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bpf: link: Refuse non-O_RDWR flags in BPF_OBJ_GET

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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: 70ed506c3b ("bpf: Introduce pinnable bpf_link abstraction")
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210326160501.46234-1-lmb@cloudflare.com
This commit is contained in:
Lorenz Bauer 2021-03-26 16:05:00 +00:00 committed by Alexei Starovoitov
parent 06ab134ce8
commit 25fc94b2f0
1 changed files with 1 additions and 1 deletions
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kernel/bpf/inode.c
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@ -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;