Merge branch 'keep-dynamic-inner-array-lookups-nullable'

Nuoqi Gui says:

====================
Keep dynamic inner array lookups nullable

An ARRAY_OF_MAPS can use an array created with BPF_F_INNER_MAP as its
inner map template. The flag allows a concrete inner array with a
different max_entries value to replace the template.

The verifier currently uses the template's max_entries to elide
nullness for a constant-key lookup through the inner map pointer. At
runtime, the lookup uses the concrete inner array's max_entries instead.
The verifier can therefore accept an unchecked dereference even though
the runtime helper returns NULL.

Patch 1 keeps lookups through BPF_F_INNER_MAP array templates nullable.
Patch 2 adds a verifier regression test for the unchecked dereference.

Before the fix, the regression program is accepted and the runtime
reproducer triggers a NULL dereference. With the fix, both programs are
rejected with an invalid map_value_or_null access.

Tested by compiling kernel/bpf/verifier.o and
verifier_map_in_map.bpf.o, and by running the regression program and
runtime reproducer in QEMU before and after the fix.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
Acked-by: Jiri Olsa <jolsa@kernel.org>
---
v1->v2:
- Update the can_elide_value_nullness() comment to match the changed
  parameter (const struct bpf_map *map).

v1: https://patch.msgid.link/20260604151153.2488051-1-gnq25@mails.tsinghua.edu.cn

To: Alexei Starovoitov <ast@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
To: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Xu <dxu@dxuuu.xyz>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ihor Solodrai <isolodrai@meta.com>
Cc: bpf@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

---
====================

Link: https://patch.msgid.link/20260607-f01-v2-v2-0-da48453146e8@mails.tsinghua.edu.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0c1cf506c219..ed7ba0e6a9ce 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8179,7 +8179,7 @@ static int get_constant_map_key(struct bpf_verifier_env *env,
 	return 0;
 }
 
-static bool can_elide_value_nullness(enum bpf_map_type type);
+static bool can_elide_value_nullness(const struct bpf_map *map);
 
 static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			  struct bpf_call_arg_meta *meta,
@@ -8298,7 +8298,7 @@ skip_type_check:
 		err = check_helper_mem_access(env, reg, argno_from_reg(regno), key_size, BPF_READ, false, NULL);
 		if (err)
 			return err;
-		if (can_elide_value_nullness(meta->map.ptr->map_type)) {
+		if (can_elide_value_nullness(meta->map.ptr)) {
 			err = get_constant_map_key(env, reg, key_size, &meta->const_map_key);
 			if (err < 0) {
 				meta->const_map_key = -1;
@@ -10068,13 +10068,16 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno
 				 state->callback_subprogno == subprogno);
 }
 
-/* Returns whether or not the given map type can potentially elide
+/* Returns whether or not the given map can potentially elide
  * lookup return value nullness check. This is possible if the key
  * is statically known.
  */
-static bool can_elide_value_nullness(enum bpf_map_type type)
+static bool can_elide_value_nullness(const struct bpf_map *map)
 {
-	switch (type) {
+	if (map->map_flags & BPF_F_INNER_MAP)
+		return false;
+
+	switch (map->map_type) {
 	case BPF_MAP_TYPE_ARRAY:
 	case BPF_MAP_TYPE_PERCPU_ARRAY:
 		return true;
@@ -10414,7 +10417,7 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		}
 
 		if (func_id == BPF_FUNC_map_lookup_elem &&
-		    can_elide_value_nullness(meta.map.ptr->map_type) &&
+		    can_elide_value_nullness(meta.map.ptr) &&
 		    meta.const_map_key >= 0 &&
 		    meta.const_map_key < meta.map.ptr->max_entries)
 			ret_flag &= ~PTR_MAYBE_NULL;