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

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Date:   Tue, 12 May 2020 13:53:26 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rafael Aquini <aquini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        mcgrof@...nel.org, keescook@...omium.org, yzaikin@...gle.com,
        tytso@....edu
Subject: Re: [PATCH] kernel: sysctl: ignore out-of-range taint bits
 introduced via kernel.tainted

On Tue, 12 May 2020 13:46:53 -0400 Rafael Aquini <aquini@...hat.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?