[PATCH stable 4.20 04/10] bpf: restrict map value pointer arithmetic for unprivileged

[Daniel Borkmann <daniel@iogearbox.net>]

[ commit 0d6303db7970e6f56ae700fa07e11eb510cda125 upstream ]

Restrict map value pointer arithmetic for unprivileged users in that
arithmetic itself must not go out of bounds as opposed to the actual
access later on. Therefore after each adjust_ptr_min_max_vals() with a
map value pointer as a destination it will simulate a check_map_access()
of 1 byte on the destination and once that fails the program is rejected
for unprivileged program loads. We use this later on for masking any
pointer arithmetic with the remainder of the map value space. The
likelihood of breaking any existing real-world unprivileged eBPF
program is very small for this corner case.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/verifier.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 88b57c518620..517ac90f9e01 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3165,6 +3165,17 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	__update_reg_bounds(dst_reg);
 	__reg_deduce_bounds(dst_reg);
 	__reg_bound_offset(dst_reg);
+
+	/* For unprivileged we require that resulting offset must be in bounds
+	 * in order to be able to sanitize access later on.
+	 */
+	if (!env->allow_ptr_leaks && dst_reg->type == PTR_TO_MAP_VALUE &&
+	    check_map_access(env, dst, dst_reg->off, 1, false)) {
+		verbose(env, "R%d pointer arithmetic of map value goes out of range, prohibited for !root\n",
+			dst);
+		return -EACCES;
+	}
+
 	return 0;
 }
 
-- 
2.17.1