Re: [PATCH] relayfs: fix out-of-bounds access in relay_file_read

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

On Wed, 19 Apr 2023 12:02:03 +0800 zhangzhengming <zhang.zhengming@h3c.com> wrote:

> From: Zhang Zhengming <zhang.zhengming@h3c.com>
> 
> There is a crash in relay_file_read, as the var from 
> point to the end of last subbuf.
> The oops looks something like:
> pc : __arch_copy_to_user+0x180/0x310
> lr : relay_file_read+0x20c/0x2c8
> Call trace:
>  __arch_copy_to_user+0x180/0x310
>  full_proxy_read+0x68/0x98
>  vfs_read+0xb0/0x1d0
>  ksys_read+0x6c/0xf0
>  __arm64_sys_read+0x20/0x28
>  el0_svc_common.constprop.3+0x84/0x108
>  do_el0_svc+0x74/0x90
>  el0_svc+0x1c/0x28
>  el0_sync_handler+0x88/0xb0
>  el0_sync+0x148/0x180
> 
> We get the condition by analyzing the vmcore:
> 1). The last produced byte and last consumed byte
>     both at the end of the last subbuf
> 2). A softirq who will call function(e.g __blk_add_trace)
>     to write relay buffer occurs when an program calling
>     function relay_file_read_avail.
>         relay_file_read
>                 relay_file_read_avail
>                         relay_file_read_consume(buf, 0, 0);
>                         //interrupted by softirq who will write subbuf
>                         ....
>                         return 1;
>                 //read_start point to the end of the last subbuf
>                 read_start = relay_file_read_start_pos
>                 //avail is equal to subsize
>                 avail = relay_file_read_subbuf_avail
>                 //from  points to an invalid memory address             
>                 from = buf->start + read_start
>                 //system is crashed
>                 copy_to_user(buffer, from, avail)

Thanks.  Hopefully Pengcheng Yang and Jens Axboe can comment.

> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -989,7 +989,8 @@ static size_t relay_file_read_start_pos(struct rchan_buf *buf)
>  	size_t subbuf_size = buf->chan->subbuf_size;
>  	size_t n_subbufs = buf->chan->n_subbufs;
>  	size_t consumed = buf->subbufs_consumed % n_subbufs;
> -	size_t read_pos = consumed * subbuf_size + buf->bytes_consumed;
> +	size_t read_pos = (consumed * subbuf_size + buf->bytes_consumed)
> +			% (n_subbufs * subbuf_size);
>  
>  	read_subbuf = read_pos / subbuf_size;
>  	padding = buf->padding[read_subbuf];

I'm thinking we should backport this into earlier kernels and that the
commit we're fixing is

Fixes: 341a7213e5c1 ("kernel/relay.c: fix read_pos error when multiple readers")