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
