Re: [PATCH] kernel: sysctl: ignore out-of-range taint bits introduced via kernel.tainted

[Rafael Aquini <aquini@redhat.com>]

On Tue, May 12, 2020 at 01:53:26PM -0700, Andrew Morton wrote:
> On Tue, 12 May 2020 13:46:53 -0400 Rafael Aquini <aquini@redhat.com> wrote:
> 
> > The sysctl knob
> 
> /proc/sys/kernel/tainted, yes?
> 
> > allows users with SYS_ADMIN capability to
> > taint the kernel with any arbitrary value, but this might
> > produce an invalid flags bitset being committed to tainted_mask.
> > 
> > This patch introduces a simple way for proc_taint() to ignore
> > any eventual invalid bit coming from the user input before
> > committing those bits to the kernel tainted_mask.
> > 
> > ...
> >
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -597,6 +597,8 @@ extern enum system_states {
> >  #define TAINT_RANDSTRUCT		17
> >  #define TAINT_FLAGS_COUNT		18
> >  
> > +#define TAINT_FLAGS_MAX			((1UL << TAINT_FLAGS_COUNT) - 1)
> > +
> >  struct taint_flag {
> >  	char c_true;	/* character printed when tainted */
> >  	char c_false;	/* character printed when not tainted */
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 8a176d8727a3..fb2d693fc08c 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -2623,11 +2623,23 @@ static int proc_taint(struct ctl_table *table, int write,
> >  		return err;
> >  
> >  	if (write) {
> > +		int i;
> > +
> > +		/*
> > +		 * Ignore user input that would cause the loop below
> > +		 * to commit arbitrary and out of valid range TAINT flags.
> > +		 */
> > +		if (tmptaint > TAINT_FLAGS_MAX) {
> > +			tmptaint &= TAINT_FLAGS_MAX;
> > +			pr_warn_once("%s: out-of-range taint input ignored."
> > +				     " tainted_mask adjusted to 0x%lx\n",
> > +				     __func__, tmptaint);
> > +		}
> > +
> >  		/*
> >  		 * 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++) {
> 
> Could simply replace BITS_PER_LONG with TAINT_FLAGS_COUNT here?
> 
> (That "&& tmptaint >> i" seems a rather silly optimization?)
> 
> >  			if ((tmptaint >> i) & 1)
> >  				add_taint(i, LOCKDEP_STILL_OK);
> 
> In fact the whole thing could be simplified down to
> 
> 	for (i = 1; i <= TAINT_FLAGS_COUNT; i <<= 1)
> 		if (i & tmptaint)
> 			add_taint(...)
> 
> and silently drop out-of-range bits?
>

Sure!

-- Rafael