[PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack check

[Oleg Nesterov <oleg@redhat.com>]

The ->start_stack check in do_shmat() looks ugly and simply wrong.

1. ->start_stack is only valid right after exec(), the application
   can switch to another stack and even unmap this area.

2. The reason for this check is not clear at all. The application
   should know what it does. And why 4 pages? And why in fact it
   requires 5 pages?

3. This wrongly assumes that the stack can only grown down.

Personally I think we should simply kill this check, but I did not
dare to do this. So the patch only fixes the 1st problem (mostly to
avoid the usage of mm->start_stack) and ignores VM_GROWSUP.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 ipc/shm.c |   23 ++++++++++++++---------
 1 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 7fc9f9f..5c35fd4 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1166,19 +1166,24 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr,
 
 	down_write(&current->mm->mmap_sem);
 	if (addr && !(shmflg & SHM_REMAP)) {
-		err = -EINVAL;
-		if (addr + size < addr)
-			goto invalid;
+		unsigned long end = addr + size;
+		struct vm_area_struct *vma;
 
-		if (find_vma_intersection(current->mm, addr, addr + size))
+		err = -EINVAL;
+		if (end < addr)
 			goto invalid;
 		/*
-		 * If shm segment goes below stack, make sure there is some
-		 * space left for the stack to grow (at least 4 pages).
+		 * Ensure this segment doesn't overlap with the next vma.
+		 * If it is stack, make sure there is some space left for
+		 * the stack to grow, at least 4 pages. (Why?)
 		 */
-		if (addr < current->mm->start_stack &&
-		    addr > current->mm->start_stack - size - PAGE_SIZE * 5)
-			goto invalid;
+		vma = find_vma(current->mm, addr);
+		if (vma) {
+			if (vma->vm_flags & VM_GROWSDOWN)
+				end += PAGE_SIZE * 4; /* can't overflow */
+			if (end > vma->vm_start)
+				goto invalid;
+		}
 	}
 
 	addr = do_mmap_pgoff(file, addr, size, prot, flags, 0, &populate);
-- 
1.5.5.1