libbpf: Fix out-of-bound read

[ Upstream commit 236d391 ]

In `set_kcfg_value_str`, an untrusted string is accessed with the assumption
that it will be at least two characters long due to the presence of checks for
opening and closing quotes. But the check for the closing quote
(value[len - 1] != '"') misses the fact that it could be checking the opening
quote itself in case of an invalid input that consists of just the opening
quote.

This commit adds an explicit check to make sure the string is at least two
characters long.

Signed-off-by: Nandakumar Edamana <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Sasha Levin <[email protected]>
(cherry picked from commit ecd205a5241dff6b9030aa69f90c65a364511adb)

Author: nandedamana

tools/lib/bpf/libbpf.c

@@ -1802,7 +1802,7 @@ static int set_kcfg_value_str(struct extern_desc *ext, char *ext_val,
 	}
 
 	len = strlen(value);
-	if (value[len - 1] != '"') {
+	if (len < 2 || value[len - 1] != '"') {
 		pr_warn("extern (kcfg) '%s': invalid string config '%s'\n",
 			ext->name, value);
 		return -EINVAL;