[PATCH RFC bpf-next v1 24/32] bpf: Permit NULL checking pointer with non-zero fixed offset

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Dave Marchevsky <davemarchevsky@fb.com>,
	Delyan Kratunov <delyank@fb.com>
Subject: [PATCH RFC bpf-next v1 24/32] bpf: Permit NULL checking pointer with non-zero fixed offset
Date: Sun,  4 Sep 2022 22:41:37 +0200	[thread overview]
Message-ID: <20220904204145.3089-25-memxor@gmail.com> (raw)
In-Reply-To: <20220904204145.3089-1-memxor@gmail.com>

Pointer increment on seeing PTR_MAYBE_NULL is already protected against,
hence make an exception for local kptrs while still keeping the warning
for other unintended cases that might creep in.

bpf_list_pop_{front,back} helpers return a local kptr with incremented
offset pointing to bpf_list_node field. The user is supposed to then
obtain the pointer to the entry using container_of after NULL checking
it. The current restrictions trigger a warning when doing the NULL
checking. Revisiting the reason, it is meant as an assertion which seems
to actually work and catch the bad case! Good job verifier!

Hence, under no other circumstances can reg->off be non-zero for a
register that has the PTR_MAYBE_NULL type flag set.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index dcbeb503c25c..5e0044796671 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11173,15 +11173,20 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 {
 	if (type_may_be_null(reg->type) && reg->id == id &&
 	    !WARN_ON_ONCE(!reg->id)) {
-		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value ||
-				 !tnum_equals_const(reg->var_off, 0) ||
-				 reg->off)) {
+		if (reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0) || reg->off) {
 			/* Old offset (both fixed and variable parts) should
 			 * have been known-zero, because we don't allow pointer
 			 * arithmetic on pointers that might be NULL. If we
 			 * see this happening, don't convert the register.
+			 *
+			 * But in some cases, some helpers that return local
+			 * kptrs advance offset for the returned pointer.
+			 * In those cases, it is fine to expect to see reg->off.
 			 */
-			return;
+			if (WARN_ON_ONCE(reg->type != (PTR_TO_BTF_ID | MEM_TYPE_LOCAL | PTR_MAYBE_NULL)))
+				return;
+			if (WARN_ON_ONCE(reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0)))
+				return;
 		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;
-- 
2.34.1

