lsm: add backing_file LSM hooks

Stacked filesystems such as overlayfs do not currently provide the
necessary mechanisms for LSMs to properly enforce access controls on the
mmap() and mprotect() operations.  In order to resolve this gap, a LSM
security blob is being added to the backing_file struct and the following
new LSM hooks are being created:

 security_backing_file_alloc()
 security_backing_file_free()
 security_mmap_backing_file()

The first two hooks are to manage the lifecycle of the LSM security blob
in the backing_file struct, while the third provides a new mmap() access
control point for the underlying backing file.  It is also expected that
LSMs will likely want to update their security_file_mprotect() callback
to address issues with their mprotect() controls, but that does not
require a change to the security_file_mprotect() LSM hook.

There are a three other small changes to support these new LSM hooks:
* Pass the user file associated with a backing file down to
alloc_empty_backing_file() so it can be included in the
security_backing_file_alloc() hook.
* Add getter and setter functions for the backing_file struct LSM blob
as the backing_file struct remains private to fs/file_table.c.
* Constify the file struct field in the LSM common_audit_data struct to
better support LSMs that need to pass a const file struct pointer into
the common LSM audit code.

Thanks to Arnd Bergmann for identifying the missing EXPORT_SYMBOL_GPL()
and supplying a fixup.

Cc: stable@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-unionfs@vger.kernel.org
Cc: linux-erofs@lists.ozlabs.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: pcmoore

fs/backing-file.c

@@ -12,6 +12,7 @@
 #include <linux/backing-file.h>
 #include <linux/splice.h>
 #include <linux/mm.h>
+#include <linux/security.h>
 
 #include "internal.h"
 
@@ -29,14 +30,15 @@
  * returned file into a container structure that also stores the stacked
  * file's path, which can be retrieved using backing_file_user_path().
  */
-struct file *backing_file_open(const struct path *user_path, int flags,
+struct file *backing_file_open(const struct file *user_file, int flags,
 			       const struct path *real_path,
 			       const struct cred *cred)
 {
+	const struct path *user_path = &user_file->f_path;
 	struct file *f;
 	int error;
 
-	f = alloc_empty_backing_file(flags, cred);
+	f = alloc_empty_backing_file(flags, cred, user_file);
 	if (IS_ERR(f))
 		return f;
 
@@ -52,15 +54,16 @@ struct file *backing_file_open(const struct path *user_path, int flags,
 }
 EXPORT_SYMBOL_GPL(backing_file_open);
 
-struct file *backing_tmpfile_open(const struct path *user_path, int flags,
+struct file *backing_tmpfile_open(const struct file *user_file, int flags,
 				  const struct path *real_parentpath,
 				  umode_t mode, const struct cred *cred)
 {
 	struct mnt_idmap *real_idmap = mnt_idmap(real_parentpath->mnt);
+	const struct path *user_path = &user_file->f_path;
 	struct file *f;
 	int error;
 
-	f = alloc_empty_backing_file(flags, cred);
+	f = alloc_empty_backing_file(flags, cred, user_file);
 	if (IS_ERR(f))
 		return f;
 
@@ -336,8 +339,13 @@ int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
 
 	vma_set_file(vma, file);
 
-	scoped_with_creds(ctx->cred)
+	scoped_with_creds(ctx->cred) {
+		ret = security_mmap_backing_file(vma, file, user_file);
+		if (ret)
+			return ret;
+
 		ret = vfs_mmap(vma->vm_file, vma);
+	}
 
 	if (ctx->accessed)
 		ctx->accessed(user_file);

fs/erofs/ishare.c

@@ -4,6 +4,7 @@
  */
 #include <linux/xxhash.h>
 #include <linux/mount.h>
+#include <linux/security.h>
 #include "internal.h"
 #include "xattr.h"
 
@@ -102,7 +103,8 @@ static int erofs_ishare_file_open(struct inode *inode, struct file *file)
 
 	if (file->f_flags & O_DIRECT)
 		return -EINVAL;
-	realfile = alloc_empty_backing_file(O_RDONLY|O_NOATIME, current_cred());
+	realfile = alloc_empty_backing_file(O_RDONLY|O_NOATIME, current_cred(),
+					    file);
 	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);
 	ihold(sharedinode);
@@ -146,8 +148,14 @@ static ssize_t erofs_ishare_file_read_iter(struct kiocb *iocb,
 static int erofs_ishare_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct file *realfile = file->private_data;
+	int err;
 
 	vma_set_file(vma, realfile);
+
+	err = security_mmap_backing_file(vma, realfile, file);
+	if (err)
+		return err;
+
 	return generic_file_readonly_mmap(file, vma);
 }
 

fs/file_table.c

@@ -50,6 +50,9 @@ struct backing_file {
 		struct path user_path;
 		freeptr_t bf_freeptr;
 	};
+#ifdef CONFIG_SECURITY
+	void *security;
+#endif
 };
 
 #define backing_file(f) container_of(f, struct backing_file, file)
@@ -66,8 +69,21 @@ void backing_file_set_user_path(struct file *f, const struct path *path)
 }
 EXPORT_SYMBOL_GPL(backing_file_set_user_path);
 
+#ifdef CONFIG_SECURITY
+void *backing_file_security(const struct file *f)
+{
+	return backing_file(f)->security;
+}
+
+void backing_file_set_security(struct file *f, void *security)
+{
+	backing_file(f)->security = security;
+}
+#endif /* CONFIG_SECURITY */
+
 static inline void backing_file_free(struct backing_file *ff)
 {
+	security_backing_file_free(&ff->file);
 	path_put(&ff->user_path);
 	kmem_cache_free(bfilp_cachep, ff);
 }
@@ -288,10 +304,12 @@ struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred)
 	return f;
 }
 
-static int init_backing_file(struct backing_file *ff)
+static int init_backing_file(struct backing_file *ff,
+			     const struct file *user_file)
 {
 	memset(&ff->user_path, 0, sizeof(ff->user_path));
-	return 0;
+	backing_file_set_security(&ff->file, NULL);
+	return security_backing_file_alloc(&ff->file, user_file);
 }
 
 /*
@@ -301,7 +319,8 @@ static int init_backing_file(struct backing_file *ff)
  * This is only for kernel internal use, and the allocate file must not be
  * installed into file tables or such.
  */
-struct file *alloc_empty_backing_file(int flags, const struct cred *cred)
+struct file *alloc_empty_backing_file(int flags, const struct cred *cred,
+				      const struct file *user_file)
 {
 	struct backing_file *ff;
 	int error;
@@ -318,7 +337,7 @@ struct file *alloc_empty_backing_file(int flags, const struct cred *cred)
 
 	/* The f_mode flags must be set before fput(). */
 	ff->file.f_mode |= FMODE_BACKING | FMODE_NOACCOUNT;
-	error = init_backing_file(ff);
+	error = init_backing_file(ff, user_file);
 	if (unlikely(error)) {
 		fput(&ff->file);
 		return ERR_PTR(error);

fs/fuse/passthrough.c

@@ -167,7 +167,7 @@ struct fuse_backing *fuse_passthrough_open(struct file *file, int backing_id)
 		goto out;
 
 	/* Allocate backing file per fuse file to store fuse path */
-	backing_file = backing_file_open(&file->f_path, file->f_flags,
+	backing_file = backing_file_open(file, file->f_flags,
 					 &fb->file->f_path, fb->cred);
 	err = PTR_ERR(backing_file);
 	if (IS_ERR(backing_file)) {

fs/internal.h

@@ -106,7 +106,8 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
  */
 struct file *alloc_empty_file(int flags, const struct cred *cred);
 struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred);
-struct file *alloc_empty_backing_file(int flags, const struct cred *cred);
+struct file *alloc_empty_backing_file(int flags, const struct cred *cred,
+				      const struct file *user_file);
 void backing_file_set_user_path(struct file *f, const struct path *path);
 
 static inline void file_put_write_access(struct file *file)

fs/overlayfs/dir.c

@@ -1374,7 +1374,7 @@ static int ovl_create_tmpfile(struct file *file, struct dentry *dentry,
 				return PTR_ERR(cred);
 
 			ovl_path_upper(dentry->d_parent, &realparentpath);
-			realfile = backing_tmpfile_open(&file->f_path, flags, &realparentpath,
+			realfile = backing_tmpfile_open(file, flags, &realparentpath,
 							mode, current_cred());
 			err = PTR_ERR_OR_ZERO(realfile);
 			pr_debug("tmpfile/open(%pd2, 0%o) = %i\n", realparentpath.dentry, mode, err);

fs/overlayfs/file.c

@@ -48,7 +48,7 @@ static struct file *ovl_open_realfile(const struct file *file,
 			if (!inode_owner_or_capable(real_idmap, realinode))
 				flags &= ~O_NOATIME;
 
-			realfile = backing_file_open(file_user_path(file),
+			realfile = backing_file_open(file,
 						     flags, realpath, current_cred());
 		}
 	}

include/linux/backing-file.h

@@ -18,10 +18,10 @@ struct backing_file_ctx {
 	void (*end_write)(struct kiocb *iocb, ssize_t);
 };
 
-struct file *backing_file_open(const struct path *user_path, int flags,
+struct file *backing_file_open(const struct file *user_file, int flags,
 			       const struct path *real_path,
 			       const struct cred *cred);
-struct file *backing_tmpfile_open(const struct path *user_path, int flags,
+struct file *backing_tmpfile_open(const struct file *user_file, int flags,
 				  const struct path *real_parentpath,
 				  umode_t mode, const struct cred *cred);
 ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,

include/linux/fs.h

@@ -2475,6 +2475,19 @@ struct file *dentry_create(struct path *path, int flags, umode_t mode,
 			   const struct cred *cred);
 const struct path *backing_file_user_path(const struct file *f);
 
+#ifdef CONFIG_SECURITY
+void *backing_file_security(const struct file *f);
+void backing_file_set_security(struct file *f, void *security);
+#else
+static inline void *backing_file_security(const struct file *f)
+{
+	return NULL;
+}
+static inline void backing_file_set_security(struct file *f, void *security)
+{
+}
+#endif /* CONFIG_SECURITY */
+
 /*
  * When mmapping a file on a stackable filesystem (e.g., overlayfs), the file
  * stored in ->vm_file is a backing file whose f_inode is on the underlying

include/linux/lsm_audit.h

@@ -94,7 +94,7 @@ struct common_audit_data {
 #endif
 		char *kmod_name;
 		struct lsm_ioctlop_audit *op;
-		struct file *file;
+		const struct file *file;
 		struct lsm_ibpkey_audit *ibpkey;
 		struct lsm_ibendport_audit *ibendport;
 		int reason;