Re: [RFC PATCH 3/3] exec: do unshare_files after de_thread

[Jeff Layton]

On Mon, 2018-08-27 at 13:47 -0400, Jeff Layton wrote:
> POSIX mandates that open fds and their associated file locks should be
> preserved across an execve. This works, unless the process is
> multithreaded at the time that execve is called.
> 
> In that case, we'll end up unsharing the files_struct but the locks will
> still have their fl_owner set to the address of the old one. Eventually,
> when the other threads die and the last reference to the old
> files_struct is put, any POSIX locks get torn down since it looks like
> a close occurred on them.
> 
> The result is that all of your open files will be intact with none of
> the locks you held before execve. The simple answer to this is "use OFD
> locks", but this is a nasty surprise and it violates the spec.
> 
> Fix this by doing unshare_files later during exec, after we've already
> killed off the other threads in the process. This helps ensure that we
> only unshare the files_struct during exec when it is truly shared with
> other processes.
> 
> Note that because the unshare_files call is now done just after
> de_thread, we need a mechanism to pass the displaced files_struct back
> up to __do_execve_file. This is done via a new displaced_files field
> inside the linux_binprm.
> 
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Reported-by: Daniel P. Berrangà <berrange@xxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/exec.c               | 19 +++++++++----------
>  include/linux/binfmts.h |  1 +
>  2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index ca25f805ebad..a45b0cae5817 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1262,6 +1262,10 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	if (retval)
>  		goto out;
>  
> +	retval = unshare_files(&bprm->displaced_files);
> +	if (retval)
> +		goto out;
> +
>  	/*
>  	 * Must be called _before_ exec_mmap() as bprm->mm is
>  	 * not visibile until then. This also enables the update
> @@ -1712,8 +1716,7 @@ static int __do_execve_file(int fd, struct filename *filename,
>  			    int flags, struct file *file)
>  {
>  	char *pathbuf = NULL;
> -	struct linux_binprm *bprm;
> -	struct files_struct *displaced;
> +	struct linux_binprm *bprm = NULL;
>  	int retval;
>  
>  	if (IS_ERR(filename))
> @@ -1735,10 +1738,6 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	 * further execve() calls fail. */
>  	current->flags &= ~PF_NPROC_EXCEEDED;
>  
> -	retval = unshare_files(&displaced);
> -	if (retval)
> -		goto out_ret;
> -
>  	retval = -ENOMEM;
>  	bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
>  	if (!bprm)
> @@ -1831,8 +1830,8 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	kfree(pathbuf);
>  	if (filename)
>  		putname(filename);
> -	if (displaced) {
> -		put_files_struct(displaced);
> +	if (bprm->displaced_files) {
> +		put_files_struct(bprm->displaced_files);

Note that this is broken (bprm is freed above this point). It's simple
enough to fix, but I'll hold off on resending until I hear some feedback
on the general approach.

>  	} else {
>  		spin_lock(&current->files->file_lock);
>  		current->files->in_exec = false;
> @@ -1855,8 +1854,8 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	kfree(pathbuf);
>  
>  out_files:
> -	if (displaced) {
> -		reset_files_struct(displaced);
> +	if (bprm && bprm->displaced_files) {
> +		reset_files_struct(bprm->displaced_files);
>  	} else {
>  		spin_lock(&current->files->file_lock);
>  		current->files->in_exec = false;
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index c05f24fac4f6..d7ec384bb1b0 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -49,6 +49,7 @@ struct linux_binprm {
>  	unsigned int taso:1;
>  #endif
>  	unsigned int recursion_depth; /* only for search_binary_handler() */
> +	struct files_struct * displaced_files;
>  	struct file * file;
>  	struct cred *cred;	/* new credentials */
>  	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */

-- 
Jeff Layton <jlayton@xxxxxxxxxx>