Commit message: bpf/verifier: fix bounds calculation on BPF_RSH

Incorrect signed bounds were being computed.
If the old upper signed bound was positive and the old lower signed bound was
negative, this could cause the new upper signed bound to be too low,
leading to security issues.

Fixes: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
[jannh@google.com: changed description to reflect bug impact]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
CVE: CVE-2017-17853
CWE: CWE-119
Vulnerability description: kernel/bpf/verifier.c in the Linux kernel through 4.14.8 allows local users to cause a denial of service (memory corruption) or possibly have unspecified other impact by leveraging incorrect BPF_RSH signed bounds calculations.

Project: linux
File: verifier.c

static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
				      struct bpf_insn *insn,
				      struct bpf_reg_state *dst_reg,
				      struct bpf_reg_state src_reg)
{
	struct bpf_reg_state *regs = cur_regs(env);
	u8 opcode = BPF_OP(insn->code);
	bool src_known, dst_known;
	s64 smin_val, smax_val;
	u64 umin_val, umax_val;

	if (BPF_CLASS(insn->code) != BPF_ALU64) {
		/* 32-bit ALU ops are (32,32)->64 */
		coerce_reg_to_32(dst_reg);
		coerce_reg_to_32(&src_reg);
	}
	smin_val = src_reg.smin_value;
	smax_val = src_reg.smax_value;
	umin_val = src_reg.umin_value;
	umax_val = src_reg.umax_value;
	src_known = tnum_is_const(src_reg.var_off);
	dst_known = tnum_is_const(dst_reg->var_off);

	switch (opcode) {
	case BPF_ADD:
		if (signed_add_overflows(dst_reg->smin_value, smin_val) ||
		    signed_add_overflows(dst_reg->smax_value, smax_val)) {
			dst_reg->smin_value = S64_MIN;
			dst_reg->smax_value = S64_MAX;
		} else {
			dst_reg->smin_value += smin_val;
			dst_reg->smax_value += smax_val;
		}
		if (dst_reg->umin_value + umin_val < umin_val ||
		    dst_reg->umax_value + umax_val < umax_val) {
			dst_reg->umin_value = 0;
			dst_reg->umax_value = U64_MAX;
		} else {
			dst_reg->umin_value += umin_val;
			dst_reg->umax_value += umax_val;
		}
		dst_reg->var_off = tnum_add(dst_reg->var_off, src_reg.var_off);
		break;
	case BPF_SUB:
		if (signed_sub_overflows(dst_reg->smin_value, smax_val) ||
		    signed_sub_overflows(dst_reg->smax_value, smin_val)) {
			/* Overflow possible, we know nothing */
			dst_reg->smin_value = S64_MIN;
			dst_reg->smax_value = S64_MAX;
		} else {
			dst_reg->smin_value -= smax_val;
			dst_reg->smax_value -= smin_val;
		}
		if (dst_reg->umin_value < umax_val) {
			/* Overflow possible, we know nothing */
			dst_reg->umin_value = 0;
			dst_reg->umax_value = U64_MAX;
		} else {
			/* Cannot overflow (as long as bounds are consistent) */
			dst_reg->umin_value -= umax_val;
			dst_reg->umax_value -= umin_val;
		}
		dst_reg->var_off = tnum_sub(dst_reg->var_off, src_reg.var_off);
		break;
	case BPF_MUL:
		dst_reg->var_off = tnum_mul(dst_reg->var_off, src_reg.var_off);
		if (smin_val < 0 || dst_reg->smin_value < 0) {
			/* Ain't nobody got time to multiply that sign */
			__mark_reg_unbounded(dst_reg);
			__update_reg_bounds(dst_reg);
			break;
		}
		/* Both values are positive, so we can work with unsigned and
		 * copy the result to signed (unless it exceeds S64_MAX).
		 */
		if (umax_val > U32_MAX || dst_reg->umax_value > U32_MAX) {
			/* Potential overflow, we know nothing */
			__mark_reg_unbounded(dst_reg);
			/* (except what we can learn from the var_off) */
			__update_reg_bounds(dst_reg);
			break;
		}
		dst_reg->umin_value *= umin_val;
		dst_reg->umax_value *= umax_val;
		if (dst_reg->umax_value > S64_MAX) {
			/* Overflow possible, we know nothing */
			dst_reg->smin_value = S64_MIN;
			dst_reg->smax_value = S64_MAX;
		} else {
			dst_reg->smin_value = dst_reg->umin_value;
			dst_reg->smax_value = dst_reg->umax_value;
		}
		break;
	case BPF_AND:
		if (src_known && dst_known) {
			__mark_reg_known(dst_reg, dst_reg->var_off.value &
						  src_reg.var_off.value);
			break;
		}
		/* We get our minimum from the var_off, since that's inherently
		 * bitwise.  Our maximum is the minimum of the operands' maxima.
		 */
		dst_reg->var_off = tnum_and(dst_reg->var_off, src_reg.var_off);
		dst_reg->umin_value = dst_reg->var_off.value;
		dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
		if (dst_reg->smin_value < 0 || smin_val < 0) {
			/* Lose signed bounds when ANDing negative numbers,
			 * ain't nobody got time for that.
			 */
			dst_reg->smin_value = S64_MIN;
			dst_reg->smax_value = S64_MAX;
		} else {
			/* ANDing two positives gives a positive, so safe to
			 * cast result into s64.
			 */
			dst_reg->smin_value = dst_reg->umin_value;
			dst_reg->smax_value = dst_reg->umax_value;
		}
		/* We may learn something more from the var_off */
		__update_reg_bounds(dst_reg);
		break;
	case BPF_OR:
		if (src_known && dst_known) {
			__mark_reg_known(dst_reg, dst_reg->var_off.value |
						  src_reg.var_off.value);
			break;
		}
		/* We get our maximum from the var_off, and our minimum is the
		 * maximum of the operands' minima
		 */
		dst_reg->var_off = tnum_or(dst_reg->var_off, src_reg.var_off);
		dst_reg->umin_value = max(dst_reg->umin_value, umin_val);
		dst_reg->umax_value = dst_reg->var_off.value |
				      dst_reg->var_off.mask;
		if (dst_reg->smin_value < 0 || smin_val < 0) {
			/* Lose signed bounds when ORing negative numbers,
			 * ain't nobody got time for that.
			 */
			dst_reg->smin_value = S64_MIN;
			dst_reg->smax_value = S64_MAX;
		} else {
			/* ORing two positives gives a positive, so safe to
			 * cast result into s64.
			 */
			dst_reg->smin_value = dst_reg->umin_value;
			dst_reg->smax_value = dst_reg->umax_value;
		}
		/* We may learn something more from the var_off */
		__update_reg_bounds(dst_reg);
		break;
	case BPF_LSH:
		if (umax_val > 63) {
			/* Shifts greater than 63 are undefined.  This includes
			 * shifts by a negative number.
			 */
			mark_reg_unknown(env, regs, insn->dst_reg);
			break;
		}
		/* We lose all sign bit information (except what we can pick
		 * up from var_off)
		 */
		dst_reg->smin_value = S64_MIN;
		dst_reg->smax_value = S64_MAX;
		/* If we might shift our top bit out, then we know nothing */
		if (dst_reg->umax_value > 1ULL << (63 - umax_val)) {
			dst_reg->umin_value = 0;
			dst_reg->umax_value = U64_MAX;
		} else {
			dst_reg->umin_value <<= umin_val;
			dst_reg->umax_value <<= umax_val;
		}
		if (src_known)
			dst_reg->var_off = tnum_lshift(dst_reg->var_off, umin_val);
		else
			dst_reg->var_off = tnum_lshift(tnum_unknown, umin_val);
		/* We may learn something more from the var_off */
		__update_reg_bounds(dst_reg);
		break;
	case BPF_RSH:
		if (umax_val > 63) {
			/* Shifts greater than 63 are undefined.  This includes
			 * shifts by a negative number.
			 */
 			mark_reg_unknown(env, regs, insn->dst_reg);
 			break;
 		}
		/* BPF_RSH is an unsigned shift, so make the appropriate casts */
		if (dst_reg->smin_value < 0) {
			if (umin_val) {
				/* Sign bit will be cleared */
				dst_reg->smin_value = 0;
			} else {
				/* Lost sign bit information */
				dst_reg->smin_value = S64_MIN;
				dst_reg->smax_value = S64_MAX;
			}
		} else {
			dst_reg->smin_value =
				(u64)(dst_reg->smin_value) >> umax_val;
		}
 		if (src_known)
 			dst_reg->var_off = tnum_rshift(dst_reg->var_off,
 						       umin_val);
		else
			dst_reg->var_off = tnum_rshift(tnum_unknown, umin_val);
		dst_reg->umin_value >>= umax_val;
		dst_reg->umax_value >>= umin_val;
		/* We may learn something more from the var_off */
		__update_reg_bounds(dst_reg);
		break;
	default:
		mark_reg_unknown(env, regs, insn->dst_reg);
		break;
	}

	__reg_deduce_bounds(dst_reg);
	__reg_bound_offset(dst_reg);
	return 0;
}