Commit d028f875 authored by Menglong Dong's avatar Menglong Dong Committed by Alexei Starovoitov
Browse files

bpf: make the verifier tracks the "not equal" for regs 



We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:

  /* The type of "a" is u32 */
  if (a > 0 && a < 100) {
    /* the range of the register for a is [0, 99], not [1, 99],
     * and will cause the following error:
     *
     *   invalid zero-sized read
     *
     * as a can be 0.
     */
    bpf_skb_store_bytes(skb, xx, xx, a, 0);
  }

In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].

For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.

Signed-off-by: default avatarMenglong Dong <menglong8.dong@gmail.com>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarShung-Hsi Yu <shung-hsi.yu@suse.com>
Link: https://lore.kernel.org/r/20231219134800.1550388-2-menglong8.dong@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 1728df7f

kernel/bpf/verifier.c

+37 −1
Original line number Diff line number Diff line
@@ -14336,7 +14336,43 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state 
		}

		break;

	case BPF_JNE:

		/* we don't derive any new information for inequality yet */

		if (!is_reg_const(reg2, is_jmp32))

			swap(reg1, reg2);

		if (!is_reg_const(reg2, is_jmp32))

			break;









		/* try to recompute the bound of reg1 if reg2 is a const and









		 * is exactly the edge of reg1.









		 */









		val = reg_const_value(reg2, is_jmp32);









		if (is_jmp32) {









			/* u32_min_value is not equal to 0xffffffff at this point,









			 * because otherwise u32_max_value is 0xffffffff as well,









			 * in such a case both reg1 and reg2 would be constants,









			 * jump would be predicted and reg_set_min_max() won't









			 * be called.









			 *









			 * Same reasoning works for all {u,s}{min,max}{32,64} cases









			 * below.









			 */









			if (reg1->u32_min_value == (u32)val)









				reg1->u32_min_value++;









			if (reg1->u32_max_value == (u32)val)









				reg1->u32_max_value--;









			if (reg1->s32_min_value == (s32)val)









				reg1->s32_min_value++;









			if (reg1->s32_max_value == (s32)val)









				reg1->s32_max_value--;









		} else {









			if (reg1->umin_value == (u64)val)









				reg1->umin_value++;









			if (reg1->umax_value == (u64)val)









				reg1->umax_value--;









			if (reg1->smin_value == (s64)val)









				reg1->smin_value++;









			if (reg1->smax_value == (s64)val)









				reg1->smax_value--;









		}









		break;









	case BPF_JSET:









		if (!is_reg_const(reg2, is_jmp32))