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?
