bpf: correct slot_type marking logic to allow more stack slot sharing

Verifier is supposed to support sharing stack slot allocated to ptr with
SCALAR_VALUE for privileged program. However this doesn't happen for some
cases.

The reason is verifier is not clearing slot_type STACK_SPILL for all bytes,
it only clears part of them, while verifier is using:

  slot_type[0] == STACK_SPILL

as a convention to check one slot is ptr type.

So, the consequence of partial clearing slot_type is verifier could treat a
partially overridden ptr slot, which should now be a SCALAR_VALUE slot,
still as ptr slot, and rejects some valid programs.

Before this patch, test_xdp_noinline.o under bpf selftests, bpf_lxc.o and
bpf_netdev.o under Cilium bpf repo, when built with -mattr=+alu32 are
rejected due to this issue. After this patch, they all accepted.

There is no processed insn number change before and after this patch on
Cilium bpf programs.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -1286,6 +1286,10 @@ static int check_stack_write(struct bpf_verifier_env *env,
 
 		/* 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. */
+		if (state->stack[spi].slot_type[0] == STACK_SPILL)
+			for (i = 0; i < BPF_REG_SIZE; i++)
+				state->stack[spi].slot_type[i] = STACK_MISC;
 
 		/* only mark the slot as written if all 8 bytes were written
 		 * otherwise read propagation may incorrectly stop too soon
@@ -1303,6 +1307,7 @@ static int check_stack_write(struct bpf_verifier_env *env,
 		    register_is_null(&cur->regs[value_regno]))
 			type = STACK_ZERO;
 
+		/* Mark slots affected by this stack write. */
 		for (i = 0; i < size; i++)
 			state->stack[spi].slot_type[(slot - i) % BPF_REG_SIZE] =
 				type;

tools/testing/selftests/bpf/test_verifier.c

@@ -1001,14 +1001,44 @@ static struct bpf_test tests[] = {
 			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
 			/* mess up with R1 pointer on stack */
 			BPF_ST_MEM(BPF_B, BPF_REG_10, -7, 0x23),
-			/* fill back into R0 should fail */
+			/* fill back into R0 is fine for priv.
+			 * R0 now becomes SCALAR_VALUE.
+			 */
 			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
+			/* Load from R0 should fail. */
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 8),
 			BPF_EXIT_INSN(),
 		},
 		.errstr_unpriv = "attempt to corrupt spilled",
-		.errstr = "corrupted spill",
+		.errstr = "R0 invalid mem access 'inv",
 		.result = REJECT,
 	},
+	{
+		"check corrupted spill/fill, LSB",
+		.insns = {
+			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
+			BPF_ST_MEM(BPF_H, BPF_REG_10, -8, 0xcafe),
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
+			BPF_EXIT_INSN(),
+		},
+		.errstr_unpriv = "attempt to corrupt spilled",
+		.result_unpriv = REJECT,
+		.result = ACCEPT,
+		.retval = POINTER_VALUE,
+	},
+	{
+		"check corrupted spill/fill, MSB",
+		.insns = {
+			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
+			BPF_ST_MEM(BPF_W, BPF_REG_10, -4, 0x12345678),
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
+			BPF_EXIT_INSN(),
+		},
+		.errstr_unpriv = "attempt to corrupt spilled",
+		.result_unpriv = REJECT,
+		.result = ACCEPT,
+		.retval = POINTER_VALUE,
+	},
 	{
 		"invalid src register in STX",
 		.insns = {