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Subject[PATCH v6 5/7] fs: Expand __receive_fd() to accept existing fd
Date
Expand __receive_fd() with support for replace_fd() for the coming seccomp
"addfd" ioctl(). Add new wrapper receive_fd_replace() for the new behavior
and update existing wrappers to retain old behavior.

Thanks to Colin Ian King <colin.king@canonical.com> for pointing out an
uninitialized variable exposure in an earlier version of this patch.

Reviewed-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
fs/file.c | 24 ++++++++++++++++++------
include/linux/file.h | 10 +++++++---
2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 0efdcf413210..11313ff36802 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -937,6 +937,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
/**
* __receive_fd() - 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: __user pointer to write new fd number to
* @o_flags: the O_* flags to apply to the new fd entry
@@ -950,7 +951,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
*
* Returns newly install fd or -ve on error.
*/
-int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
+int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flags)
{
struct socket *sock;
int new_fd;
@@ -960,18 +961,30 @@ int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
if (error)
return error;

- new_fd = get_unused_fd_flags(o_flags);
- if (new_fd < 0)
- return new_fd;
+ if (fd < 0) {
+ new_fd = get_unused_fd_flags(o_flags);
+ if (new_fd < 0)
+ return new_fd;
+ } else
+ new_fd = fd;

if (ufd) {
error = put_user(new_fd, ufd);
if (error) {
- put_unused_fd(new_fd);
+ if (fd < 0)
+ put_unused_fd(new_fd);
return error;
}
}

+ if (fd < 0)
+ fd_install(new_fd, get_file(file));
+ else {
+ error = replace_fd(new_fd, file, o_flags);
+ if (error)
+ return error;
+ }
+
/*
* Bump the usage count and install the file. The resulting value of
* "error" is ignored here since we only need to take action when
@@ -982,7 +995,6 @@ int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
sock_update_netprioidx(&sock->sk->sk_cgrp_data);
sock_update_classid(&sock->sk->sk_cgrp_data);
}
- fd_install(new_fd, get_file(file));
return new_fd;
}

diff --git a/include/linux/file.h b/include/linux/file.h
index d9fee9f5c8da..225982792fa2 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -92,18 +92,22 @@ extern void put_unused_fd(unsigned int fd);

extern void fd_install(unsigned int fd, struct file *file);

-extern int __receive_fd(struct file *file, int __user *ufd,
+extern int __receive_fd(int fd, struct file *file, int __user *ufd,
unsigned int o_flags);
static inline int receive_fd_user(struct file *file, int __user *ufd,
unsigned int o_flags)
{
if (ufd == NULL)
return -EFAULT;
- return __receive_fd(file, ufd, o_flags);
+ return __receive_fd(-1, file, ufd, o_flags);
}
static inline int receive_fd(struct file *file, unsigned int o_flags)
{
- return __receive_fd(file, NULL, o_flags);
+ return __receive_fd(-1, file, NULL, o_flags);
+}
+static inline int receive_fd_replace(int fd, struct file *file, unsigned int o_flags)
+{
+ return __receive_fd(fd, file, NULL, o_flags);
}

extern void flush_delayed_fput(void);
--
2.25.1
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