[SRU][Focal][PATCH 1/1] relayfs: fix out-of-bounds access in relay_file_read

[Yuxuan Luo]

From: Zhang Zhengming <zhang.zhengming at 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 calls function(e.g __blk_add_trace)
    to write relay buffer occurs when an program is calling
    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)

Link: https://lkml.kernel.org/r/20230419040203.37676-1-zhang.zhengming@h3c.com
Fixes: 8d62fdebdaf9 ("relay file read: start-pos fix")
Signed-off-by: Zhang Zhengming <zhang.zhengming at h3c.com>
Reviewed-by: Zhao Lei <zhao_lei1 at hoperun.com>
Reviewed-by: Zhou Kete <zhou.kete at h3c.com>
Reviewed-by: Pengcheng Yang <yangpc at wangsu.com>
Cc: Jens Axboe <axboe at kernel.dk>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
(backported from commit 43ec16f1450f4936025a9bdf1a273affdb9732c1)
[yuxuan.luo: modify the read_pos = consumed * subbuf_size +
 buf->bytes_consumed; line directly.
]
CVE-2023-3268
Signed-off-by: Yuxuan Luo <yuxuan.luo at canonical.com>
---
 kernel/relay.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index 9b1cfcd8dc6b1..29ac38e42a297 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -1081,7 +1081,8 @@ static size_t relay_file_read_start_pos(size_t read_pos,
 	size_t consumed = buf->subbufs_consumed % n_subbufs;
 
 	if (!read_pos)
-		read_pos = consumed * subbuf_size + buf->bytes_consumed;
+		read_pos = (consumed * subbuf_size + buf->bytes_consumed)
+			 % (n_subbufs * subbuf_size);
 	read_subbuf = read_pos / subbuf_size;
 	padding = buf->padding[read_subbuf];
 	padding_start = (read_subbuf + 1) * subbuf_size - padding;
-- 
2.34.1