From: Yonghong Song <yhs@fb.com>
Date: Fri, 21 Aug 2020 12:10:54 -0700
Subject: bpf: Fix a buffer out-of-bound access when filling raw_tp link_info
Patch-mainline: v5.9-rc4
Git-commit: b474959d5afda6e341a02c85f9595d85d39189ae
References: bsc#1177028

Commit f2e10bff16a0 ("bpf: Add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link")
added link query for raw_tp. One of fields in link_info is to
fill a user buffer with tp_name. The Scurrent checking only
declares "ulen && !ubuf" as invalid. So "!ulen && ubuf" will be
valid. Later on, we do "copy_to_user(ubuf, tp_name, ulen - 1)" which
may overwrite user memory incorrectly.

This patch fixed the problem by disallowing "!ulen && ubuf" case as well.

Fixes: f2e10bff16a0 ("bpf: Add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200821191054.714731-1-yhs@fb.com
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/syscall.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2626,7 +2626,7 @@ static int bpf_raw_tp_link_fill_link_inf
 	u32 ulen = info->raw_tracepoint.tp_name_len;
 	size_t tp_len = strlen(tp_name);
 
-	if (ulen && !ubuf)
+	if (!ulen ^ !ubuf)
 		return -EINVAL;
 
 	info->raw_tracepoint.tp_name_len = tp_len + 1;