[PATCH 2/3] userfaultfd: prevent unregistering VMAs through a different userfaultfd

[Tal Zussman <tz2294@columbia.edu>]

Currently, a VMA registered with a uffd can be unregistered through a
different uffd asssociated with the same mm_struct.

Change this behavior to be stricter by requiring VMAs to be unregistered
through the same uffd they were registered with.

While at it, correct the comment for the no userfaultfd case. This seems
to be a copy-paste artifact from the analagous userfaultfd_register()
check.

Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 fs/userfaultfd.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 22f4bf956ba1..9289e30b24c4 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1477,6 +1477,16 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		if (!vma_can_userfault(cur, cur->vm_flags, wp_async))
 			goto out_unlock;
 
+		/*
+		 * Check that this vma isn't already owned by a different
+		 * userfaultfd. This provides for more strict behavior by
+		 * preventing a VMA registered with a userfaultfd from being
+		 * unregistered through a different userfaultfd.
+		 */
+		if (cur->vm_userfaultfd_ctx.ctx &&
+		    cur->vm_userfaultfd_ctx.ctx != ctx)
+			goto out_unlock;
+
 		found = true;
 	} for_each_vma_range(vmi, cur, end);
 	BUG_ON(!found);
@@ -1491,10 +1501,11 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		cond_resched();
 
 		BUG_ON(!vma_can_userfault(vma, vma->vm_flags, wp_async));
+		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
+		       vma->vm_userfaultfd_ctx.ctx != ctx);
 
 		/*
-		 * Nothing to do: this vma is already registered into this
-		 * userfaultfd and with the right tracking mode too.
+		 * Nothing to do: this vma is not registered with userfaultfd.
 		 */
 		if (!vma->vm_userfaultfd_ctx.ctx)
 			goto skip;

-- 
2.39.5