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From:   Kees Cook <keescook@chromium.org>
To:     linux-kernel@vger.kernel.org
Cc:     Kees Cook <keescook@chromium.org>,
        Sargun Dhillon <sargun@sargun.me>,
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Subject: [PATCH v4 05/11] fs: Expand __fd_install_received() to accept fd
Date:   Mon, 15 Jun 2020 20:25:18 -0700
Message-Id: <20200616032524.460144-6-keescook@chromium.org>
In-Reply-To: <20200616032524.460144-1-keescook@chromium.org>
References: <20200616032524.460144-1-keescook@chromium.org>
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Sender: linux-kernel-owner@vger.kernel.org
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Expand __fd_install_received() with support for replace_fd() for the
coming seccomp "addfd" ioctl(). Add new wrapper fd_replace_received()
for the new mode and update existing wrappers to retain old mode.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/file.c            | 22 ++++++++++++++++------
 include/linux/file.h | 10 +++++++---
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 14a8ef74efb2..b583e7c60571 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -950,8 +950,8 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
  *
  * Returns newly install fd or -ve on error.
  */
-int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
-			  unsigned int o_flags)
+int __fd_install_received(int fd, struct file *file, bool ufd_required,
+			  int __user *ufd, unsigned int o_flags)
 {
 	struct socket *sock;
 	int new_fd;
@@ -961,9 +961,11 @@ int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
 	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;
+	}
 
 	if (ufd_required) {
 		error = put_user(new_fd, ufd);
@@ -973,6 +975,15 @@ int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
 		}
 	}
 
+	if (fd < 0)
+		fd_install(new_fd, get_file(file));
+	else {
+		new_fd = fd;
+		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
 	 * the file is a socket and testing "sock" for NULL is sufficient.
@@ -982,7 +993,6 @@ int __fd_install_received(struct file *file, bool ufd_required, int __user *ufd,
 		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 999a2c56db07..f1d16e24a12e 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -91,16 +91,20 @@ 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, bool ufd_required,
+extern int __fd_install_received(int fd, 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, true, ufd, o_flags);
+	return __fd_install_received(-1, 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);
+	return __fd_install_received(-1, file, false, NULL, o_flags);
+}
+static inline int fd_replace_received(int fd, struct file *file, unsigned int o_flags)
+{
+	return __fd_install_received(fd, file, false, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);
-- 
2.25.1