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

[zhangzhengming]

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)

Signed-off-by: Zhang Zhengming <zhang.zhengming@xxxxxxx>
Reviewed-by: Zhao Lei <zhao_lei1@xxxxxxxxxxx>
Reviewed-by: Zhou Kete <zhou.kete@xxxxxxx>
---
 kernel/relay.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index 9aa70ae..a80fa01 100644
--- 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];
-- 
2.17.1