Commit message: bpf: don't prune branches when a scalar is replaced with a pointer

This could be made safe by passing through a reference to env and checking
for env->allow_ptr_leaks, but it would only work one way and is probably
not worth the hassle - not doing it will not directly lead to program
rejection.

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
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-2017-17864
CWE: CWE-119
Vulnerability description: kernel/bpf/verifier.c in the Linux kernel through 4.14.8 mishandles states_equal comparisons between the pointer data type and the UNKNOWN_VALUE data type, which allows local users to obtain potentially sensitive address information, aka a "pointer leak."

Project: linux
File: verifier.c

static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
		    struct idpair *idmap)
{
	if (!(rold->live & REG_LIVE_READ))
		/* explored state didn't use this */
		return true;

	if (memcmp(rold, rcur, offsetof(struct bpf_reg_state, live)) == 0)
		return true;

	if (rold->type == NOT_INIT)
		/* explored state can't have used this */
		return true;
	if (rcur->type == NOT_INIT)
		return false;
	switch (rold->type) {
	case SCALAR_VALUE:
		if (rcur->type == SCALAR_VALUE) {
			/* new val must satisfy old val knowledge */
 			return range_within(rold, rcur) &&
 			       tnum_in(rold->var_off, rcur->var_off);
 		} else {
			/* if we knew anything about the old value, we're not
			 * equal, because we can't know anything about the
			 * scalar value of the pointer in the new value.
 			 */
			return rold->umin_value == 0 &&
			       rold->umax_value == U64_MAX &&
			       rold->smin_value == S64_MIN &&
			       rold->smax_value == S64_MAX &&
			       tnum_is_unknown(rold->var_off);
 		}
 	case PTR_TO_MAP_VALUE:
 		/* If the new min/max/var_off satisfy the old ones and
		 * everything else matches, we are OK.
		 * We don't care about the 'id' value, because nothing
		 * uses it for PTR_TO_MAP_VALUE (only for ..._OR_NULL)
		 */
		return memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)) == 0 &&
		       range_within(rold, rcur) &&
		       tnum_in(rold->var_off, rcur->var_off);
	case PTR_TO_MAP_VALUE_OR_NULL:
		/* a PTR_TO_MAP_VALUE could be safe to use as a
		 * PTR_TO_MAP_VALUE_OR_NULL into the same map.
		 * However, if the old PTR_TO_MAP_VALUE_OR_NULL then got NULL-
		 * checked, doing so could have affected others with the same
		 * id, and we can't check for that because we lost the id when
		 * we converted to a PTR_TO_MAP_VALUE.
		 */
		if (rcur->type != PTR_TO_MAP_VALUE_OR_NULL)
			return false;
		if (memcmp(rold, rcur, offsetof(struct bpf_reg_state, id)))
			return false;
		/* Check our ids match any regs they're supposed to */
		return check_ids(rold->id, rcur->id, idmap);
	case PTR_TO_PACKET_META:
	case PTR_TO_PACKET:
		if (rcur->type != rold->type)
			return false;
		/* We must have at least as much range as the old ptr
		 * did, so that any accesses which were safe before are
		 * still safe.  This is true even if old range < old off,
		 * since someone could have accessed through (ptr - k), or
		 * even done ptr -= k in a register, to get a safe access.
		 */
		if (rold->range > rcur->range)
			return false;
		/* If the offsets don't match, we can't trust our alignment;
		 * nor can we be sure that we won't fall out of range.
		 */
		if (rold->off != rcur->off)
			return false;
		/* id relations must be preserved */
		if (rold->id && !check_ids(rold->id, rcur->id, idmap))
			return false;
		/* new val must satisfy old val knowledge */
		return range_within(rold, rcur) &&
		       tnum_in(rold->var_off, rcur->var_off);
	case PTR_TO_CTX:
	case CONST_PTR_TO_MAP:
	case PTR_TO_STACK:
	case PTR_TO_PACKET_END:
		/* Only valid matches are exact, which memcmp() above
		 * would have accepted
		 */
	default:
		/* Don't know what's going on, just say it's not safe */
		return false;
	}

	/* Shouldn't get here; if we do, say it's not safe */
	WARN_ON_ONCE(1);
	return false;
}