From: Jiri Pirko <jpirko@redhat.com> Date: Thu, 19 Aug 2010 11:28:33 -0400 Subject: [mm] keep a guard page below a grow-down stack segment Message-id: <1282217317-11853-2-git-send-email-jpirko@redhat.com> Patchwork-id: 27711 O-Subject: [PATCH RHEL5.6 1/5] mm: keep a guard page below a grow-down stack segment Bugzilla: 607858 RH-Acked-by: Rik van Riel <riel@redhat.com> RH-Acked-by: Jarod Wilson <jarod@redhat.com> 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. Signed-off-by: Jiri Pirko <jpirko@redhat.com> diff --git a/mm/memory.c b/mm/memory.c index 1ebc53e..fef663d 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2353,6 +2353,26 @@ 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. @@ -2365,6 +2385,9 @@ 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 (write_access) { /* Allocate our own private page. */ pte_unmap(page_table);