From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:51040 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751817AbeCVK54 (ORCPT ); Thu, 22 Mar 2018 06:57:56 -0400 Message-ID: <1521716273.4722.10.camel@kernel.org> Subject: Re: [PATCH v2] locks: change POSIX lock ownership on execve when files_struct is displaced From: Jeff Layton To: "Eric W. Biederman" Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Viro , "J. Bruce Fields" , Thomas Gleixner , Daniel =?ISO-8859-1?Q?P=2EBerrang=E9?= , Kate Stewart , Dan Williams , Philippe Ombredanne , Greg Kroah-Hartman Date: Thu, 22 Mar 2018 06:57:53 -0400 In-Reply-To: <871sgcvfh7.fsf@xmission.com> References: <20180317142520.30520-1-jlayton@kernel.org> <20180317165859.26200-1-jlayton@kernel.org> <87bmfgvg8w.fsf@xmission.com> <871sgcvfh7.fsf@xmission.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, 2018-03-22 at 00:36 -0500, Eric W. Biederman wrote: > ebiederm@xmission.com (Eric W. Biederman) writes: > > > Jeff Layton writes: > > > > > From: Jeff Layton > > > > > > 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. > > Would this perhaps work better if we moved unshare_files to after or > > inside of de_thread. That would remove any cases where fd->count is > 1 > > simply because you are multi-threaded. It would only leave the strange > > cases where files struct is shared between different processes. > > The fact we have a problem here appears to be a regression caused by: > fd8328be874f ("[PATCH] sanitize handling of shared descriptor tables in > failing execve()") > > So I really think we are calling unshare_files in the wrong location. > > We could perhaps keep the benefit of being able to fail exec cleanly > if we freeze the threads and then only unshare if the count of threads > differs from the fd->count. I don't know if it is worth it. > I'm certainly open to your idea, and that does sound simpler. I looked at a couple of different solutions here, and considered moving the unshare_files call, but the problem is just what you state: the error handling in here is _really_ difficult to manage. Would you be willing to draft a patch that does what you're suggesting? execve/thread handling is not really my area of expertise, so I'd appreciate some guidance on how best to fix this. Daniel wrote a testcase for the problem he reported, so we should be able to quickly verify that the original problem is fixed if you have a patch that does this. We probably we ought to get that testcase into LTP or something too. > > > 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. > > > > > > On a successful execve, change ownership of any POSIX file_locks > > > associated with the old files_struct to the new one, if we ended up > > > swapping it out. > > > > If we can move unshare_files I believe the need for changing the > > ownership would go away. Which seems like easier to understand > > and simpler code in the end. With fewer surprises. > > > > Eric > > > > > Reported-by: Daniel P. Berrangé > > > Signed-off-by: Jeff Layton > > > > > > > --- > > > fs/exec.c | 4 +++- > > > fs/locks.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > include/linux/fs.h | 8 ++++++++ > > > 3 files changed, 71 insertions(+), 1 deletion(-) > > > > > > diff --git a/fs/exec.c b/fs/exec.c > > > index 7eb8d21bcab9..35b05376bf78 100644 > > > --- a/fs/exec.c > > > +++ b/fs/exec.c > > > @@ -1812,8 +1812,10 @@ static int do_execveat_common(int fd, struct filename *filename, > > > free_bprm(bprm); > > > kfree(pathbuf); > > > putname(filename); > > > - if (displaced) > > > + if (displaced) { > > > + posix_change_lock_owners(current->files, displaced); > > > put_files_struct(displaced); > > > + } > > > return retval; > > > > > > out: > > > diff --git a/fs/locks.c b/fs/locks.c > > > index d6ff4beb70ce..ab428ca8bb11 100644 > > > --- a/fs/locks.c > > > +++ b/fs/locks.c > > > @@ -993,6 +993,66 @@ static int flock_lock_inode(struct inode *inode, struct file_lock *request) > > > return error; > > > } > > > > > > +struct posix_change_lock_owners_arg { > > > + fl_owner_t old; > > > + fl_owner_t new; > > > +}; > > > + > > > +static int posix_change_lock_owners_cb(const void *varg, struct file *file, > > > + unsigned int fd) > > > +{ > > > + const struct posix_change_lock_owners_arg *arg = varg; > > > + struct inode *inode = file_inode(file); > > > + struct file_lock_context *ctx; > > > + struct file_lock *fl, *tmp; > > > + > > > + /* If there is no context, then no locks need to be changed */ > > > + ctx = locks_get_lock_context(inode, F_UNLCK); > > > + if (!ctx) > > > + return 0; > > > + > > > + percpu_down_read_preempt_disable(&file_rwsem); > > > + spin_lock(&ctx->flc_lock); > > > + /* Find the first lock with the old owner */ > > > + list_for_each_entry(fl, &ctx->flc_posix, fl_list) { > > > + if (fl->fl_owner == arg->old) > > > + break; > > > + } > > > + > > > + list_for_each_entry_safe_from(fl, tmp, &ctx->flc_posix, fl_list) { > > > + if (fl->fl_owner != arg->old) > > > + break; > > > + > > > + /* This should only be used for normal userland lockmanager */ > > > + if (fl->fl_lmops) { > > > + WARN_ON_ONCE(1); > > > + break; > > > + } > > > + fl->fl_owner = arg->new; > > > + } > > > + spin_unlock(&ctx->flc_lock); > > > + percpu_up_read_preempt_enable(&file_rwsem); > > > + return 0; > > > +} > > > + > > > +/** > > > + * posix_change_lock_owners - change lock owners from old files_struct to new > > > + * @files: new files struct to own locks > > > + * @old: old files struct that previously held locks > > > + * > > > + * On execve, a process may end up with a new files_struct. In that case, we > > > + * must change all of the locks that were owned by the previous files_struct > > > + * to the new one. > > > + */ > > > +void posix_change_lock_owners(struct files_struct *new, > > > + struct files_struct *old) > > > +{ > > > + struct posix_change_lock_owners_arg arg = { .old = old, > > > + .new = new }; > > > + > > > + iterate_fd(new, 0, posix_change_lock_owners_cb, &arg); > > > +} > > > + > > > static int posix_lock_inode(struct inode *inode, struct file_lock *request, > > > struct file_lock *conflock) > > > { > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > > index 79c413985305..65fa99707bf9 100644 > > > --- a/include/linux/fs.h > > > +++ b/include/linux/fs.h > > > @@ -1098,6 +1098,8 @@ extern int lease_modify(struct file_lock *, int, struct list_head *); > > > struct files_struct; > > > extern void show_fd_locks(struct seq_file *f, > > > struct file *filp, struct files_struct *files); > > > +extern void posix_change_lock_owners(struct files_struct *new, > > > + struct files_struct *old); > > > #else /* !CONFIG_FILE_LOCKING */ > > > static inline int fcntl_getlk(struct file *file, unsigned int cmd, > > > struct flock __user *user) > > > @@ -1232,6 +1234,12 @@ static inline int lease_modify(struct file_lock *fl, int arg, > > > struct files_struct; > > > static inline void show_fd_locks(struct seq_file *f, > > > struct file *filp, struct files_struct *files) {} > > > + > > > +static inline void posix_change_lock_owners(struct files_struct *new, > > > + struct files_struct *old) > > > +{ > > > +} > > > + > > > #endif /* !CONFIG_FILE_LOCKING */ > > > > > > static inline struct inode *file_inode(const struct file *f) -- Jeff Layton