Bug#674411: linux-image-2.6.32-5-xen-686: Kernel crashes if AIO is used on pages belonging to guests

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 30 May 2012, Ian Campbell wrote:
> On Thu, 2012-05-24 at 11:57 +0000, George Dunlap wrote:
> > Package: linux-2.6
> > Version: 2.6.32-45
> > Severity: normal
> > 
> > The kernel version in this package does not handle AIO to pages 
> > owned by guest domains ("foreign domains") properly.  Newer versions of qemu
> > will attempt to use AIO for disk reads and writes; this causes domain 0 to crash
> > if you're running.
> 
> Thanks George. Do you happen to have a log of the actual kernel crash?
> 
> I think Stefano (CCd) has been fixing this issue (or at least one which
> sounds very similar) in the upstream kernels recently. Stefano is there
> anything specific I can backport to a Jeremy xen.git 2.6.32 style pvops
> kernel? Or is there another appropriate fix?
> 
> FYI Squeeze's kernel is based on e73f4955a821 from Jeremy's tree,
> although with a bunch of fixes backported too.

We need to backport the m2p_override, this is a list of the relevant
upstream commits:

289b777eac19c811b474593b4d2fd14e46340c23
b5eafe924bb054d7c56e6ebd18106352e8a3f916
448f2831934381e9d3c4d93e700ba7bbe14612dc
9b705f0e98c489b18ba22a6eab9d694b546c8552
87f1d40a706bdebdc8f959b9ac291d0d8fdfcc7e
e1b478e4ec4477520767d1a920433626263a2a6b
b254244d2682fe975630f176c25a4444cc4e088d
cf8d91633ddef9e816ccbf3da833c79ce508988d
0f4b49eaf25e661fbe63a5370b7781166b34d616
0930bba674e248b921ea659b036ff02564e5a5f4
dc4972a4e2f3fee1663bd0670dfc4cd798d5f9b2


The backport is non-trivial, but I can help.

As an alternative we could add a simple check to spot an attempt to use
AIO on a granted page and return an error (still better than crashing):

---

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index a33e443..f060770 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -573,6 +573,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	vma->vm_flags |= VM_RESERVED;
 	vma->vm_flags |= VM_DONTCOPY;
 	vma->vm_flags |= VM_DONTEXPAND;
+	vma->vm_flags |= VM_FOREIGN;
 
 	vma->vm_private_data = map;
 	map->vma = vma;
diff --git a/fs/aio.c b/fs/aio.c
index 02a2c93..70b8854 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1655,6 +1655,8 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 	for (i=0; i<nr; i++) {
 		struct iocb __user *user_iocb;
 		struct iocb tmp;
+		struct vm_area_struct *vma = NULL;
+		struct iovec *v = NULL;
 
 		if (unlikely(__get_user(user_iocb, iocbpp + i))) {
 			ret = -EFAULT;
@@ -1666,6 +1668,19 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 			break;
 		}
 
+		down_read(&current->mm->mmap_sem);
+		v = (struct iovec *) tmp.aio_buf;
+		/* just checking the first iovec is enough for now */
+		if (v != NULL)
+			vma = find_vma(current->mm, (unsigned long) v->iov_base);
+		if (vma != NULL && vma->vm_flags & (VM_FOREIGN|VM_DONTEXPAND)) {
+			WARN_ONCE(1, "missing AIO support on granted pages\n");
+			ret = -EFAULT;
+			up_read(&current->mm->mmap_sem);
+			break;
+		}
+		up_read(&current->mm->mmap_sem);
+
 		ret = io_submit_one(ctx, user_iocb, &tmp);
 		if (ret)
 			break;