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Date: Thu, 30 Aug 2018 17:39:35 +0200 From: Jann Horn <jannh@...gle.com> To: yu-cheng.yu@...el.com Cc: "the arch/x86 maintainers" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, kernel list <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>, linux-arch <linux-arch@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...capital.net>, Balbir Singh <bsingharora@...il.com>, Cyrill Gorcunov <gorcunov@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com, Jonathan Corbet <corbet@....net>, keescook@...omiun.org, Mike Kravetz <mike.kravetz@...cle.com>, Nadav Amit <nadav.amit@...il.com>, Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>, Peter Zijlstra <peterz@...radead.org>, ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com Subject: Re: [RFC PATCH v3 19/24] x86/cet/shstk: Introduce WRUSS instruction On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@...el.com> 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. > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com> [...] > +static inline int write_user_shstk_64(unsigned long addr, unsigned long val) > +{ > + int err = 0; > + > + asm volatile("1: wrussq %1, (%0)\n" > + "2:\n" > + _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wruss) > + : > + : "r" (addr), "r" (val)); > + > + return err; > +} What's up with "err"? You set it to zero, and then you return it, but nothing can ever set it to non-zero, right? > +__visible bool ex_handler_wruss(const struct exception_table_entry *fixup, > + struct pt_regs *regs, int trapnr) > +{ > + regs->ip = ex_fixup_addr(fixup); > + regs->ax = -1; > + return true; > +} And here you just write into regs->ax, but your "asm volatile" doesn't reserve that register. This looks wrong to me. I think you probably want to add something like an explicit `"+&a"(err)` output to the asm statements. > @@ -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 Nits: typo ("instrcution"), weird grammar ("and but writes")
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