Messages in this thread |  | | Date | Wed, 3 Oct 2018 06:15:57 +0200 | From | Eugene Syromiatnikov <> | Subject | Re: [RFC PATCH v4 19/27] x86/cet/shstk: Introduce WRUSS instruction |
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On Fri, Sep 21, 2018 at 08:03:43AM -0700, Yu-cheng Yu wrote: > WRUSS is a new kernel-mode instruction but writes directly > to user shadow stack memory. This is used to construct > a return address on the shadow stack for the signal > handler. > > This instruction can fault if the user shadow stack is > invalid shadow stack memory. In that case, the kernel does > fixup.
"a fixup"
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com> > --- > arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++ > arch/x86/mm/fault.c | 9 ++++++++ > 2 files changed, 41 insertions(+) > > diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h > index 317fc59b512c..c04e68ef47da 100644 > --- a/arch/x86/include/asm/special_insns.h > +++ b/arch/x86/include/asm/special_insns.h > @@ -237,6 +237,38 @@ static inline void clwb(volatile void *__p) > : [pax] "a" (p)); > } > > +#ifdef CONFIG_X86_INTEL_CET > +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32) > +static inline int write_user_shstk_32(unsigned long addr, unsigned int val) > +{ > + asm_volatile_goto("1: wrussd %1, (%0)\n" > + _ASM_EXTABLE(1b, %l[fail]) > + :: "r" (addr), "r" (val) > + :: fail); > + return 0; > +fail: > + return -1;
Should it...
> +} > +#else > +static inline int write_user_shstk_32(unsigned long addr, unsigned int val) > +{ > + WARN_ONCE(1, "write_user_shstk_32 used but not supported.\n");
"is/was used"
> + return -EFAULT; > +} > +#endif > + > +static inline int write_user_shstk_64(unsigned long addr, unsigned long val) > +{ > + asm_volatile_goto("1: wrussq %1, (%0)\n" > + _ASM_EXTABLE(1b, %l[fail]) > + :: "r" (addr), "r" (val) > + :: fail); > + return 0; > +fail: > + return -1;
...and it be -EPERM, if -EFAULT was returned earlier for write_user_shstk_32?
> +} > +#endif /* CONFIG_X86_INTEL_CET */ > + > #define nop() asm volatile ("nop") > > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 7c3877a982f4..4d4ac57a4ba2 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code, > error_code |= X86_PF_USER; > flags |= FAULT_FLAG_USER; > } else { > + /* > + * WRUSS is a kernel instrcution and but writes
"WRUSS is a kernel instruction but writes"
> + * to user shadow stack. When a fault occurs, > + * both X86_PF_USER and X86_PF_SHSTK are set. > + * Clear X86_PF_USER here. > + */ > + if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) == > + (X86_PF_USER | X86_PF_SHSTK)) > + error_code &= ~X86_PF_USER; > if (regs->flags & X86_EFLAGS_IF) > local_irq_enable(); > } > -- > 2.17.1 >
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