netdev - [PATCH bpf v2 1/2] bpf: link: refuse non-O_RDWR flags in BPF_OBJ

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Message-Id: <20210326160501.46234-1-lmb@cloudflare.com>
Date:   Fri, 26 Mar 2021 16:05:00 +0000
From:   Lorenz Bauer <lmb@...udflare.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org
Cc:     kernel-team@...udflare.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Lorenz Bauer <lmb@...udflare.com>
Subject: [PATCH bpf v2 1/2] bpf: link: refuse non-O_RDWR flags in BPF_OBJ_GET

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@...udflare.com>
---
 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..dc56237d6960 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 = (f_flags != O_RDWR) ? -EINVAL : bpf_link_new_fd(raw);
 	else
 		return -ENOENT;

-- 
2.27.0