Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

On Thu, 2018-10-11 at 13:55 -0700, Andy Lutomirski wrote:
> On Thu, Oct 11, 2018 at 1:39 PM Jann Horn <jannh@google.com> wrote:
> > 
> > On Thu, Oct 11, 2018 at 5:20 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > > Create a guard area between VMAs to detect memory corruption.
> > 
> > [...]
> > > +config VM_AREA_GUARD
> > > +       bool "VM area guard"
> > > +       default n
> > > +       help
> > > +         Create a guard area between VM areas so that access beyond
> > > +         limit can be detected.
> > > +
> > >  endmenu
> > 
> > Sorry to bring this up so late, but Daniel Micay pointed out to me
> > that, given that VMA guards will raise the number of VMAs by
> > inhibiting vma_merge(), people are more likely to run into
> > /proc/sys/vm/max_map_count (which limits the number of VMAs to ~65k by
> > default, and can't easily be raised without risking an overflow of
> > page->_mapcount on systems with over ~800GiB of RAM, see
> > https://lore.kernel.org/lkml/20180208021112.GB14918@bombadil.infradead.org/
> > and replies) with this change.
> > 
> > Playing with glibc's memory allocator, it looks like glibc will use
> > mmap() for 128KB allocations; so at 65530*128KB=8GB of memory usage in
> > 128KB chunks, an application could run out of VMAs.
> 
> Ugh.
> 
> Do we have a free VM flag so we could do VM_GUARD to force a guard
> page?  (And to make sure that, when a new VMA is allocated, it won't
> be directly adjacent to a VM_GUARD VMA.)

Maybe something like the following?  These vm_start_gap()/vm_end_gap() are used
in many architectures.  Do we want to put them in a different series?  Comments?

Yu-cheng




diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0416a7204be3..92b580542411 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -224,11 +224,13 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_HIGH_ARCH_BIT_2	34	/* bit only usable on 64-bit
architectures */
 #define VM_HIGH_ARCH_BIT_3	35	/* bit only usable on 64-bit
architectures */
 #define VM_HIGH_ARCH_BIT_4	36	/* bit only usable on 64-bit
architectures */
+#define VM_HIGH_ARCH_BIT_5	37	/* bit only usable on 64-bit
architectures */
 #define VM_HIGH_ARCH_0	BIT(VM_HIGH_ARCH_BIT_0)
 #define VM_HIGH_ARCH_1	BIT(VM_HIGH_ARCH_BIT_1)
 #define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
 #define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
 #define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
+#define VM_HIGH_ARCH_5	BIT(VM_HIGH_ARCH_BIT_5)
 #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
 
 #ifdef CONFIG_ARCH_HAS_PKEYS
@@ -266,6 +268,12 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_MPX		VM_NONE
 #endif
 
+#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
+#define VM_GUARD	VM_HIGH_ARCH_5
+#else
+#define VM_GUARD	VM_NONE
+#endif
+
 #ifndef VM_GROWSUP
 # define VM_GROWSUP	VM_NONE
 #endif
@@ -2417,24 +2425,34 @@ 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)
+static inline unsigned long vm_start_gap(struct vm_area_struct *vma, vm_flags_t
flags)
 {
 	unsigned long vm_start = vma->vm_start;
+	unsigned long gap = 0;
+
+	if (vma->vm_flags & VM_GROWSDOWN)
+		gap = stack_guard_gap;
+	else if ((vma->vm_flags & VM_GUARD) || (flags & VM_GUARD))
+		gap = PAGE_SIZE;
+
+	vm_start -= gap;
+	if (vm_start > vma->vm_start)
+		vm_start = 0;
 
-	if (vma->vm_flags & VM_GROWSDOWN) {
-		vm_start -= stack_guard_gap;
-		if (vm_start > vma->vm_start)
-			vm_start = 0;
-	}
 	return vm_start;
 }
 
-static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
+static inline unsigned long vm_end_gap(struct vm_area_struct *vma, vm_flags_t
flags)
 {
 	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_GUARD) || (flags & VM_GUARD))
+		gap = PAGE_SIZE;
+
+	vm_end += gap;
+	if (vm_end < vma->vm_end)
+		vm_end = -PAGE_SIZE;
 
-	if (vma->vm_flags & VM_GROWSUP) {
-		vm_end += stack_guard_gap;
-		if (vm_end < vma->vm_end)
-			vm_end = -PAGE_SIZE;
-	}
 	return vm_end;
 }