mm: keep a guard page below a grow-down stack segment

This is a rather minimally invasive patch to solve the problem of the
user stack growing into a memory mapped area below it.  Whenever we fill
the first page of the stack segment, expand the segment down by one
page.

Now, admittedly some odd application might _want_ the stack to grow down
into the preceding memory mapping, and so we may at some point need to
make this a process tunable (some people might also want to have more
than a single page of guarding), but let's try the minimal approach
first.

Tested with trivial application that maps a single page just below the
stack, and then starts recursing.  Without this, we will get a SIGSEGV
_after_ the stack has smashed the mapping.  With this patch, we'll get a
nice SIGBUS just as the stack touches the page just above the mapping.

Requested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 858829d..9606ceb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2760,6 +2760,26 @@ out_release:
 }
 
 /*
+ * This is like a special single-page "expand_downwards()",
+ * except we must first make sure that 'address-PAGE_SIZE'
+ * doesn't hit another vma.
+ *
+ * The "find_vma()" will do the right thing even if we wrap
+ */
+static inline int check_stack_guard_page(struct vm_area_struct *vma, unsigned long address)
+{
+	address &= PAGE_MASK;
+	if ((vma->vm_flags & VM_GROWSDOWN) && address == vma->vm_start) {
+		address -= PAGE_SIZE;
+		if (find_vma(vma->vm_mm, address) != vma)
+			return -ENOMEM;
+
+		expand_stack(vma, address);
+	}
+	return 0;
+}
+
+/*
  * We enter with non-exclusive mmap_sem (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
  * We return with mmap_sem still held, but pte unmapped and unlocked.
@@ -2772,6 +2792,9 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	spinlock_t *ptl;
 	pte_t entry;
 
+	if (check_stack_guard_page(vma, address) < 0)
+		return VM_FAULT_SIGBUS;
+
 	if (!(flags & FAULT_FLAG_WRITE)) {
 		entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
 						vma->vm_page_prot));