Re: [RFC][PATCH] procfs: Always expose /proc/<pid>/map_files/ and make it readable

[Calvin Owens <calvinowens@fb.com>]

On Thursday 01/22 at 13:02 +0200, Kirill A. Shutemov wrote:
> On Wed, Jan 21, 2015 at 06:45:54PM -0800, Calvin Owens wrote:
> > Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> > is only exposed if CONFIG_CHECKPOINT_RESTORE is set. This interface
> > is very useful for enumerating the files mapped into a process when
> > the more verbose information in /proc/<pid>/maps is not needed.
> > 
> > This patch moves the folder out from behind CHECKPOINT_RESTORE, and
> > removes the CAP_SYS_ADMIN restrictions. To avoid exposing files to
> > processes for whom they may not be visible, a follow_link() stub is
> > added to the inode_operations struct attached to the symlinks that
> > prevent them from being followed without CAP_SYS_ADMIN.
> > 
> > Signed-off-by: Calvin Owens <calvinowens@fb.com>
> > ---
> >  fs/proc/base.c | 42 +++++++++++++++++++++++-------------------
> >  1 file changed, 23 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 3f3d7ae..7d48003 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1632,8 +1632,6 @@ end_instantiate:
> >  	return dir_emit(ctx, name, len, 1, DT_UNKNOWN);
> >  }
> >  
> > -#ifdef CONFIG_CHECKPOINT_RESTORE
> > -
> >  /*
> >   * dname_to_vma_addr - maps a dentry name into two unsigned longs
> >   * which represent vma start and end addresses.
> > @@ -1660,11 +1658,6 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
> >  	if (flags & LOOKUP_RCU)
> >  		return -ECHILD;
> >  
> > -	if (!capable(CAP_SYS_ADMIN)) {
> > -		status = -EPERM;
> > -		goto out_notask;
> > -	}
> > -
> >  	inode = dentry->d_inode;
> >  	task = get_proc_task(inode);
> >  	if (!task)
> > @@ -1753,6 +1746,28 @@ struct map_files_info {
> >  	unsigned char	name[4*sizeof(long)+2]; /* max: %lx-%lx\0 */
> >  };
> >  
> > +/*
> > + * Allowing any user to follow the symlinks in /proc/<pid>/map_files/ could
> > + * allow processes to access files that should not be visible to them, so
> > + * restrict follow_link() to CAP_SYS_ADMIN for these files.
> > + */
> > +static void *proc_map_files_follow_link(struct dentry *d, struct nameidata *n)
> > +{
> > +	if (!capable(CAP_SYS_ADMIN))
> > +		return ERR_PTR(-EPERM);
> > +
> > +	return proc_pid_follow_link(d, n);
> > +}
> 
> I have thought a bit more about this and not sure it's reasonable to
> limit it to CAP_SYS_ADMIN. What scenario are we protecting from?
> 
> Initially, I thought about something like this: privileged process opens a
> file, map part of it, closes the file and drop privileges with hope to
> limit further access to mapped window of the file. But I don't see what
> would stop the unprivileged process from accessing rest of the file using
> mremap(2). And if a process can do this, anybody who can ptrace(2) the
> process can do this.
> 
> Am I missing something?

The specific case I was thinking of is a process in a chroot with a
mounted /proc inside of it: if a process inside the chroot has the same
UID as a process outside of it, the chroot'ed process could follow the
symlinks in map_files/ and poke files it can't actually see, right?

I don't personally care about that use case, but it seemed like
something that might surprise somebody.
 
Calvin
 
> -- 
>  Kirill A. Shutemov