[1/3] mm: keep a guard page below a grow-down stack segment

From: Greg KH
Date: Fri Aug 13 2010 - 17:49:23 EST


2.6.27-stable review patch. If anyone has any objections, please let us know.

------------------

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

commit 320b2b8de12698082609ebbc1a17165727f4c893 upstream.

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@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
mm/memory.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2396,6 +2396,26 @@ out_nomap:
}

/*
+ * 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.
@@ -2408,6 +2428,9 @@ static int do_anonymous_page(struct mm_s
spinlock_t *ptl;
pte_t entry;

+ if (check_stack_guard_page(vma, address) < 0)
+ return VM_FAULT_SIGBUS;
+
/* Allocate our own private page. */
pte_unmap(page_table);



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