.37.1
>
Bump.
If there are no further comments/complaints, can we get this queued up
via x86/urgent (or something)? We're eager to get this moving and
eventually onto stable to fix rr users on 5.15.
- Kyle