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@xxxxxxxxxx> 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
