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Date:   Tue, 21 Jul 2020 18:16:27 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-raid@vger.kernel.org,
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        Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH 05/24] devtmpfs: open code ksys_chdir and ksys_chroot
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On Tue, Jul 21, 2020 at 09:49:17AM -0700, Linus Torvalds wrote:
> On Tue, Jul 21, 2020 at 9:28 AM Christoph Hellwig <hch@lst.de> wrote:
> >
> > +
> > +       /* traverse into overmounted root and then chroot to it */
> > +       if (!kern_path("/..", LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path) &&
> > +           !inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR) &&
> > +           ns_capable(current_user_ns(), CAP_SYS_CHROOT) &&
> > +           !security_path_chroot(&path)) {
> > +               set_fs_pwd(current->fs, &path);
> > +               set_fs_root(current->fs, &path);
> > +       }
> > +       path_put(&path);
> 
> This looks wrong.

It is wrong.  kern_path() leaves *path unmodified in case of error, and
that struct path is uninitialized here.