bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH

When BPF_FETCH is set, atomic instructions load a value from memory
into a register. The current verifier code first checks via
check_mem_access whether we can access the memory, and then checks
via check_reg_arg whether we can write into the register.

For loads, check_reg_arg has the side-effect of marking the
register's value as unkonwn, and check_mem_access has the side effect
of propagating bounds from memory to the register. This currently only
takes effect for stack memory.

Therefore with the current order, bounds information is thrown away,
but by simply reversing the order of check_reg_arg
vs. check_mem_access, we can instead propagate bounds smartly.

A simple test is added with an infinite loop that can only be proved
unreachable if this propagation is present. This is implemented both
with C and directly in test_verifier using assembly.

Suggested-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210202135002.4024825-1-jackmanb@google.com

4 files changed, 84 insertions, 14 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 972fc38eb62d..5e09632efddb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3665,9 +3665,26 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 		return -EACCES;
 	}
 
+	if (insn->imm & BPF_FETCH) {
+		if (insn->imm == BPF_CMPXCHG)
+			load_reg = BPF_REG_0;
+		else
+			load_reg = insn->src_reg;
+
+		/* check and record load of old value */
+		err = check_reg_arg(env, load_reg, DST_OP);
+		if (err)
+			return err;
+	} else {
+		/* This instruction accesses a memory location but doesn't
+		 * actually load it into a register.
+		 */
+		load_reg = -1;
+	}
+
 	/* check whether we can read the memory */
 	err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
-			       BPF_SIZE(insn->code), BPF_READ, -1, true);
+			       BPF_SIZE(insn->code), BPF_READ, load_reg, true);
 	if (err)
 		return err;
 
@@ -3677,19 +3694,6 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 	if (err)
 		return err;
 
-	if (!(insn->imm & BPF_FETCH))
-		return 0;
-
-	if (insn->imm == BPF_CMPXCHG)
-		load_reg = BPF_REG_0;
-	else
-		load_reg = insn->src_reg;
-
-	/* check and record load of old value */
-	err = check_reg_arg(env, load_reg, DST_OP);
-	if (err)
-		return err;
-
 	return 0;
 }
 
diff --git a/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c b/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c
new file mode 100644
index 000000000000..addf127068e4
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+
+#include "atomic_bounds.skel.h"
+
+void test_atomic_bounds(void)
+{
+	struct atomic_bounds *skel;
+	__u32 duration = 0;
+
+	skel = atomic_bounds__open_and_load();
+	if (CHECK(!skel, "skel_load", "couldn't load program\n"))
+		return;
+}
diff --git a/tools/testing/selftests/bpf/progs/atomic_bounds.c b/tools/testing/selftests/bpf/progs/atomic_bounds.c
new file mode 100644
index 000000000000..e5fff7fc7f8f
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/atomic_bounds.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <stdbool.h>
+
+#ifdef ENABLE_ATOMICS_TESTS
+bool skip_tests __attribute((__section__(".data"))) = false;
+#else
+bool skip_tests = true;
+#endif
+
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(sub, int x)
+{
+#ifdef ENABLE_ATOMICS_TESTS
+	int a = 0;
+	int b = __sync_fetch_and_add(&a, 1);
+	/* b is certainly 0 here. Can the verifier tell? */
+	while (b)
+		continue;
+#endif
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/verifier/atomic_bounds.c b/tools/testing/selftests/bpf/verifier/atomic_bounds.c
new file mode 100644
index 000000000000..e82183e4914f
--- /dev/null
+++ b/tools/testing/selftests/bpf/verifier/atomic_bounds.c
@@ -0,0 +1,27 @@
+{
+	"BPF_ATOMIC bounds propagation, mem->reg",
+	.insns = {
+		/* a = 0; */
+		/*
+		 * Note this is implemented with two separate instructions,
+		 * where you might think one would suffice:
+		 *
+		 * BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+		 *
+		 * This is because BPF_ST_MEM doesn't seem to set the stack slot
+		 * type to 0 when storing an immediate.
+		 */
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
+		/* b = atomic_fetch_add(&a, 1); */
+		BPF_MOV64_IMM(BPF_REG_1, 1),
+		BPF_ATOMIC_OP(BPF_DW, BPF_ADD | BPF_FETCH, BPF_REG_10, BPF_REG_1, -8),
+		/* Verifier should be able to tell that this infinite loop isn't reachable. */
+		/* if (b) while (true) continue; */
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, -1),
+		BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.result_unpriv = REJECT,
+	.errstr_unpriv = "back-edge",
+},