[PATCH v5 3/3] Wire UFFD up to SELinux

[Daniel Colascione <dancol@google.com>]

This change gives userfaultfd file descriptors a real security
context, allowing policy to act on them.

Signed-off-by: Daniel Colascione <dancol@google.com>
---
 fs/userfaultfd.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 37df7c9eedb1..78ff5d898733 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -76,6 +76,8 @@ struct userfaultfd_ctx {
 	bool mmap_changing;
 	/* mm with one ore more vmas attached to this userfaultfd_ctx */
 	struct mm_struct *mm;
+	/* The inode that owns this context --- not a strong reference.  */
+	const struct inode *owner;
 };
 
 struct userfaultfd_fork_ctx {
@@ -1022,8 +1024,10 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 {
 	int fd;
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
-			      O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
+	fd = anon_inode_getfd_secure(
+		"[userfaultfd]", &userfaultfd_fops, new,
+		O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS),
+		ctx->owner);
 	if (fd < 0)
 		return fd;
 
@@ -1945,6 +1949,7 @@ static void init_once_userfaultfd_ctx(void *mem)
 
 SYSCALL_DEFINE1(userfaultfd, int, flags)
 {
+	struct file *file;
 	struct userfaultfd_ctx *ctx;
 	int fd;
 
@@ -1974,8 +1979,25 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	/* prevent the mm struct to be freed */
 	mmgrab(ctx->mm);
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
-			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+	file = anon_inode_getfile_secure(
+		"[userfaultfd]", &userfaultfd_fops, ctx,
+		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
+		NULL);
+	if (IS_ERR(file)) {
+		fd = PTR_ERR(file);
+		goto out;
+	}
+
+	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		fput(file);
+		goto out;
+	}
+
+	ctx->owner = file_inode(file);
+	fd_install(fd, file);
+
+out:
 	if (fd < 0) {
 		mmdrop(ctx->mm);
 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
-- 
2.26.0.rc2.310.g2932bb562d-goog