Allow procstat to view current working directory? [xfce4-terminal, linprocfs, ...]

[Damjan Jovanovic]

On Wed, May 6, 2020 at 11:46 PM Konstantin Belousov <kib at freebsd.org> wrote:

> On Wed, May 06, 2020 at 08:32:08PM +0200, Damjan Jovanovic wrote:
> > Hi
> >
> > Currently "procstat fd [pid]" cannot view anything, even for other
> > processes owned by the user making the call, not even their current
> working
> > directory (CWD), unless it has PGET_CANDEBUG permission.
> >
> > linprocfs however allows reading the CWD for any process because it
> doesn't
> > perform that check (sys/compat/linprocfs/linprocfs.c, function
> > linprocfs_doproccwd()).
> >
> > Applications use this, eg. xfce4-terminal relies on
> > /compat/linux/proc/<pid>/cwd to find the shell's CWD, so that when you
> open
> > a new tab, it starts in the same CWD as the tab you opened it from (
> >
> https://github.com/xfce-mirror/xfce4-terminal/blob/master/terminal/terminal-screen.c#L2343
> ).
> > I would like to patch xfce4-terminal to use libprocstat for that instead
> of
> > needing linprocfs to be mounted, but since procstat is more restrictive,
> it
> > will break it.
> >
> > Can we please downgrade PGET_CANDEBUG to at least PGET_CANSEE, so you can
> > view the CWD for processes you own? Maybe other open files still need to
> be
> > hidden, but the CWD doesn't seem like a major security concern.
> Can you explain why CANDEBUG vs CANSEE matters for your case ?
> Or better, why xfce4-terminal cannot debug shells it spawns ?
> Is it suid ?
>
>
xfce4-terminal is a normal process, no suid. It wants to read CWD of the
current tab so the new tab opens in the same directory, which is often what
you want. On Linux /proc/<pid>/cwd is lrwxrwxrwx for all processes. On
FreeBSD /compat/linux/proc/<pid>/cwd is lrwxrwxrwx for all procesess. It
works as intended at present, as long as you have linprocfs mounted.

But I'd rather not have to mount linprocfs. If I patch xfce4-terminal to
use libprocstat instead, then it will break, as it needs CANDEBUG to see
the CWD, a restriction that seems extreme.


>
> My initial reaction was that linprocfs should be patched to match FreeBSD
> native behaviour instead.
>

Why does Linux allow everyone to see CWD for all processes?


>
> >
> > Linux's own /proc filesystem never hides the CWD (lrwxrwxrwx), and only
> > hides file descriptors for processes you don't own.
> >
> > A patch along the following lines could be a start:
> >
> > diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
> > index 423968b2e1cc..f487232d2cff 100644
> > --- a/sys/kern/kern_descrip.c
> > +++ b/sys/kern/kern_descrip.c
> > @@ -3692,7 +3692,7 @@ sysctl_kern_proc_filedesc(SYSCTL_HANDLER_ARGS)
> >
> >         sbuf_new_for_sysctl(&sb, NULL, FILEDESC_SBUF_SIZE, req);
> >         sbuf_clear_flags(&sb, SBUF_INCLUDENUL);
> > -   error = pget((pid_t)name[0], PGET_CANDEBUG | PGET_NOTWEXIT, &p);
> > + error = pget((pid_t)name[0], PGET_CANSEE | PGET_NOTWEXIT, &p);
> >         if (error != 0) {
> >                 sbuf_delete(&sb);
> >                 return (error);
> > @@ -3768,7 +3768,7 @@ sysctl_kern_proc_ofiledesc(SYSCTL_HANDLER_ARGS)
> >         struct proc *p;
> >
> >         name = (int *)arg1;
> > -   error = pget((pid_t)name[0], PGET_CANDEBUG | PGET_NOTWEXIT, &p);
> > + error = pget((pid_t)name[0], PGET_CANSEE | PGET_NOTWEXIT, &p);
> >         if (error != 0)
> >                 return (error);
> >         fdp = fdhold(p);
> >
> >
> >
> >
> > Thank you
> > Damjan
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