[PATCH bpf-next v2 09/23] bpf: Extend liveness analysis to track stack argument slots

[Yonghong Song <yonghong.song@linux.dev>]

BPF_REG_PARAMS (R11) is at index MAX_BPF_REG, which is beyond the
register tracking arrays in const_fold.c and liveness.c. Handle it
explicitly to avoid out-of-bounds accesses.

Extend the arg tracking dataflow to cover stack arg slots. Otherwise,
pointers passed through stack args are invisible to liveness, causing
the pointed-to stack slots to be incorrectly poisoned.

Extend the at_out tracking array to MAX_AT_TRACK_REGS (registers
plus stack arg slots) so that outgoing stack arg stores are tracked
alongside registers. Add a separate at_stack_arg_entry array in
arg_track_xfer() to restore FP-derived values on incoming stack arg
reads.

Extend record_call_access() to check stack arg slots for FP-derived
pointers at kfunc call sites, reusing the record_arg_access() helper
extracted in the previous patch. Pass stack arg state from caller to
callee in analyze_subprog() so that callees can track pointers received
through stack args, hence avoid poisoning.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/const_fold.c |  8 ++++
 kernel/bpf/liveness.c   | 83 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 77 insertions(+), 14 deletions(-)

diff --git a/kernel/bpf/const_fold.c b/kernel/bpf/const_fold.c
index db73c4740b1e..b2a19acadb91 100644
--- a/kernel/bpf/const_fold.c
+++ b/kernel/bpf/const_fold.c
@@ -58,6 +58,14 @@ static void const_reg_xfer(struct bpf_verifier_env *env, struct const_arg_info *
 	u8 opcode = BPF_OP(insn->code) | BPF_SRC(insn->code);
 	int r;
 
+	/* Stack arg stores (r11-based) are outside the tracked register set. */
+	if (is_stack_arg_st(insn) || is_stack_arg_stx(insn))
+		return;
+	if (is_stack_arg_ldx(insn)) {
+		ci_out[insn->dst_reg] = unknown;
+		return;
+	}
+
 	switch (class) {
 	case BPF_ALU:
 	case BPF_ALU64:
diff --git a/kernel/bpf/liveness.c b/kernel/bpf/liveness.c
index de0b4c6263ff..eed7260f2bb0 100644
--- a/kernel/bpf/liveness.c
+++ b/kernel/bpf/liveness.c
@@ -610,6 +610,24 @@ enum arg_track_state {
 /* Track callee stack slots fp-8 through fp-512 (64 slots of 8 bytes each) */
 #define MAX_ARG_SPILL_SLOTS 64
 
+/* Track stack arg slots: outgoing starts at -(i+1)*8, incoming at +(i+1)*8 */
+#define MAX_STACK_ARG_SLOTS (MAX_BPF_FUNC_ARGS - MAX_BPF_FUNC_REG_ARGS)
+
+/*
+ * Combined register + stack arg tracking: R0-R10 at indices 0-10,
+ * outgoing stack arg slots at indices MAX_BPF_REG..MAX_BPF_REG+6.
+ */
+#define MAX_AT_TRACK_REGS (MAX_BPF_REG + MAX_STACK_ARG_SLOTS)
+
+static int stack_arg_off_to_slot(s16 off)
+{
+	int aoff = off < 0 ? -off : off;
+
+	if (aoff / 8 > MAX_STACK_ARG_SLOTS)
+		return -1;
+	return aoff / 8 - 1;
+}
+
 static bool arg_is_visited(const struct arg_track *at)
 {
 	return at->frame != ARG_UNVISITED;
@@ -1062,6 +1080,7 @@ static bool can_be_local_fp(int depth, int regno, struct arg_track *at)
 static void arg_track_xfer(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			   int insn_idx,
 			   struct arg_track *at_out, struct arg_track *at_stack_out,
+			   const struct arg_track *at_stack_arg_entry,
 			   struct func_instance *instance,
 			   u32 *callsites)
 {
@@ -1071,8 +1090,24 @@ static void arg_track_xfer(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	struct arg_track *dst = &at_out[insn->dst_reg];
 	struct arg_track *src = &at_out[insn->src_reg];
 	struct arg_track none = { .frame = ARG_NONE };
-	int r;
-
+	int r, slot;
+
+	/* Handle stack arg stores and loads. */
+	if (is_stack_arg_st(insn) || is_stack_arg_stx(insn)) {
+		slot = stack_arg_off_to_slot(insn->off);
+		if (slot >= 0) {
+			if (is_stack_arg_stx(insn))
+				at_out[MAX_BPF_REG + slot] = at_out[insn->src_reg];
+			else
+				at_out[MAX_BPF_REG + slot] = none;
+		}
+		return;
+	}
+	if (is_stack_arg_ldx(insn)) {
+		slot = stack_arg_off_to_slot(insn->off);
+		at_out[insn->dst_reg] = (slot >= 0) ? at_stack_arg_entry[slot] : none;
+		return;
+	}
 	if (class == BPF_ALU64 && BPF_SRC(insn->code) == BPF_K) {
 		if (code == BPF_MOV) {
 			*dst = none;
@@ -1395,11 +1430,18 @@ static int record_call_access(struct bpf_verifier_env *env,
 	if (bpf_get_call_summary(env, insn, &cs))
 		num_params = cs.num_params;
 
-	for (r = BPF_REG_1; r < BPF_REG_1 + num_params; r++) {
+	for (r = BPF_REG_1; r < BPF_REG_1 + min(num_params, MAX_BPF_FUNC_REG_ARGS); r++) {
 		err = record_arg_access(env, instance, insn, &at[r], r - 1, insn_idx);
 		if (err)
 			return err;
 	}
+
+	for (r = 0; r < MAX_STACK_ARG_SLOTS && r < num_params - MAX_BPF_FUNC_REG_ARGS; r++) {
+		err = record_arg_access(env, instance, insn, &at[MAX_BPF_REG + r],
+					r + MAX_BPF_FUNC_REG_ARGS, insn_idx);
+		if (err)
+			return err;
+	}
 	return 0;
 }
 
@@ -1456,7 +1498,7 @@ static int find_callback_subprog(struct bpf_verifier_env *env,
 
 /* Per-subprog intermediate state kept alive across analysis phases */
 struct subprog_at_info {
-	struct arg_track (*at_in)[MAX_BPF_REG];
+	struct arg_track (*at_in)[MAX_AT_TRACK_REGS];
 	int len;
 };
 
@@ -1554,10 +1596,11 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 	int end = env->subprog_info[subprog + 1].start;
 	int po_end = env->subprog_info[subprog + 1].postorder_start;
 	int len = end - start;
-	struct arg_track (*at_in)[MAX_BPF_REG] = NULL;
-	struct arg_track at_out[MAX_BPF_REG];
+	struct arg_track (*at_in)[MAX_AT_TRACK_REGS] = NULL;
+	struct arg_track at_out[MAX_AT_TRACK_REGS];
 	struct arg_track (*at_stack_in)[MAX_ARG_SPILL_SLOTS] = NULL;
 	struct arg_track *at_stack_out = NULL;
+	struct arg_track at_stack_arg_entry[MAX_STACK_ARG_SLOTS];
 	struct arg_track unvisited = { .frame = ARG_UNVISITED };
 	struct arg_track none = { .frame = ARG_NONE };
 	bool changed;
@@ -1576,19 +1619,19 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 		goto err_free;
 
 	for (i = 0; i < len; i++) {
-		for (r = 0; r < MAX_BPF_REG; r++)
+		for (r = 0; r < MAX_AT_TRACK_REGS; r++)
 			at_in[i][r] = unvisited;
 		for (r = 0; r < MAX_ARG_SPILL_SLOTS; r++)
 			at_stack_in[i][r] = unvisited;
 	}
 
-	for (r = 0; r < MAX_BPF_REG; r++)
+	for (r = 0; r < MAX_AT_TRACK_REGS; r++)
 		at_in[0][r] = none;
 
 	/* Entry: R10 is always precisely the current frame's FP */
 	at_in[0][BPF_REG_FP] = arg_single(depth, 0);
 
-	/* R1-R5: from caller or ARG_NONE for main */
+	/* R1-R5 and outgoing stack args: from caller or ARG_NONE for main */
 	if (callee_entry) {
 		for (r = BPF_REG_1; r <= BPF_REG_5; r++)
 			at_in[0][r] = callee_entry[r];
@@ -1598,6 +1641,10 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 	for (r = 0; r < MAX_ARG_SPILL_SLOTS; r++)
 		at_stack_in[0][r] = none;
 
+	/* Entry: incoming stack args from caller, or ARG_NONE for main */
+	for (r = 0; r < MAX_STACK_ARG_SLOTS; r++)
+		at_stack_arg_entry[r] = callee_entry ? callee_entry[MAX_BPF_REG + r] : none;
+
 	if (env->log.level & BPF_LOG_LEVEL2)
 		verbose(env, "subprog#%d: analyzing (depth %d)...\n", subprog, depth);
 
@@ -1616,7 +1663,8 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 		memcpy(at_out, at_in[i], sizeof(at_out));
 		memcpy(at_stack_out, at_stack_in[i], MAX_ARG_SPILL_SLOTS * sizeof(*at_stack_out));
 
-		arg_track_xfer(env, insn, idx, at_out, at_stack_out, instance, callsites);
+		arg_track_xfer(env, insn, idx, at_out, at_stack_out,
+			       at_stack_arg_entry, instance, callsites);
 		arg_track_log(env, insn, idx, at_in[i], at_stack_in[i], at_out, at_stack_out);
 
 		/* Propagate to successors within this subprogram */
@@ -1630,7 +1678,7 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 				continue;
 			ti = target - start;
 
-			for (r = 0; r < MAX_BPF_REG; r++)
+			for (r = 0; r < MAX_AT_TRACK_REGS; r++)
 				changed |= arg_track_join(env, idx, target, r,
 							  &at_in[ti][r], at_out[r]);
 
@@ -1685,12 +1733,15 @@ static int compute_subprog_args(struct bpf_verifier_env *env,
 	return err;
 }
 
-/* Return true if any of R1-R5 is derived from a frame pointer. */
+/* Return true if any of R1-R5 or stack args is derived from a frame pointer. */
 static bool has_fp_args(struct arg_track *args)
 {
 	for (int r = BPF_REG_1; r <= BPF_REG_5; r++)
 		if (args[r].frame != ARG_NONE)
 			return true;
+	for (int r = 0; r < MAX_STACK_ARG_SLOTS; r++)
+		if (arg_is_fp(&args[MAX_BPF_REG + r]))
+			return true;
 	return false;
 }
 
@@ -1814,7 +1865,7 @@ static int analyze_subprog(struct bpf_verifier_env *env,
 	/* For each reachable call site in the subprog, recurse into callees */
 	for (int p = po_start; p < po_end; p++) {
 		int idx = env->cfg.insn_postorder[p];
-		struct arg_track callee_args[BPF_REG_5 + 1];
+		struct arg_track callee_args[MAX_AT_TRACK_REGS] = {};
 		struct arg_track none = { .frame = ARG_NONE };
 		struct bpf_insn *insn = &insns[idx];
 		struct func_instance *callee_instance;
@@ -1829,9 +1880,11 @@ static int analyze_subprog(struct bpf_verifier_env *env,
 			if (callee < 0)
 				continue;
 
-			/* Build entry args: R1-R5 from at_in at call site */
+			/* Build entry args: R1-R5 and stack args from at_in at call site */
 			for (int r = BPF_REG_1; r <= BPF_REG_5; r++)
 				callee_args[r] = info[subprog].at_in[j][r];
+			for (int r = 0; r < MAX_STACK_ARG_SLOTS; r++)
+				callee_args[MAX_BPF_REG + r] = info[subprog].at_in[j][MAX_BPF_REG + r];
 		} else if (bpf_calls_callback(env, idx)) {
 			callee = find_callback_subprog(env, insn, idx, &caller_reg, &cb_callee_reg);
 			if (callee == -2) {
@@ -1853,6 +1906,8 @@ static int analyze_subprog(struct bpf_verifier_env *env,
 
 			for (int r = BPF_REG_1; r <= BPF_REG_5; r++)
 				callee_args[r] = none;
+			for (int r = 0; r < MAX_STACK_ARG_SLOTS; r++)
+				callee_args[MAX_BPF_REG + r] = none;
 			callee_args[cb_callee_reg] = info[subprog].at_in[j][caller_reg];
 		} else {
 			continue;
-- 
2.53.0-Meta