[PATCH 5.10 046/188] bpf: link: Refuse non-O_RDWR flags in BPF_OBJ_GET

[Greg Kroah-Hartman]

From: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>

commit 25fc94b2f02d832fa8e29419699dcc20b0b05c6a upstream.

Invoking BPF_OBJ_GET on a pinned bpf_link checks the path access
permissions based on file_flags, but the returned fd ignores flags.
This means that any user can acquire a "read-write" fd for a pinned
link with mode 0664 by invoking BPF_OBJ_GET with BPF_F_RDONLY in
file_flags. The fd can be used to invoke BPF_LINK_DETACH, etc.

Fix this by refusing non-O_RDWR flags in BPF_OBJ_GET. This works
because OBJ_GET by default returns a read write mapping and libbpf
doesn't expose a way to override this behaviour for programs
and links.

Fixes: 70ed506c3bbc ("bpf: Introduce pinnable bpf_link abstraction")
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20210326160501.46234-1-lmb@xxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/inode.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -546,7 +546,7 @@ int bpf_obj_get_user(const char __user *
 	else if (type == BPF_TYPE_MAP)
 		ret = bpf_map_new_fd(raw, f_flags);
 	else if (type == BPF_TYPE_LINK)
-		ret = bpf_link_new_fd(raw);
+		ret = (f_flags != O_RDWR) ? -EINVAL : bpf_link_new_fd(raw);
 	else
 		return -ENOENT;