[PATCH v4 03/11] fs: Add fd_install_received() wrapper for __fd_install_received()

Kees Cook
2020-06-16T03:27:13Z

For both pidfd and seccomp, the __user pointer is not used. Update
__fd_install_received() to make writing to ufd optional. (ufd
itself cannot checked for NULL because this changes the SCM_RIGHTS
interface behavior.) In these cases, the new fd needs to be returned
on success. Update the existing callers to handle it. Add new wrapper
fd_install_received() for pidfd and seccomp that does not use the ufd
argument.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
fs/file.c | 26 +++++++++++++++++---------
include/linux/file.h | 10 +++++++---
net/compat.c | 2 +-
net/core/scm.c | 2 +-
4 files changed, 26 insertions(+), 14 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index fcfddae0d252..14a8ef74efb2 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -944,11 +944,14 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
* @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.
+ * checks and count updates. Optionally writes the fd number to userspace, if
+ * @ufd_required is true (@ufd cannot just be tested for NULL because NULL may
+ * actually get passed into SCM_RIGHTS).
*
- * Returns -ve on error.
+ * Returns newly install fd or -ve on error.
*/
-int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_flags)
+int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
+ unsigned int o_flags)
{
struct socket *sock;
int new_fd;
@@ -962,20 +965,25 @@ int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_fla
if (new_fd < 0)
return new_fd;

- error = put_user(new_fd, ufd);
- if (error) {
- put_unused_fd(new_fd);
- return error;
+ if (ufd_required) {
+ error = put_user(new_fd, ufd);
+ if (error) {
+ put_unused_fd(new_fd);
+ return error;
+ }
}

- /* Bump the usage count and install the file. */
+ /* Bump the usage count and install the file. The resulting value of
+ * "error" is ignored here since we only need to take action when
+ * the file is a socket and testing "sock" for NULL is sufficient.
+ */
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;
+ return new_fd;
}

static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
diff --git a/include/linux/file.h b/include/linux/file.h
index fe18a1a0d555..999a2c56db07 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -91,12 +91,16 @@ 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);
+extern int __fd_install_received(struct file *file, bool ufd_required,
+ 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);
+ return __fd_install_received(file, true, ufd, o_flags);
+}
+static inline int fd_install_received(struct file *file, unsigned int o_flags)
+{
+ return __fd_install_received(file, false, NULL, o_flags);
}

extern void flush_delayed_fput(void);
diff --git a/net/compat.c b/net/compat.c
index 94f288e8dac5..71494337cca7 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -299,7 +299,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)

for (i = 0; i < fdmax; i++) {
err = fd_install_received_user(scm->fp->fp[i], cmsg_data + i, o_flags);
- if (err)
+ if (err < 0)
break;
}

diff --git a/net/core/scm.c b/net/core/scm.c
index df190f1fdd28..b9a0442ebd26 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -307,7 +307,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)

for (i = 0; i < fdmax; i++) {
err = fd_install_received_user(scm->fp->fp[i], cmsg_data + i, o_flags);
- if (err)
+ if (err < 0)
break;
}

--
2.25.1