From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support Date: Thu, 7 Jun 2018 09:37:05 -0700 Message-ID: References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-2-yu-cheng.yu@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20180607143807.3611-2-yu-cheng.yu@intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Yu-cheng Yu Cc: LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "H. J. Lu" , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com List-Id: linux-arch.vger.kernel.org On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: > > This patch adds basic shadow stack enabling/disabling routines. > A task's shadow stack is allocated from memory with VM_SHSTK > flag set and read-only protection. The shadow stack is > allocated to a fixed size and that can be changed by the system > admin. How do threads work? Can a user program mremap() its shadow stack to make it bigger? Also, did you add all the needed checks to make get_user_pages(), access_process_vm(), etc fail when called on the shadow stack? (Or at least fail if they're requesting write access and the FORCE bit isn't set.) > +#define SHSTK_SIZE (0x8000 * (test_thread_flag(TIF_IA32) ? 4 : 8)) Please don't add more mode-dependent #defines. Also, please try to avoid adding any new code that looks at TIF_IA32 or similar. Uses of that bit are generally bugs, and the bit itself should get removed some day. If you need to make a guess, use in_compat_syscall() or similar if appropriate. > + > +static inline int cet_set_shstk_ptr(unsigned long addr) > +{ > + u64 r; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return -1; > + > + if ((addr >= TASK_SIZE) || (!IS_ALIGNED(addr, 4))) > + return -1;' TASK_SIZE_MAX, please. TASK_SIZE is weird and is usually the wrong thing to use. > +static unsigned long shstk_mmap(unsigned long addr, unsigned long len) > +{ > + struct mm_struct *mm = current->mm; > + unsigned long populate; > + > + down_write(&mm->mmap_sem); > + addr = do_mmap(NULL, addr, len, PROT_READ, > + MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK, > + 0, &populate, NULL); > + up_write(&mm->mmap_sem); > + > + if (populate) > + mm_populate(addr, populate); Please don't populate if do_mmap() failed. > +int cet_setup_shstk(void) > +{ > + unsigned long addr, size; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return -EOPNOTSUPP; > + > + size = SHSTK_SIZE; > + addr = shstk_mmap(0, size); > + > + if (addr >= TASK_SIZE) > + return -ENOMEM; Please check the actual value that do_mmap() would return on error. (IS_ERR, 0, MAP_FAILED -- I don't remember.) > + > + cet_set_shstk_ptr(addr + size - sizeof(void *)); > + current->thread.cet.shstk_base = addr; > + current->thread.cet.shstk_size = size; > + current->thread.cet.shstk_enabled = 1; > + return 0; > +} > + > +void cet_disable_shstk(void) > +{ > + u64 r; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return; > + > + rdmsrl(MSR_IA32_U_CET, r); > + r &= ~(MSR_IA32_CET_SHSTK_EN); > + wrmsrl(MSR_IA32_U_CET, r); > + wrmsrl(MSR_IA32_PL3_SSP, 0); > + current->thread.cet.shstk_enabled = 0; > +} > + > +void cet_disable_free_shstk(struct task_struct *tsk) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK) || > + !tsk->thread.cet.shstk_enabled) > + return; > + > + if (tsk == current) > + cet_disable_shstk(); if tsk != current, then this will malfunction, right? What is it intended to do? > + > + /* > + * Free only when tsk is current or shares mm > + * with current but has its own shstk. > + */ > + if (tsk->mm && (tsk->mm == current->mm) && > + (tsk->thread.cet.shstk_base)) { > + vm_munmap(tsk->thread.cet.shstk_base, > + tsk->thread.cet.shstk_size); > + tsk->thread.cet.shstk_base = 0; > + tsk->thread.cet.shstk_size = 0; > + } I'm having trouble imagining why the kernel would ever want to automatically free the shadow stack vma. What is this for? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:35264 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753534AbeFGQhT (ORCPT ); Thu, 7 Jun 2018 12:37:19 -0400 Received: from mail-wr0-f170.google.com (mail-wr0-f170.google.com [209.85.128.170]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3AC3C208B0 for ; Thu, 7 Jun 2018 16:37:19 +0000 (UTC) Received: by mail-wr0-f170.google.com with SMTP id l10-v6so10535740wrn.2 for ; Thu, 07 Jun 2018 09:37:19 -0700 (PDT) MIME-Version: 1.0 References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-2-yu-cheng.yu@intel.com> In-Reply-To: <20180607143807.3611-2-yu-cheng.yu@intel.com> From: Andy Lutomirski Date: Thu, 7 Jun 2018 09:37:05 -0700 Message-ID: Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Yu-cheng Yu Cc: LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "H. J. Lu" , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com Message-ID: <20180607163705.8TD3K7AzpXRjh7pOQxSe91etzwmfA07KUPSyDR3toZY@z> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: > > This patch adds basic shadow stack enabling/disabling routines. > A task's shadow stack is allocated from memory with VM_SHSTK > flag set and read-only protection. The shadow stack is > allocated to a fixed size and that can be changed by the system > admin. How do threads work? Can a user program mremap() its shadow stack to make it bigger? Also, did you add all the needed checks to make get_user_pages(), access_process_vm(), etc fail when called on the shadow stack? (Or at least fail if they're requesting write access and the FORCE bit isn't set.) > +#define SHSTK_SIZE (0x8000 * (test_thread_flag(TIF_IA32) ? 4 : 8)) Please don't add more mode-dependent #defines. Also, please try to avoid adding any new code that looks at TIF_IA32 or similar. Uses of that bit are generally bugs, and the bit itself should get removed some day. If you need to make a guess, use in_compat_syscall() or similar if appropriate. > + > +static inline int cet_set_shstk_ptr(unsigned long addr) > +{ > + u64 r; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return -1; > + > + if ((addr >= TASK_SIZE) || (!IS_ALIGNED(addr, 4))) > + return -1;' TASK_SIZE_MAX, please. TASK_SIZE is weird and is usually the wrong thing to use. > +static unsigned long shstk_mmap(unsigned long addr, unsigned long len) > +{ > + struct mm_struct *mm = current->mm; > + unsigned long populate; > + > + down_write(&mm->mmap_sem); > + addr = do_mmap(NULL, addr, len, PROT_READ, > + MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK, > + 0, &populate, NULL); > + up_write(&mm->mmap_sem); > + > + if (populate) > + mm_populate(addr, populate); Please don't populate if do_mmap() failed. > +int cet_setup_shstk(void) > +{ > + unsigned long addr, size; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return -EOPNOTSUPP; > + > + size = SHSTK_SIZE; > + addr = shstk_mmap(0, size); > + > + if (addr >= TASK_SIZE) > + return -ENOMEM; Please check the actual value that do_mmap() would return on error. (IS_ERR, 0, MAP_FAILED -- I don't remember.) > + > + cet_set_shstk_ptr(addr + size - sizeof(void *)); > + current->thread.cet.shstk_base = addr; > + current->thread.cet.shstk_size = size; > + current->thread.cet.shstk_enabled = 1; > + return 0; > +} > + > +void cet_disable_shstk(void) > +{ > + u64 r; > + > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) > + return; > + > + rdmsrl(MSR_IA32_U_CET, r); > + r &= ~(MSR_IA32_CET_SHSTK_EN); > + wrmsrl(MSR_IA32_U_CET, r); > + wrmsrl(MSR_IA32_PL3_SSP, 0); > + current->thread.cet.shstk_enabled = 0; > +} > + > +void cet_disable_free_shstk(struct task_struct *tsk) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK) || > + !tsk->thread.cet.shstk_enabled) > + return; > + > + if (tsk == current) > + cet_disable_shstk(); if tsk != current, then this will malfunction, right? What is it intended to do? > + > + /* > + * Free only when tsk is current or shares mm > + * with current but has its own shstk. > + */ > + if (tsk->mm && (tsk->mm == current->mm) && > + (tsk->thread.cet.shstk_base)) { > + vm_munmap(tsk->thread.cet.shstk_base, > + tsk->thread.cet.shstk_size); > + tsk->thread.cet.shstk_base = 0; > + tsk->thread.cet.shstk_size = 0; > + } I'm having trouble imagining why the kernel would ever want to automatically free the shadow stack vma. What is this for?