[PATCH bpf] bpf: link: refuse non-zero file_flags in BPF_OBJ_GET

[Lorenz Bauer]

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-zero flags in BPF_OBJ_GET. Since zero flags
imply O_RDWR this requires users to have read-write access to the
pinned file, which matches the behaviour of the link primitive.

libbpf doesn't expose a way to set file_flags for links, so this
change is unlikely to break users.

Fixes: 70ed506c3bbc ("bpf: Introduce pinnable bpf_link abstraction")
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
---
 kernel/bpf/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 1576ff331ee4..2f9e8115ad58 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -547,7 +547,7 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
 	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 = (flags) ? -EINVAL : bpf_link_new_fd(raw);
 	else
 		return -ENOENT;
 
-- 
2.27.0