bpf: Do not reject when the stack read size is different from the tracked scalar size

Below is a simplified case from a report in bcc [0]:

  r4 = 20
  *(u32 *)(r10 -4) = r4
  *(u32 *)(r10 -8) = r4  /* r4 state is tracked */
  r4 = *(u64 *)(r10 -8)  /* Read more than the tracked 32bit scalar.
			  * verifier rejects as 'corrupted spill memory'.
			  */

After commit 354e8f19 ("bpf: Support <8-byte scalar spill and refill"),
the 8-byte aligned 32bit spill is also tracked by the verifier and the
register state is stored.

However, if 8 bytes are read from the stack instead of the tracked 4 byte
scalar, then verifier currently rejects the program as "corrupted spill
memory". This patch fixes this case by allowing it to read but marks the
register as unknown.

Also note that, if the prog is trying to corrupt/leak an earlier spilled
pointer by spilling another <8 bytes register on top, this has already
been rejected in the check_stack_write_fixed_off().

  [0] https://github.com/iovisor/bcc/pull/3683

Fixes: 354e8f19 ("bpf: Support <8-byte scalar spill and refill")
Reported-by: Hengqi Chen <hengqi.chen@gmail.com>
Reported-by: Yonghong Song <yhs@gmail.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211102064535.316018-1-kafai@fb.com

kernel/bpf/verifier.c

@@ -3088,9 +3088,12 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 	reg = &reg_state->stack[spi].spilled_ptr;
 
 	if (is_spilled_reg(&reg_state->stack[spi])) {
-		if (size != BPF_REG_SIZE) {
-			u8 scalar_size = 0;
+		u8 spill_size = 1;
+
+		for (i = BPF_REG_SIZE - 1; i > 0 && stype[i - 1] == STACK_SPILL; i--)
+			spill_size++;
 
+		if (size != BPF_REG_SIZE || spill_size != BPF_REG_SIZE) {
 			if (reg->type != SCALAR_VALUE) {
 				verbose_linfo(env, env->insn_idx, "; ");
 				verbose(env, "invalid size of register fill\n");
@@ -3101,10 +3104,7 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 			if (dst_regno < 0)
 				return 0;
 
-			for (i = BPF_REG_SIZE; i > 0 && stype[i - 1] == STACK_SPILL; i--)
-				scalar_size++;
-
-			if (!(off % BPF_REG_SIZE) && size == scalar_size) {
+			if (!(off % BPF_REG_SIZE) && size == spill_size) {
 				/* The earlier check_reg_arg() has decided the
 				 * subreg_def for this insn.  Save it first.
 				 */
@@ -3128,12 +3128,6 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 			state->regs[dst_regno].live |= REG_LIVE_WRITTEN;
 			return 0;
 		}
-		for (i = 1; i < BPF_REG_SIZE; i++) {
-			if (stype[(slot - i) % BPF_REG_SIZE] != STACK_SPILL) {
-				verbose(env, "corrupted spill memory\n");
-				return -EACCES;
-			}
-		}
 
 		if (dst_regno >= 0) {
 			/* restore register state from stack */