linux-kernel - [PATCH 43/82] bpf: Refactor intentional wrap-around test

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Message-Id: <20240123002814.1396804-43-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:27:18 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	bpf@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 43/82] bpf: Refactor intentional wrap-around test

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Daniel Borkmann <daniel@...earbox.net>
Cc: John Fastabend <john.fastabend@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Song Liu <song@...nel.org>
Cc: Yonghong Song <yonghong.song@...ux.dev>
Cc: KP Singh <kpsingh@...nel.org>
Cc: Stanislav Fomichev <sdf@...gle.com>
Cc: Hao Luo <haoluo@...gle.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: bpf@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 kernel/bpf/verifier.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 65f598694d55..21e3f30c8757 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12901,8 +12901,8 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 			dst_reg->smin_value = smin_ptr + smin_val;
 			dst_reg->smax_value = smax_ptr + smax_val;
 		}
-		if (umin_ptr + umin_val < umin_ptr ||
-		    umax_ptr + umax_val < umax_ptr) {
+		if (add_would_overflow(umin_ptr, umin_val) ||
+		    add_would_overflow(umax_ptr, umax_val)) {
 			dst_reg->umin_value = 0;
 			dst_reg->umax_value = U64_MAX;
 		} else {
@@ -13023,8 +13023,8 @@ static void scalar32_min_max_add(struct bpf_reg_state *dst_reg,
 		dst_reg->s32_min_value += smin_val;
 		dst_reg->s32_max_value += smax_val;
 	}
-	if (dst_reg->u32_min_value + umin_val < umin_val ||
-	    dst_reg->u32_max_value + umax_val < umax_val) {
+	if (add_would_overflow(umin_val, dst_reg->u32_min_value) ||
+	    add_would_overflow(umax_val, dst_reg->u32_max_value)) {
 		dst_reg->u32_min_value = 0;
 		dst_reg->u32_max_value = U32_MAX;
 	} else {
@@ -13049,8 +13049,8 @@ static void scalar_min_max_add(struct bpf_reg_state *dst_reg,
 		dst_reg->smin_value += smin_val;
 		dst_reg->smax_value += smax_val;
 	}
-	if (dst_reg->umin_value + umin_val < umin_val ||
-	    dst_reg->umax_value + umax_val < umax_val) {
+	if (add_would_overflow(umin_val, dst_reg->umin_value) ||
+	    add_would_overflow(umax_val, dst_reg->umax_value)) {
 		dst_reg->umin_value = 0;
 		dst_reg->umax_value = U64_MAX;
 	} else {
-- 
2.34.1