bpf: Fix variable length stack write over spilled pointers

[ Upstream commit 4639eb9 ]

Scrub slots if variable-offset stack write goes over spilled pointers.
Otherwise is_spilled_reg() may == true && spilled_ptr.type == NOT_INIT
and valid program is rejected by check_stack_read_fixed_off()
with obscure "invalid size of register fill" message.

Fixes: 01f810a ("bpf: Allow variable-offset stack access")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260324215938.81733-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Alexei Starovoitov

kernel/bpf/verifier.c

@@ -5241,6 +5241,18 @@ static void check_fastcall_stack_contract(struct bpf_verifier_env *env,
 	}
 }
 
+static void scrub_special_slot(struct bpf_func_state *state, int spi)
+{
+	int i;
+
+	/* regular write of data into stack destroys any spilled ptr */
+	state->stack[spi].spilled_ptr.type = NOT_INIT;
+	/* Mark slots as STACK_MISC if they belonged to spilled ptr/dynptr/iter. */
+	if (is_stack_slot_special(&state->stack[spi]))
+		for (i = 0; i < BPF_REG_SIZE; i++)
+			scrub_spilled_slot(&state->stack[spi].slot_type[i]);
+}
+
 /* check_stack_{read,write}_fixed_off functions track spill/fill of registers,
  * stack boundary and alignment are checked in check_mem_access()
  */
@@ -5338,12 +5350,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 	} else {
 		u8 type = STACK_MISC;
 
-		/* regular write of data into stack destroys any spilled ptr */
-		state->stack[spi].spilled_ptr.type = NOT_INIT;
-		/* Mark slots as STACK_MISC if they belonged to spilled ptr/dynptr/iter. */
-		if (is_stack_slot_special(&state->stack[spi]))
-			for (i = 0; i < BPF_REG_SIZE; i++)
-				scrub_spilled_slot(&state->stack[spi].slot_type[i]);
+		scrub_special_slot(state, spi);
 
 		/* when we zero initialize stack slots mark them as such */
 		if ((reg && register_is_null(reg)) ||
@@ -5467,8 +5474,13 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
 			}
 		}
 
-		/* Erase all other spilled pointers. */
-		state->stack[spi].spilled_ptr.type = NOT_INIT;
+		/*
+		 * Scrub slots if variable-offset stack write goes over spilled pointers.
+		 * Otherwise is_spilled_reg() may == true && spilled_ptr.type == NOT_INIT
+		 * and valid program is rejected by check_stack_read_fixed_off()
+		 * with obscure "invalid size of register fill" message.
+		 */
+		scrub_special_slot(state, spi);
 
 		/* Update the slot type. */
 		new_type = STACK_MISC;