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

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