From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yu-cheng Yu Subject: [RFC PATCH v9 07/27] Add guard pages around a Shadow Stack. Date: Wed, 5 Feb 2020 10:19:15 -0800 Message-ID: <20200205181935.3712-8-yu-cheng.yu@intel.com> References: <20200205181935.3712-1-yu-cheng.yu@intel.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20200205181935.3712-1-yu-cheng.yu@intel.com> Sender: linux-kernel-owner@vger.kernel.org To: x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit Cc: Yu-cheng Yu List-Id: linux-api@vger.kernel.org INCSSPD/INCSSPQ instruction is used to unwind a Shadow Stack (SHSTK). It performs 'pop and discard' of the first and last element from SHSTK in the range specified in the operand. The maximum value of the operand is 255, and the maximum moving distance of the SHSTK pointer is 255 * 4 for INCSSPD, 255 * 8 for INCSSPQ. Since SHSTK has a fixed size, creating a guard page above prevents INCSSP/RET from moving beyond. Likewise, creating a guard page below prevents CALL from underflowing the SHSTK. Signed-off-by: Yu-cheng Yu --- include/linux/mm.h | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index b5145fbe102e..75de07674649 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2464,9 +2464,15 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m static inline unsigned long vm_start_gap(struct vm_area_struct *vma) { unsigned long vm_start = vma->vm_start; + unsigned long gap = 0; - if (vma->vm_flags & VM_GROWSDOWN) { - vm_start -= stack_guard_gap; + if (vma->vm_flags & VM_GROWSDOWN) + gap = stack_guard_gap; + else if (vma->vm_flags & VM_SHSTK) + gap = PAGE_SIZE; + + if (gap != 0) { + vm_start -= gap; if (vm_start > vma->vm_start) vm_start = 0; } @@ -2476,9 +2482,15 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma) static inline unsigned long vm_end_gap(struct vm_area_struct *vma) { unsigned long vm_end = vma->vm_end; + unsigned long gap = 0; + + if (vma->vm_flags & VM_GROWSUP) + gap = stack_guard_gap; + else if (vma->vm_flags & VM_SHSTK) + gap = PAGE_SIZE; - if (vma->vm_flags & VM_GROWSUP) { - vm_end += stack_guard_gap; + if (gap != 0) { + vm_end += gap; if (vm_end < vma->vm_end) vm_end = -PAGE_SIZE; } -- 2.21.0