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

Submitted by Kees Cook on June 17, 2020, 7:58 p.m.

Details

Message ID 202006171141.4DA1174979@keescook
State New
Series "Add seccomp notifier ioctl that enables adding fds"
Headers show

Commit Message

Kees Cook June 17, 2020, 7:58 p.m.
On Wed, Jun 17, 2020 at 03:35:20PM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 16 June 2020 04:25
> > 
> > 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.
> ...> 
> >  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);
> > +}
> 
> Can you get rid of the 'return user' parameter by adding
> 	if (!ufd) return -EFAULT;
> to the above wrapper, then checking for NULL in the function?
> 
> Or does that do the wrong horrid things in the fail path?

Oh, hm. No, that shouldn't break the failure path, since everything gets
unwound in __fd_install_received if the ufd write fails.

Effectively this (I'll chop it up into the correct patches):

Patch hide | download patch | download mbox

diff --git a/fs/file.c b/fs/file.c
index b583e7c60571..3b80324a31cc 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -939,18 +939,16 @@  int replace_fd(unsigned fd, struct file *file, unsigned flags)
  *
  * @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, if
- * @ufd_required is true (@ufd cannot just be tested for NULL because NULL may
- * actually get passed into SCM_RIGHTS).
+ * @ufd is non-NULL.
  *
  * Returns newly install fd or -ve on error.
  */
-int __fd_install_received(int fd, struct file *file, bool ufd_required,
+int __fd_install_received(int fd, struct file *file,
 			  int __user *ufd, unsigned int o_flags)
 {
 	struct socket *sock;
@@ -967,7 +965,7 @@  int __fd_install_received(int fd, struct file *file, bool ufd_required,
 			return new_fd;
 	}
 
-	if (ufd_required) {
+	if (ufd) {
 		error = put_user(new_fd, ufd);
 		if (error) {
 			put_unused_fd(new_fd);
diff --git a/include/linux/file.h b/include/linux/file.h
index f1d16e24a12e..2ade0d90bc5e 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -91,20 +91,22 @@  extern void put_unused_fd(unsigned int fd);
 
 extern void fd_install(unsigned int fd, struct file *file);
 
-extern int __fd_install_received(int fd, struct file *file, bool ufd_required,
+extern int __fd_install_received(int fd, 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(-1, file, true, ufd, o_flags);
+	if (ufd == NULL)
+		return -EFAULT;
+	return __fd_install_received(-1, file, ufd, o_flags);
 }
 static inline int fd_install_received(struct file *file, unsigned int o_flags)
 {
-	return __fd_install_received(-1, file, false, NULL, o_flags);
+	return __fd_install_received(-1, file, 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);
+	return __fd_install_received(fd, file, NULL, o_flags);
 }
 
 extern void flush_delayed_fput(void);

Comments

David Laight June 18, 2020, 8:19 a.m.
From: Kees Cook
> Sent: 17 June 2020 20:58
> On Wed, Jun 17, 2020 at 03:35:20PM +0000, David Laight wrote:
> > From: Kees Cook
> > > Sent: 16 June 2020 04:25
> > >
> > > 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.
> > ...>
> > >  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);
> > > +}
> >
> > Can you get rid of the 'return user' parameter by adding
> > 	if (!ufd) return -EFAULT;
> > to the above wrapper, then checking for NULL in the function?
> >
> > Or does that do the wrong horrid things in the fail path?
> 
> Oh, hm. No, that shouldn't break the failure path, since everything gets
> unwound in __fd_install_received if the ufd write fails.
> 
> Effectively this (I'll chop it up into the correct patches):

Yep, that's what i was thinking...

Personally I'm not sure that it matters whether the file is left
attached to a process fd when the copy_to_user() fails.
The kernel data structures are consistent either way.
So sane code relies on catching SIGSEGV, fixing thigs up,
and carrying on.
(IIRC the original /bin/sh code called sbrk() in its SIGSEGV
handler instead of doing the limit check in malloc()!)

The important error path is 'failing to get an fd number'.
In that case the caller needs to keep the 'file *' or close it.

I've not looked at the code, but I wonder if you need to pass
the 'file *' by reference so that you can consume it (write NULL)
and return an error.

	David

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