linux-kernel - Re: [PATCH] kernel: sysctl: ignore invalid taint bits introduced via kernel.tainted and taint the kernel with TAINT

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Message-ID: <20200512010313.GA725253@optiplex-lnx>
Date:   Mon, 11 May 2020 21:03:13 -0400
From:   Rafael Aquini <aquini@...hat.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Tso Ted <tytso@....edu>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, keescook@...omium.org,
        yzaikin@...gle.com
Subject: Re: [PATCH] kernel: sysctl: ignore invalid taint bits introduced via
 kernel.tainted and taint the kernel with TAINT_USER on writes

On Tue, May 12, 2020 at 12:17:03AM +0000, Luis Chamberlain wrote:
> On Mon, May 11, 2020 at 07:59:14PM -0400, Rafael Aquini wrote:
> > On Mon, May 11, 2020 at 11:10:45PM +0000, Luis Chamberlain wrote:
> > > On Mon, May 11, 2020 at 05:59:04PM -0400, Rafael Aquini wrote:
> > > > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > > > index 8a176d8727a3..f0a4fb38ac62 100644
> > > > --- a/kernel/sysctl.c
> > > > +++ b/kernel/sysctl.c
> > > > @@ -2623,17 +2623,32 @@ static int proc_taint(struct ctl_table *table, int write,
> > > >  		return err;
> > > >  
> > > >  	if (write) {
> > > > +		int i;
> > > > +
> > > > +		/*
> > > > +		 * Ignore user input that would make us committing
> > > > +		 * arbitrary invalid TAINT flags in the loop below.
> > > > +		 */
> > > > +		tmptaint &= (1UL << TAINT_FLAGS_COUNT) - 1;
> > > 
> > > This looks good but we don't pr_warn() of information lost on intention.
> > >
> > 
> > Are you thinking in sth like:
> > 
> > +               if (tmptaint > TAINT_FLAGS_MAX) {
> > +                       tmptaint &= TAINT_FLAGS_MAX;
> > +                       pr_warn("proc_taint: out-of-range invalid input ignored"
> > +                               " tainted_mask adjusted to 0x%x\n", tmptaint);
> > +               }
> > ?
> 
> Sure that would clarify this.
> 
> > > > +
> > > >  		/*
> > > >  		 * Poor man's atomic or. Not worth adding a primitive
> > > >  		 * to everyone's atomic.h for this
> > > >  		 */
> > > > -		int i;
> > > >  		for (i = 0; i < BITS_PER_LONG && tmptaint >> i; i++) {
> > > >  			if ((tmptaint >> i) & 1)
> > > >  				add_taint(i, LOCKDEP_STILL_OK);
> > > >  		}
> > > > +
> > > > +		/*
> > > > +		 * Users with SYS_ADMIN capability can include any arbitrary
> > > > +		 * taint flag by writing to this interface. If that's the case,
> > > > +		 * we also need to mark the kernel "tainted by user".
> > > > +		 */
> > > > +		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> > > 
> > > I'm in favor of this however I'd like to hear from Ted on if it meets
> > > the original intention. I would think he had a good reason not to add
> > > it here.
> > >
> > 
> > Fair enough. The impression I got by reading Ted's original commit
> > message is that the intent was to have TAINT_USER as the flag set 
> > via this interface, even though the code was allowing for any 
> > arbitrary value.
> 
> That wasn't my reading, it was that the user did something very odd
> with user input which we don't like as kernel developers, and it gives
> us a way to prove: hey you did something stupid, sorry but I cannot
> support your kernel panic.
> 
> > I think it's OK to let the user fiddle with
> > the flags, as it's been allowed since the introduction of
> > this interface, but we need to reflect that fact in the
> > tainting itself. Since TAINT_USER is not used anywhere,
> 
> I see users of TAINT_USER sprinkled around
>

I meant in the original commit that introduced it
(commit 34f5a39899f3f3e815da64f48ddb72942d86c366). Sorry I
miscomunicated that.

In its current usage, it seems that the other places adding TAINT_USER
match with what is being proposed here: To signal when we have user 
fiddling with kernel / module parameters.

 
> > this change perfectly communicates that fact without
> > the need for introducing yet another taint flag.
> 
> I'd be happy if we don't have introduce yet-anothe flag as well.
> But since Ted introduced it, without using the flag on the proc_taint()
> I'd like confirmation we won't screw things up with existing test cases
> which assume proc_taint() won't set this up. We'd therefore regress
> userspace.
> 
> This is why I'd like for us to be careful with this flag.
> 
>   Luis
>