netdev - [PATCH bpf v4 2/2] selftests/bpf: Add test cases to assert proper ID tracking on spill

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Message-Id: <20230607123951.558971-3-maxtram95@gmail.com>
Date: Wed,  7 Jun 2023 15:39:51 +0300
From: Maxim Mikityanskiy <maxtram95@...il.com>
To: bpf@...r.kernel.org
Cc: netdev@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Maxim Mikityanskiy <maxim@...valent.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yhs@...com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Mykola Lysenko <mykolal@...com>,
	Shuah Khan <shuah@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>
Subject: [PATCH bpf v4 2/2] selftests/bpf: Add test cases to assert proper ID tracking on spill

From: Maxim Mikityanskiy <maxim@...valent.com>

The previous commit fixed a verifier bypass by ensuring that ID is not
preserved on narrowing spills. Add the test cases to check the
problematic patterns.

Signed-off-by: Maxim Mikityanskiy <maxim@...valent.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 79 +++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 136e5530b72c..6115520154e3 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -371,4 +371,83 @@ __naked void and_then_at_fp_8(void)
 "	::: __clobber_all);
 }
 
+SEC("xdp")
+__description("32-bit spill of 64-bit reg should clear ID")
+__failure __msg("math between ctx pointer and 4294967295 is not allowed")
+__naked void spill_32bit_of_64bit_fail(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	/* Roll one bit to force the verifier to track both branches. */\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0x8;					\
+	/* Put a large number into r1. */		\
+	r1 = 0xffffffff;				\
+	r1 <<= 32;					\
+	r1 += r0;					\
+	/* Assign an ID to r1. */			\
+	r2 = r1;					\
+	/* 32-bit spill r1 to stack - should clear the ID! */\
+	*(u32*)(r10 - 8) = r1;				\
+	/* 32-bit fill r2 from stack. */		\
+	r2 = *(u32*)(r10 - 8);				\
+	/* Compare r2 with another register to trigger find_equal_scalars.\
+	 * Having one random bit is important here, otherwise the verifier cuts\
+	 * the corners. If the ID was mistakenly preserved on spill, this would\
+	 * cause the verifier to think that r1 is also equal to zero in one of\
+	 * the branches, and equal to eight on the other branch.\
+	 */						\
+	r3 = 0;						\
+	if r2 != r3 goto l0_%=;				\
+l0_%=:	r1 >>= 32;					\
+	/* At this point, if the verifier thinks that r1 is 0, an out-of-bounds\
+	 * read will happen, because it actually contains 0xffffffff.\
+	 */						\
+	r6 += r1;					\
+	r0 = *(u32*)(r6 + 0);				\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("xdp")
+__description("16-bit spill of 32-bit reg should clear ID")
+__failure __msg("dereference of modified ctx ptr R6 off=65535 disallowed")
+__naked void spill_16bit_of_32bit_fail(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	/* Roll one bit to force the verifier to track both branches. */\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0x8;					\
+	/* Put a large number into r1. */		\
+	w1 = 0xffff0000;				\
+	r1 += r0;					\
+	/* Assign an ID to r1. */			\
+	r2 = r1;					\
+	/* 16-bit spill r1 to stack - should clear the ID! */\
+	*(u16*)(r10 - 8) = r1;				\
+	/* 16-bit fill r2 from stack. */		\
+	r2 = *(u16*)(r10 - 8);				\
+	/* Compare r2 with another register to trigger find_equal_scalars.\
+	 * Having one random bit is important here, otherwise the verifier cuts\
+	 * the corners. If the ID was mistakenly preserved on spill, this would\
+	 * cause the verifier to think that r1 is also equal to zero in one of\
+	 * the branches, and equal to eight on the other branch.\
+	 */						\
+	r3 = 0;						\
+	if r2 != r3 goto l0_%=;				\
+l0_%=:	r1 >>= 16;					\
+	/* At this point, if the verifier thinks that r1 is 0, an out-of-bounds\
+	 * read will happen, because it actually contains 0xffff.\
+	 */						\
+	r6 += r1;					\
+	r0 = *(u32*)(r6 + 0);				\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.40.1