Re: [RFC PATCH 2/2] mm: readahead: handle LARGE input to get_init_ra_size()

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 20 Dec 2020 13:10:51 -0800 Randy Dunlap <rdunlap@infradead.org> wrote:

> Add a test to detect if the input ra request size has its high order
> bit set (is negative when tested as a signed long). This would be a
> really Huge readahead.
> 
> If so, WARN() with the value and a stack trace so that we can see
> where this is happening and then make further corrections later.
> Then adjust the size value so that it is not so Huge (although
> this may not be needed).

What motivates this change?  Is there any reason to think this can
happen?

Also, everything in there *should* be unsigned, because a negative
readahead is semantically nonsensical.  Is our handling of this
inherently unsigned quantity incorrect somewhere?

> --- linux-5.10.1.orig/mm/readahead.c
> +++ linux-5.10.1/mm/readahead.c
> 
> ...
>
> @@ -303,14 +304,21 @@ void force_page_cache_ra(struct readahea
>  }
>  
>  /*
> - * Set the initial window size, round to next power of 2 and square
> + * Set the initial window size, round to next power of 2
>   * for small size, x 4 for medium, and x 2 for large
>   * for 128k (32 page) max ra
>   * 1-8 page = 32k initial, > 8 page = 128k initial
>   */
>  static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
>  {
> -	unsigned long newsize = roundup_pow_of_two(size);
> +	unsigned long newsize;
> +
> +	if ((signed long)size < 0) { /* high bit is set: ultra-large ra req */
> +		WARN_ONCE(1, "%s: size=0x%lx\n", __func__, size);
> +		size = -size;	/* really only need to flip the high/sign bit */
> +	}
> +
> +	newsize = roundup_pow_of_two(size);

Is there any way in which userspace can deliberately trigger warning?
Via sys_readadhead() or procfs tuning or whatever?

I guess that permitting a user-triggerable WARN_ONCE() isn't a huuuuge
problem - it isn't a DoS if it only triggers a single time.  It does
permit the malicious user to disable future valid warnings, but I don't
see what incentive there would be for this.  But still, it seems
desirable to avoid it.