libbpf: Avoid reading past ELF data section end when copying license

Fix possible read beyond ELF "license" data section if the license
string is not properly zero-terminated. Use the fact that libbpf_strlcpy
never accesses the (N-1)st byte of the source string because it's
replaced with '\0' anyways.

If this happens, it's a violation of contract between libbpf and a user,
but not handling this more robustly upsets CIFuzz, so given the fix is
trivial, let's fix the potential issue.

Fixes: 9fc205b413b3 ("libbpf: Add sane strncpy alternative and use it internally")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211214232054.3458774-1-andrii@kernel.org

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 684c34c..cf862a1 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1320,7 +1320,10 @@ static int bpf_object__check_endianness(struct bpf_object *obj)
 static int
 bpf_object__init_license(struct bpf_object *obj, void *data, size_t size)
 {
-       libbpf_strlcpy(obj->license, data, sizeof(obj->license));
+       /* libbpf_strlcpy() only copies first N - 1 bytes, so size + 1 won't
+        * go over allowed ELF data section buffer
+        */
+       libbpf_strlcpy(obj->license, data, min(size + 1, sizeof(obj->license)));
        pr_debug("license of %s is %s\n", obj->path, obj->license);
        return 0;
 }