Re: [PATCH] ring-buffer: fix array bounds checking

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Dec 17, 2024 at 01:49:30AM +0900, Jeongjun Park wrote:
> If there is a case where the variable s is greater than or equal to nr_subbufs
> before entering the loop, oob read or use-after-free will occur. This problem
> occurs because the variable s is used as an index to dereference the
> struct page before the variable value range check. This logic prevents the
> wrong address value from being copied to the pages array through the subsequent
> range check, but oob read still occurs, so the code needs to be modified.

Hi Jeongjun, thanks for the patch.

Did you find a reproducer for that problem or has it just been found by code
inspection?

As discussed here [1], s >= nr_subbufs should really never happen as we already
cap nr_pages. 

[1] https://lore.kernel.org/all/78e20e98-bdfc-4d7b-a59c-988b81fcc58b@redhat.com/, 

> 
> Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> ---
>  kernel/trace/ring_buffer.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7e257e855dd1..83da74bf7bd6 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6994,9 +6994,9 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
>  {
>  	unsigned long nr_subbufs, nr_pages, nr_vma_pages, pgoff = vma->vm_pgoff;
>  	unsigned int subbuf_pages, subbuf_order;
> -	struct page **pages;
> +	struct page **pages, *page;
>  	int p = 0, s = 0;
> -	int err;
> +	int err, off;
>  
>  	/* Refuse MP_PRIVATE or writable mappings */
>  	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
> @@ -7055,14 +7055,14 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
>  	}
>  
>  	while (p < nr_pages) {
> -		struct page *page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
> -		int off = 0;
> -

I believe we can keep the struct page and off declaration within the while loop.

>  		if (WARN_ON_ONCE(s >= nr_subbufs)) {
>  			err = -EINVAL;
>  			goto out;
>  		}
>  
> +		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
> +		off = 0;
> +
>  		for (; off < (1 << (subbuf_order)); off++, page++) {
>  			if (p >= nr_pages)
>  				break;
> --