diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2010-08-12 17:54:33 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-12 17:54:33 -0700
commit320b2b8de12698082609ebbc1a17165727f4c893 (patch)
parent2069601b3f0ea38170d4b509b89f3ca0a373bdc1 (diff)
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 858829d06a9..9606ceb3c16 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)
if (!(flags & FAULT_FLAG_WRITE)) {
entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),