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

[Pengcheng Yang]

On April 20, 2023 5:04 AM, Andrew Morton wrote:
> On Wed, 19 Apr 2023 12:02:03 +0800 zhangzhengming <zhang.zhengming@xxxxxxx> wrote:
>
>> From: Zhang Zhengming <zhang.zhengming@xxxxxxx>
>> 
>> 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.

This patch looks good to me.

Reviewed-by: Pengcheng Yang <yangpc@xxxxxxxxxx>

>
> 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")

I suggest starting backport with this tag:

Fixes: 8d62fdebdaf9 ("relay file read: start-pos fix")