On Thu, Jun 18, 2020 at 10:56:14AM +0200, Christian Brauner wrote:

On Mon, Jun 15, 2020 at 08:25:15PM -0700, Kees Cook wrote:

quoted

In preparation for users of the "install a received file" logic outside
of net/ (pidfd and seccomp), relocate and rename __scm_install_fd() from
net/core/scm.c to __fd_install_received() in fs/file.c, and provide a
wrapper named fd_install_received_user(), as future patches will change
the interface to __fd_install_received().

Signed-off-by: Kees Cook <redacted>
---
 fs/file.c            | 47 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/file.h |  8 ++++++++
 include/net/scm.h    |  1 -
 net/compat.c         |  2 +-
 net/core/scm.c       | 32 +-----------------------------
 5 files changed, 57 insertions(+), 33 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index abb8b7081d7a..fcfddae0d252 100644
--- a/fs/file.c
+++ b/fs/file.c

@@ -11,6 +11,7 @@
 #include <linux/export.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
+#include <linux/net.h>
 #include <linux/sched/signal.h>
 #include <linux/slab.h>
 #include <linux/file.h>

@@ -18,6 +19,8 @@
 #include <linux/bitops.h>
 #include <linux/spinlock.h>
 #include <linux/rcupdate.h>
+#include <net/cls_cgroup.h>
+#include <net/netprio_cgroup.h>
 
 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 unsigned int sysctl_nr_open_min = BITS_PER_LONG;

@@ -931,6 +934,50 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
 	return err;
 }
 
+/**
+ * __fd_install_received() - Install received file into file descriptor table
+ *
+ * @fd: fd to install into (if negative, a new fd will be allocated)
+ * @file: struct file that was received from another process
+ * @ufd_required: true to use @ufd for writing fd number to userspace
+ * @ufd: __user pointer to write new fd number to
+ * @o_flags: the O_* flags to apply to the new fd entry
+ *
+ * Installs a received file into the file descriptor table, with appropriate
+ * checks and count updates. Optionally writes the fd number to userspace.
+ *
+ * Returns -ve on error.
+ */
+int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_flags)
+{
+	struct socket *sock;
+	int new_fd;
+	int error;
+
+	error = security_file_receive(file);
+	if (error)
+		return error;
+
+	new_fd = get_unused_fd_flags(o_flags);
+	if (new_fd < 0)
+		return new_fd;
+
+	error = put_user(new_fd, ufd);
+	if (error) {
+		put_unused_fd(new_fd);
+		return error;
+	}
+
+	/* Bump the usage count and install the file. */
+	sock = sock_from_file(file, &error);
+	if (sock) {
+		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
+		sock_update_classid(&sock->sk->sk_cgrp_data);
+	}
+	fd_install(new_fd, get_file(file));
+	return 0;
+}
+
 static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 {
 	int err = -EBADF;

diff --git a/include/linux/file.h b/include/linux/file.h
index 122f80084a3e..fe18a1a0d555 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h

@@ -91,6 +91,14 @@ extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
+extern int __fd_install_received(struct file *file, int __user *ufd,
+				 unsigned int o_flags);
+static inline int fd_install_received_user(struct file *file, int __user *ufd,
+					   unsigned int o_flags)
+{
+	return __fd_install_received(file, ufd, o_flags);
+}

Shouldn't this be the other way around such that
fd_install_received_user() is the workhorse that has a "ufd" argument
and fd_install_received() is the static inline function that doesn't?

extern int fd_install_received_user(struct file *file, int __user *ufd, unsigned int o_flags)
static inline int fd_install_received(struct file *file, unsigned int o_flags)
{
	return fd_install_received_user(file, NULL, o_flags);
}

So, I think it's all worked out in v5[1], so the helper argument handling
is better for the ufd case, as David pointed out earlier. (As in,
I think you're reacting to the same general problem here.)

(So I'm on vacation this week some my reviews are selective and spotty
but I promise to be back next week. :))

No worries!

-Kees

[1] https://lore.kernel.org/lkml/20200617220327.3731559-1-keescook@chromium.org/ (local)

-- 
Kees Cook