linux-kernel - Re: [PATCH v8 3/5] x86/pkeys: Update PKRU to enable all pkeys before XSAVE

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Message-ID: <CABi2SkV6AfxDNnuSKr_PCydZSyvKSmzmEeMnAXw3w1=w-kz7ow@mail.gmail.com>
Date: Fri, 9 Aug 2024 10:30:51 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, dave.hansen@...ux.intel.com, 
	tglx@...utronix.de, mingo@...nel.org, linux-mm@...ck.org, 
	keith.lucas@...cle.com, rick.p.edgecombe@...el.com, jorgelo@...omium.org, 
	keescook@...omium.org, sroettger@...gle.com, jannh@...gle.com
Subject: Re: [PATCH v8 3/5] x86/pkeys: Update PKRU to enable all pkeys before XSAVE

On Thu, Aug 1, 2024 at 11:13 PM Aruna Ramakrishna
<aruna.ramakrishna@...cle.com> wrote:
>
> If the alternate signal stack is protected by a different pkey than the
> current execution stack, copying xsave data to the sigaltstack will fail
> if its pkey is not enabled in the PKRU register.
>
> We do not know which pkey was used by the application for the altstack,
> so enable all pkeys before xsave.
>
> But this updated PKRU value is also pushed onto the sigframe, which
> means the register value restored from sigcontext will be different from
> the user-defined one, which is unexpected. Fix that by overwriting the
> PKRU value on the sigframe with the original, user-defined PKRU.
>
> Signed-off-by: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
> ---
>  arch/x86/kernel/fpu/signal.c | 11 +++++++++--
>  arch/x86/kernel/signal.c     | 12 ++++++++++--
>  2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 931c5469d7f3..1065ab995305 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -168,8 +168,15 @@ static inline bool save_xstate_epilog(void __user *buf, int ia32_frame,
>
>  static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf, u32 pkru)
>  {
> -       if (use_xsave())
> -               return xsave_to_user_sigframe(buf);
> +       int err = 0;
> +
> +       if (use_xsave()) {
> +               err = xsave_to_user_sigframe(buf);
> +               if (!err)
> +                       err = update_pkru_in_sigframe(buf, pkru);
> +               return err;
> +       }
> +
>         if (use_fxsr())
>                 return fxsave_to_user_sigframe((struct fxregs_state __user *) buf);
>         else
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 9dc77ad03a0e..5f441039b572 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -102,7 +102,7 @@ get_sigframe(struct ksignal *ksig, struct pt_regs *regs, size_t frame_size,
>         unsigned long math_size = 0;
>         unsigned long sp = regs->sp;
>         unsigned long buf_fx = 0;
> -       u32 pkru = read_pkru();
> +       u32 pkru;
>
>         /* redzone */
>         if (!ia32_frame)
> @@ -157,9 +157,17 @@ get_sigframe(struct ksignal *ksig, struct pt_regs *regs, size_t frame_size,
>                 return (void __user *)-1L;
>         }
>
> +       /* Update PKRU to enable access to the alternate signal stack. */
> +       pkru = sig_prepare_pkru();
I think, the better place to call sig_prepare_pkru() at begging of the
get_sigframe:

get_sigframe()
{
   /* ... */
    if (ka->sa.sa_flags & SA_ONSTACK) {
         if (sas_ss_flags(sp) == 0) {
                // set pkru = 0 <--- set pkru = 0 here.
                entering_altstack = true;
         }
}
For two reasons:
- We probably don't want all signaling handling going through "PKRU=0"
, this includes the regular stack and nested signaling handling. The
best place is when "entering the altstack". IIUC, this feature only
enabled when sigaltstack() is used.
 - The thread might not have read-access to the altstack, so we will
want to make sure that pkru=0 is set before any read to the altstack.
And IIRC,  fpu__alloc_mathframe needs read-access to the altstack.
To help with testing, I will send a test case to demo the usage.
(please give me sometime to organize the test code, I'm hoping to send
out before the end of next week)

Also, could you please consider adding a new flag SS_PKEYALTSTACK (see
SS_AUTODISARM for example). Most existing apps that use sigaltstack()
don't use PKEY and  don't care about setting PKRU=0, and don't need to
overwrite the sig frame after XSAVE.  The flag will limit the impact
of this patch.

Thanks
Best regards,
-Jeff





>         /* save i387 and extended state */
> -       if (!copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size, pkru))
> +       if (!copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size, pkru)) {
> +               /*
> +                * Restore PKRU to the original, user-defined value; disable
> +                * extra pkeys enabled for the alternate signal stack, if any.
> +                */
> +               write_pkru(pkru);
>                 return (void __user *)-1L;
> +       }
>
>         return (void __user *)sp;
>  }
> --
> 2.39.3
>