[PATCH v16 16/26] mm: Add guard pages around a shadow stack.
From: Yu-cheng Yu <hidden>
Date: 2020-12-09 22:29:05
Also in:
linux-arch, linux-doc, linux-mm, lkml
Subsystem:
memory management - core, the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the first and the last elements in the range, effectively touches those memory areas. The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and 255 * 4 = 1020 bytes by INCSSPD. Both ranges are far from PAGE_SIZE. Thus, putting a gap page on both ends of a shadow stack prevents INCSSP, CALL, and RET from going beyond. Signed-off-by: Yu-cheng Yu <redacted> --- arch/x86/include/asm/page_64_types.h | 10 ++++++++++ include/linux/mm.h | 24 ++++++++++++++++++++---- 2 files changed, 30 insertions(+), 4 deletions(-)
diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
index 3f49dac03617..2b2991e5f344 100644
--- a/arch/x86/include/asm/page_64_types.h
+++ b/arch/x86/include/asm/page_64_types.h@@ -97,6 +97,16 @@ #define STACK_TOP TASK_SIZE_LOW #define STACK_TOP_MAX TASK_SIZE_MAX +/* + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D). INCSSPQ + * moves shadow stack pointer up to 255 * 8 = ~2 KB (~1KB for INCSSPD) and + * touches the first and the last element in the range, which triggers a + * page fault if the range is not in a shadow stack. Because of this, + * creating 4-KB guard pages around a shadow stack prevents these + * instructions from going beyond. + */ +#define ARCH_SHADOW_STACK_GUARD_GAP PAGE_SIZE + /* * Maximum kernel image size is limited to 1 GiB, due to the fixmap living * in the next 1 GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
diff --git a/include/linux/mm.h b/include/linux/mm.h
index b111f23a1be9..0bb6c265446d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h@@ -2639,6 +2639,10 @@ extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf); int __must_check write_one_page(struct page *page); void task_dirty_inc(struct task_struct *tsk); +#ifndef ARCH_SHADOW_STACK_GUARD_GAP +#define ARCH_SHADOW_STACK_GUARD_GAP 0 +#endif + extern unsigned long stack_guard_gap; /* Generic expand stack which grows the stack according to GROWS{UP,DOWN} */ extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
@@ -2671,9 +2675,15 @@ 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 = ARCH_SHADOW_STACK_GUARD_GAP; + + if (gap != 0) { + vm_start -= gap; if (vm_start > vma->vm_start) vm_start = 0; }
@@ -2683,9 +2693,15 @@ 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 = ARCH_SHADOW_STACK_GUARD_GAP; - 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