bpf: Add precision marking and backtracking for stack argument slots

Extend the precision marking and backtracking infrastructure to
support stack argument slots (r11-based accesses). Without this,
precision demands for scalar values passed through stack arguments
are silently dropped, which could allow the verifier to incorrectly
prune states with different constant values in stack arg slots.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260513045025.2387526-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 57 insertions, 1 deletions

diff --git a/kernel/bpf/backtrack.c b/kernel/bpf/backtrack.c
index 5e93e57fb7ae..2e4ae0ef0860 100644
--- a/kernel/bpf/backtrack.c
+++ b/kernel/bpf/backtrack.c
@@ -129,11 +129,21 @@ static inline u32 bt_empty(struct backtrack_state *bt)
 	int i;
 
 	for (i = 0; i <= bt->frame; i++)
-		mask |= bt->reg_masks[i] | bt->stack_masks[i];
+		mask |= bt->reg_masks[i] | bt->stack_masks[i] | bt->stack_arg_masks[i];
 
 	return mask == 0;
 }
 
+static inline void bt_clear_frame_stack_arg_slot(struct backtrack_state *bt, u32 frame, u32 slot)
+{
+	bt->stack_arg_masks[frame] &= ~(1 << slot);
+}
+
+static inline bool bt_is_frame_stack_arg_slot_set(struct backtrack_state *bt, u32 frame, u32 slot)
+{
+	return bt->stack_arg_masks[frame] & (1 << slot);
+}
+
 static inline int bt_subprog_enter(struct backtrack_state *bt)
 {
 	if (bt->frame == MAX_CALL_FRAMES - 1) {
@@ -194,6 +204,11 @@ static inline u64 bt_stack_mask(struct backtrack_state *bt)
 	return bt->stack_masks[bt->frame];
 }
 
+static inline u8 bt_stack_arg_mask(struct backtrack_state *bt)
+{
+	return bt->stack_arg_masks[bt->frame];
+}
+
 static inline bool bt_is_reg_set(struct backtrack_state *bt, u32 reg)
 {
 	return bt->reg_masks[bt->frame] & (1 << reg);
@@ -335,6 +350,19 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 			return 0;
 		bt_clear_reg(bt, load_reg);
 
+		if (hist && hist->flags & INSN_F_STACK_ARG_ACCESS) {
+			spi = hist->spi;
+			/*
+			 * Stack arg read: callee reads from r11+off, but
+			 * the data lives in the caller's stack_arg_regs.
+			 * Set the mask in the caller frame so precision
+			 * is marked in the caller's slot at the callee
+			 * entry checkpoint.
+			 */
+			bt_set_frame_stack_arg_slot(bt, bt->frame - 1, spi);
+			return 0;
+		}
+
 		/* scalars can only be spilled into stack w/o losing precision.
 		 * Load from any other memory can be zero extended.
 		 * The desire to keep that precision is already indicated
@@ -357,6 +385,17 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 			 * encountered a case of pointer subtraction.
 			 */
 			return -ENOTSUPP;
+
+		if (hist && hist->flags & INSN_F_STACK_ARG_ACCESS) {
+			spi = hist->spi;
+			if (!bt_is_frame_stack_arg_slot_set(bt, bt->frame, spi))
+				return 0;
+			bt_clear_frame_stack_arg_slot(bt, bt->frame, spi);
+			if (class == BPF_STX)
+				bt_set_reg(bt, sreg);
+			return 0;
+		}
+
 		/* scalars can only be spilled into stack */
 		if (!hist || !(hist->flags & INSN_F_STACK_ACCESS))
 			return 0;
@@ -425,6 +464,12 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 						bpf_bt_set_frame_reg(bt, bt->frame - 1, i);
 					}
 				}
+				if (bt_stack_arg_mask(bt)) {
+					verifier_bug(env,
+						     "static subprog leftover stack arg slots %x",
+						     bt_stack_arg_mask(bt));
+					return -EFAULT;
+				}
 				if (bt_subprog_exit(bt))
 					return -EFAULT;
 				return 0;
@@ -895,6 +940,17 @@ int bpf_mark_chain_precision(struct bpf_verifier_env *env,
 					*changed = true;
 				}
 			}
+			for (i = 0; i < func->out_stack_arg_cnt; i++) {
+				if (!bt_is_frame_stack_arg_slot_set(bt, fr, i))
+					continue;
+				reg = &func->stack_arg_regs[i];
+				if (reg->type != SCALAR_VALUE || reg->precise) {
+					bt_clear_frame_stack_arg_slot(bt, fr, i);
+				} else {
+					reg->precise = true;
+					*changed = true;
+				}
+			}
 			if (env->log.level & BPF_LOG_LEVEL2) {
 				fmt_reg_mask(env->tmp_str_buf, TMP_STR_BUF_LEN,
 					     bt_frame_reg_mask(bt, fr));