Code: 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.  If the value in dst_reg might
		 * be negative, then either:
		 * 1) src_reg might be zero, so the sign bit of the result is
		 *    unknown, so we lose our signed bounds
		 * 2) it's known negative, thus the unsigned bounds capture the
		 *    signed bounds
		 * 3) the signed bounds cross zero, so they tell us nothing
		 *    about the result
		 * If the value in dst_reg is known nonnegative, then again the
		 * unsigned bounts capture the signed bounds.
		 * Thus, in all cases it suffices to blow away our signed bounds
		 * and rely on inferring new ones from the unsigned bounds and
		 * var_off of the result.
		 */
		dst_reg->smin_value = S64_MIN;
		dst_reg->smax_value = S64_MAX;
		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;
}

Vulnerability Type: DoS Overflow Mem. Corr. 
CWE ID: CWE-119
Summary: 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 register truncation mishandling.
Commit Message: bpf: fix incorrect tracking of register size truncation

Properly handle register truncation to a smaller size.

The old code first mirrors the clearing of the high 32 bits in the bitwise
tristate representation, which is correct. But then, it computes the new
arithmetic bounds as the intersection between the old arithmetic bounds and
the bounds resulting from the bitwise tristate representation. Therefore,
when coerce_reg_to_32() is called on a number with bounds
[0xffff'fff8, 0x1'0000'0007], the verifier computes
[0xffff'fff8, 0xffff'ffff] as bounds of the truncated number.
This is incorrect: The truncated number could also be in the range [0, 7],
and no meaningful arithmetic bounds can be computed in that case apart from
the obvious [0, 0xffff'ffff].

Starting with v4.14, this is exploitable by unprivileged users as long as
the unprivileged_bpf_disabled sysctl isn't set.

Debian assigned CVE-2017-16996 for this issue.

v2:
 - flip the mask during arithmetic bounds calculation (Ben Hutchings)
v3:
 - add CVE number (Ben Hutchings)

Fixes: b03c9f9fdc37 ("bpf/verifier: track signed and unsigned min/max values")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>