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",
+},