From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f70.google.com (mail-pl0-f70.google.com [209.85.160.70]) by kanga.kvack.org (Postfix) with ESMTP id 77DF26B0005 for ; Thu, 7 Jun 2018 13:50:05 -0400 (EDT) Received: by mail-pl0-f70.google.com with SMTP id e1-v6so5730679pld.23 for ; Thu, 07 Jun 2018 10:50:05 -0700 (PDT) Received: from mga01.intel.com (mga01.intel.com. [192.55.52.88]) by mx.google.com with ESMTPS id v65-v6si541046pfb.324.2018.06.07.10.50.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Jun 2018 10:50:04 -0700 (PDT) Message-ID: <1528393611.4636.70.camel@2b52.sc.intel.com> Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support From: Yu-cheng Yu Date: Thu, 07 Jun 2018 10:46:51 -0700 In-Reply-To: References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-2-yu-cheng.yu@intel.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski 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 On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote: > 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? A pthread's shadow stack is allocated/freed by the kernel. This patch has the supporting routines that handle both non-pthread and pthread. In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread shadow stack in copy_thread_tls(), and free it in deactivate_mm(). If clone of a pthread fails, shadow stack is freed in cet_disable_free_shstk() below (I will add more comments): If (Current thread existing) Disable and free shadow stack If (Clone of a pthread fails) Free the pthread shadow stack We block mremap, mprotect, madvise, and munmap on a vma that has VM_SHSTK (in separate patches). > 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.) Currently if FORCE bit is set, these functions can write to shadow stack, otherwise write access will fail. I will test it. > > +#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. OK. > > > + > > +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. OK. > > > +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. I will fix it. > > > +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.) OK. > > > + > > + 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? We get here when clone fails. In this condition, we don't disable the calling task's shadow stack. I will add comments. > > > + > > + /* > > + * 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? This is for pthreads. When a pthread exits, its shadow stack needs to be freed.