[PATCH v1 1/2] fs: make all new mount api fds cloexec by default

[Christian Brauner]

This makes all file descriptors returned from new syscalls of the new mount
api cloexec by default.

>From a userspace perspective it is rarely the case that fds are supposed to
be inherited across exec. In fact, most of the time userspace either needs
to remember to pass the <SPECIFIC>_CLOEXEC flag along or needs to invoke
fcntl() on the fd to prevent leaking the fd. This is a much bigger issue
than forgetting to remove the cloexec flag to inherit the fd.
For old fd types we can't break userspace. But for new ones we should
whenever reasonable make them cloexec by default (Examples of this policy
are the new seccomp notify fds and also pidfds.). If userspace wants to
inherit fds across exec they can remove the O_CLOEXEC flag and need to opt
in to inheritance explicitly.

Note, this also has the advantage that we can get rid of all the special
flags per file descriptor type for the new mount api. In total this lets us
remove 4 flags:
- FSMOUNT_CLOEXEC
- FSOPEN_CLOEXEC
- FSPICK_CLOEXEC
- OPEN_TREE_CLOEXEC

Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
---
v1:
- David Howells <dhowells@xxxxxxxxxx>:
  - ensure that only O_CLOEXEC is passed so that fd allocation doesn't
    break when new flags are added to a syscall
---
 fs/fsopen.c                | 13 ++++++-------
 fs/namespace.c             | 11 ++++-------
 include/uapi/linux/mount.h | 18 +++---------------
 3 files changed, 13 insertions(+), 29 deletions(-)

diff --git a/fs/fsopen.c b/fs/fsopen.c
index 3bb9c0c8cbcc..a38fa8c616cf 100644
--- a/fs/fsopen.c
+++ b/fs/fsopen.c
@@ -88,12 +88,12 @@ const struct file_operations fscontext_fops = {
 /*
  * Attach a filesystem context to a file and an fd.
  */
-static int fscontext_create_fd(struct fs_context *fc, unsigned int o_flags)
+static int fscontext_create_fd(struct fs_context *fc)
 {
 	int fd;
 
 	fd = anon_inode_getfd("fscontext", &fscontext_fops, fc,
-			      O_RDWR | o_flags);
+			      O_RDWR | O_CLOEXEC);
 	if (fd < 0)
 		put_fs_context(fc);
 	return fd;
@@ -126,7 +126,7 @@ SYSCALL_DEFINE2(fsopen, const char __user *, _fs_name, unsigned int, flags)
 	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
-	if (flags & ~FSOPEN_CLOEXEC)
+	if (flags)
 		return -EINVAL;
 
 	fs_name = strndup_user(_fs_name, PAGE_SIZE);
@@ -149,7 +149,7 @@ SYSCALL_DEFINE2(fsopen, const char __user *, _fs_name, unsigned int, flags)
 	if (ret < 0)
 		goto err_fc;
 
-	return fscontext_create_fd(fc, flags & FSOPEN_CLOEXEC ? O_CLOEXEC : 0);
+	return fscontext_create_fd(fc);
 
 err_fc:
 	put_fs_context(fc);
@@ -169,8 +169,7 @@ SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags
 	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
-	if ((flags & ~(FSPICK_CLOEXEC |
-		       FSPICK_SYMLINK_NOFOLLOW |
+	if ((flags & ~(FSPICK_SYMLINK_NOFOLLOW |
 		       FSPICK_NO_AUTOMOUNT |
 		       FSPICK_EMPTY_PATH)) != 0)
 		return -EINVAL;
@@ -203,7 +202,7 @@ SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags
 		goto err_fc;
 
 	path_put(&target);
-	return fscontext_create_fd(fc, flags & FSPICK_CLOEXEC ? O_CLOEXEC : 0);
+	return fscontext_create_fd(fc);
 
 err_fc:
 	put_fs_context(fc);
diff --git a/fs/namespace.c b/fs/namespace.c
index 3357c3d65475..b024e2a05384 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2369,11 +2369,8 @@ SYSCALL_DEFINE3(open_tree, int, dfd, const char *, filename, unsigned, flags)
 	int error;
 	int fd;
 
-	BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);
-
 	if (flags & ~(AT_EMPTY_PATH | AT_NO_AUTOMOUNT | AT_RECURSIVE |
-		      AT_SYMLINK_NOFOLLOW | OPEN_TREE_CLONE |
-		      OPEN_TREE_CLOEXEC))
+		      AT_SYMLINK_NOFOLLOW | OPEN_TREE_CLONE))
 		return -EINVAL;
 
 	if ((flags & (AT_RECURSIVE | OPEN_TREE_CLONE)) == AT_RECURSIVE)
@@ -2389,7 +2386,7 @@ SYSCALL_DEFINE3(open_tree, int, dfd, const char *, filename, unsigned, flags)
 	if (detached && !may_mount())
 		return -EPERM;
 
-	fd = get_unused_fd_flags(flags & O_CLOEXEC);
+	fd = get_unused_fd_flags(O_CLOEXEC);
 	if (fd < 0)
 		return fd;
 
@@ -3352,7 +3349,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	if (!may_mount())
 		return -EPERM;
 
-	if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
+	if (flags)
 		return -EINVAL;
 
 	if (attr_flags & ~(MOUNT_ATTR_RDONLY |
@@ -3457,7 +3454,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	}
 	file->f_mode |= FMODE_NEED_UNMOUNT;
 
-	ret = get_unused_fd_flags((flags & FSMOUNT_CLOEXEC) ? O_CLOEXEC : 0);
+	ret = get_unused_fd_flags(O_CLOEXEC);
 	if (ret >= 0)
 		fd_install(ret, file);
 	else
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 96a0240f23fe..c688e4ac843b 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -59,7 +59,6 @@
  * open_tree() flags.
  */
 #define OPEN_TREE_CLONE		1		/* Clone the target tree and attach the clone */
-#define OPEN_TREE_CLOEXEC	O_CLOEXEC	/* Close the file on execve() */
 
 /*
  * move_mount() flags.
@@ -72,18 +71,12 @@
 #define MOVE_MOUNT_T_EMPTY_PATH		0x00000040 /* Empty to path permitted */
 #define MOVE_MOUNT__MASK		0x00000077
 
-/*
- * fsopen() flags.
- */
-#define FSOPEN_CLOEXEC		0x00000001
-
 /*
  * fspick() flags.
  */
-#define FSPICK_CLOEXEC		0x00000001
-#define FSPICK_SYMLINK_NOFOLLOW	0x00000002
-#define FSPICK_NO_AUTOMOUNT	0x00000004
-#define FSPICK_EMPTY_PATH	0x00000008
+#define FSPICK_SYMLINK_NOFOLLOW	0x00000001
+#define FSPICK_NO_AUTOMOUNT	0x00000002
+#define FSPICK_EMPTY_PATH	0x00000004
 
 /*
  * The type of fsconfig() call made.
@@ -99,11 +92,6 @@ enum fsconfig_command {
 	FSCONFIG_CMD_RECONFIGURE = 7,	/* Invoke superblock reconfiguration */
 };
 
-/*
- * fsmount() flags.
- */
-#define FSMOUNT_CLOEXEC		0x00000001
-
 /*
  * Mount attributes.
  */
-- 
2.21.0