Re: [PATCH] fs: split receive_fd_replace from __receive_fd
From: Al Viro
Date: Fri Apr 16 2021 - 00:15:55 EST
On Fri, Apr 02, 2021 at 12:01:05PM -0700, Kees Cook wrote:
> On Thu, Mar 25, 2021 at 09:22:09AM +0100, Christoph Hellwig wrote:
> > receive_fd_replace shares almost no code with the general case, so split
> > it out. Also remove the "Bump the sock usage counts" comment from
> > both copies, as that is now what __receive_sock actually does.
> >
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>
> I'm okay with repeating code in fs/file.c. What I wanted to avoid was
> open coded combinations in various callers.
... and that got you a lovely userland ABI, where you have
(1) newfd >= 0, SECCOMP_ADDFD_FLAG_SETFD is present => replace
(2) newfd < 0, SECCOMP_ADDFD_FLAG_SETFD is present => insert
(3) newfd == 0, SECCOMP_ADDFD_FLAG_SETFD not present => insert
(4) newfd != 0, SECCOMP_ADDFD_FLAG_SETFD not present => -EINVAL
IMO (2) is a bug. Whether we still can fix it or not... no idea, depends
on whether the actual userland has come to depend upon it.
I suggest turning (2) into an error (-EBADF is what you'd get from
attempt to set something at such descriptor) and seeing if anything
breaks. And having SECCOMP_ADDFD_FLAG_SETFD status passed into kaddfd
explicitly, with explicit check in seccomp_handle_addfd(). As in
commit 42eb0d54c08a0331d6d295420f602237968d792b
Author: Christoph Hellwig <hch@xxxxxx>
Date: Thu Mar 25 09:22:09 2021 +0100
fs: split receive_fd_replace from __receive_fd
receive_fd_replace shares almost no code with the general case, so split
it out. Also remove the "Bump the sock usage counts" comment from
both copies, as that is now what __receive_sock actually does.
[AV: ... and make the only user of receive_fd_replace() choose between
it and receive_fd() according to what userland had passed to it in
flags]
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
diff --git a/fs/file.c b/fs/file.c
index f3a4bac2cbe9..d8ccb95a7f41 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1068,8 +1068,6 @@ 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
@@ -1083,7 +1081,7 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
*
* Returns newly install fd or -ve on error.
*/
-int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flags)
+int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
{
int new_fd;
int error;
@@ -1092,32 +1090,33 @@ int __receive_fd(int fd, struct file *file, int __user *ufd, unsigned int o_flag
if (error)
return error;
- if (fd < 0) {
- new_fd = get_unused_fd_flags(o_flags);
- if (new_fd < 0)
- return new_fd;
- } else {
- new_fd = fd;
- }
+ new_fd = get_unused_fd_flags(o_flags);
+ if (new_fd < 0)
+ return new_fd;
if (ufd) {
error = put_user(new_fd, ufd);
if (error) {
- if (fd < 0)
- put_unused_fd(new_fd);
+ 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;
- }
+ fd_install(new_fd, get_file(file));
+ __receive_sock(file);
+ return new_fd;
+}
- /* Bump the sock usage counts, if any. */
+int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
+{
+ int error;
+
+ error = security_file_receive(file);
+ if (error)
+ return error;
+ error = replace_fd(new_fd, file, o_flags);
+ if (error)
+ return error;
__receive_sock(file);
return new_fd;
}
diff --git a/include/linux/file.h b/include/linux/file.h
index 225982792fa2..2de2e4613d7b 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -92,23 +92,20 @@ extern void put_unused_fd(unsigned int fd);
extern void fd_install(unsigned int fd, struct file *file);
-extern int __receive_fd(int fd, struct file *file, int __user *ufd,
+extern int __receive_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(-1, file, ufd, o_flags);
+ return __receive_fd(file, ufd, o_flags);
}
static inline int receive_fd(struct file *file, unsigned int 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);
+ return __receive_fd(file, NULL, o_flags);
}
+int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags);
extern void flush_delayed_fput(void);
extern void __fput_sync(struct file *);
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 1d60fc2c9987..4fe19cecaa94 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -119,8 +119,11 @@ struct seccomp_kaddfd {
int fd;
unsigned int flags;
- /* To only be set on reply */
- int ret;
+ union {
+ bool setfd;
+ /* To only be set on reply */
+ int ret;
+ };
struct completion completion;
struct list_head list;
};
@@ -1069,7 +1072,11 @@ static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
* that it has been handled.
*/
list_del_init(&addfd->list);
- addfd->ret = receive_fd_replace(addfd->fd, addfd->file, addfd->flags);
+ if (!addfd->setfd)
+ addfd->ret = receive_fd(addfd->file, addfd->flags);
+ else
+ addfd->ret = receive_fd_replace(addfd->fd, addfd->file,
+ addfd->flags);
complete(&addfd->completion);
}
@@ -1583,8 +1590,8 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter,
return -EBADF;
kaddfd.flags = addfd.newfd_flags;
- kaddfd.fd = (addfd.flags & SECCOMP_ADDFD_FLAG_SETFD) ?
- addfd.newfd : -1;
+ kaddfd.setfd = addfd.flags & SECCOMP_ADDFD_FLAG_SETFD;
+ kaddfd.fd = addfd.newfd;
init_completion(&kaddfd.completion);
ret = mutex_lock_interruptible(&filter->notify_lock);