Merge branch 'bpf-fix-null-ptr-derefs-when-showing-a-void-btf-type'

Jiayuan Chen says:

====================
bpf: Fix NULL-ptr-derefs when showing a void BTF type

This series fixes three NULL-ptr-derefs in BTF handling.

Patch 1 handles the syzbot report. A key-less BTF (btf_key_type_id == 0) used
to be rejected for hash maps, until htab and rhtab gained a ->map_check_btf
(to register a dtor) that does not look at the key, so a key-less hash map is
now accepted. Dumping it through bpffs feeds the key type_id 0 into
btf_type_seq_show() and NULL-derefs in btf_type_show(). Reject it again.

Patches 2 and 3 fix two related, pre-existing crashes reachable via
bpf_snprintf_btf(), which renders a type_id taken straight from the BPF
program against the vmlinux BTF. A "const void" (a modifier resolving to
void) NULL-derefs in btf_modifier_show() - void has no ->show op; a
BTF_KIND_VAR NULL-derefs in btf_var_show() - the vmlinux base BTF has no
resolved_ids. Patch 2 falls back to btf_df_show(), the "<unsupported kind:N>"
placeholder already used for FWD/FUNC/FLOAT/DECL_TAG; patch 3 resolves the
var's type directly, mirroring the existing guard in btf_modifier_show().

Patches 4 and 5 add selftests. Patch 4 checks a key-less hash and rhash map is
rejected at creation. Patch 5 extends the snprintf_btf test to render a
"const void" and a BTF_KIND_VAR from the vmlinux BTF and checks they resolve
without crashing.

v3 -> v4: Drop the pin-and-read reproducer from the key-less map test, fold
the void/VAR test into the existing snprintf_btf test, and simplify patch 2's
wording (review comments).

v2 -> v3: Fold in a third fix for the same class of bug, btf_var_show(),
reported while reviewing v2. Address review comments (Fixes attribution,
verbatim syzbot trace, skip instead of fail).

v1 -> v2: AI reported a pre-exist issue. Let's fold it in this series.

v3: https://lore.kernel.org/bpf/20260831110314.150870-1-jiayuan.chen@linux.dev/
v2: https://lore.kernel.org/bpf/20260830073242.148092-1-jiayuan.chen@linux.dev/
v1: https://lore.kernel.org/bpf/20260828093142.179856-1-jiayuan.chen@linux.dev/
====================

Link: https://patch.msgid.link/20260901104924.346187-1-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -2911,14 +2911,29 @@ static void btf_modifier_show(const struct btf *btf,
 	else
 		t = btf_type_skip_modifiers(btf, type_id, NULL);
 
-	btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
+	/*
+	 * A modifier can resolve to void, which has no show op; print a
+	 * placeholder rather than dereferencing NULL.
+	 */
+	if (!btf_type_ops(t))
+		btf_df_show(btf, t, type_id, data, bits_offset, show);
+	else
+		btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
 }
 
 static void btf_var_show(const struct btf *btf, const struct btf_type *t,
 			 u32 type_id, void *data, u8 bits_offset,
 			 struct btf_show *show)
 {
-	t = btf_type_id_resolve(btf, &type_id);
+	/*
+	 * btf_type_id_resolve() dereferences btf->resolved_ids, which is NULL
+	 * for a base BTF (e.g. the vmlinux BTF that bpf_snprintf_btf() uses).
+	 * Resolve the var's type directly in that case.
+	 */
+	if (btf->resolved_ids)
+		t = btf_type_id_resolve(btf, &type_id);
+	else
+		t = btf_type_skip_modifiers(btf, t->type, &type_id);
 
 	btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
 }

kernel/bpf/hashtab.c

@@ -530,6 +530,9 @@ static int htab_map_check_btf(struct bpf_map *map, const struct btf *btf,
 {
 	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
 
+	if (btf_type_is_void(key_type))
+		return -EINVAL;
+
 	if (htab_is_prealloc(htab))
 		return 0;
 	/*
@@ -3111,6 +3114,9 @@ static int rhtab_map_check_btf(struct bpf_map *map, const struct btf *btf,
 {
 	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
 
+	if (btf_type_is_void(key_type))
+		return -EINVAL;
+
 	return bpf_ma_set_dtor(map, &rhtab->ma, rhtab_mem_dtor);
 }
 

tools/testing/selftests/bpf/prog_tests/btf_map_keyless.c

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+/*
+ * A hash map with a key-less BTF (btf_key_type_id == 0) used to be accepted
+ * and then NULL-deref in btf_type_show() when dumped through bpffs. A fixed
+ * kernel rejects it at creation; verify that rejection, with a keyed positive
+ * control so the -EINVAL is about the missing key type and not some unrelated
+ * failure.
+ */
+static void check_keyless(int map_type, __u32 map_flags, int btf_fd, int val_id)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts);
+	int map_fd;
+
+	opts.map_flags = map_flags;
+	opts.btf_fd = btf_fd;
+	opts.btf_value_type_id = val_id;
+
+	/* Positive control: the same map with a real key type is accepted. */
+	opts.btf_key_type_id = val_id;
+	map_fd = bpf_map_create(map_type, "keyed_map", 4, 4, 8, &opts);
+	if (!ASSERT_GE(map_fd, 0, "keyed create is accepted"))
+		return;
+	close(map_fd);
+
+	/* A key-less BTF must be rejected. */
+	opts.btf_key_type_id = 0;
+	map_fd = bpf_map_create(map_type, "keyless_map", 4, 4, 8, &opts);
+	ASSERT_EQ(map_fd, -EINVAL, "key-less create is rejected");
+	if (map_fd >= 0)
+		close(map_fd);
+}
+
+void test_btf_map_keyless(void)
+{
+	int btf_fd, val_id;
+	struct btf *btf;
+
+	btf = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf, "btf__new_empty"))
+		return;
+
+	val_id = btf__add_int(btf, "int", 4, BTF_INT_SIGNED);
+	if (!ASSERT_GT(val_id, 0, "btf__add_int"))
+		goto out;
+
+	if (!ASSERT_OK(btf__load_into_kernel(btf), "btf__load_into_kernel"))
+		goto out;
+	btf_fd = btf__fd(btf);
+
+	if (test__start_subtest("hash"))
+		check_keyless(BPF_MAP_TYPE_HASH, 0, btf_fd, val_id);
+	if (test__start_subtest("rhash"))
+		check_keyless(BPF_MAP_TYPE_RHASH, BPF_F_NO_PREALLOC, btf_fd, val_id);
+out:
+	btf__free(btf);
+}

tools/testing/selftests/bpf/prog_tests/snprintf_btf.c

@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
 #include <linux/btf.h>
+#include <bpf/btf.h>
 #include "netif_receive_skb.skel.h"
+#include "snprintf_btf_void.skel.h"
 
 /* Demonstrate that bpf_snprintf_btf succeeds and that various data types
  * are formatted correctly.
@@ -58,3 +60,80 @@ void serial_test_snprintf_btf(void)
 cleanup:
 	netif_receive_skb__destroy(skel);
 }
+
+/*
+ * bpf_snprintf_btf() renders a type_id taken straight from the vmlinux BTF.
+ * Two such type_ids used to NULL-deref in the BTF show path:
+ *   - a "const void" (a modifier resolving to void) in btf_modifier_show()
+ *   - a BTF_KIND_VAR in btf_var_show() (base BTF has no resolved_ids)
+ * A fixed kernel renders both without crashing.
+ */
+static long run(struct snprintf_btf_void *skel, __u32 type_id)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	char ctx[8] = {};
+
+	skel->bss->type_id = type_id;
+	topts.ctx_in = ctx;
+	topts.ctx_size_in = sizeof(ctx);
+	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(skel->progs.dump_type),
+					      &topts), "test_run"))
+		return -1;
+	return skel->bss->ret;
+}
+
+void test_snprintf_btf_void(void)
+{
+	const struct btf_type *t;
+	struct snprintf_btf_void *skel;
+	int i, n, cv = 0, var = 0;
+	struct btf *btf;
+
+	btf = btf__parse("/sys/kernel/btf/vmlinux", NULL);
+	if (!btf) {
+		test__skip();
+		return;
+	}
+
+	skel = snprintf_btf_void__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		goto out_btf;
+
+	n = btf__type_cnt(btf);
+	for (i = 1; i < n && !(cv && var); i++) {
+		t = btf__type_by_id(btf, i);
+		if (!cv && btf_kind(t) == BTF_KIND_CONST && t->type == 0)
+			cv = i;
+		/* Pick a VAR small enough to render from the program's buffer. */
+		if (!var && btf_kind(t) == BTF_KIND_VAR) {
+			long sz = btf__resolve_size(btf, t->type);
+
+			if (sz > 0 && sz <= (long)sizeof(skel->bss->obj))
+				var = i;
+		}
+	}
+
+	/* "const void" renders the "<unsupported kind:0>" placeholder. */
+	if (test__start_subtest("const_void")) {
+		if (cv) {
+			ASSERT_EQ(run(skel, cv),
+				  sizeof("<unsupported kind:0>") - 1, "ret");
+			ASSERT_STREQ(skel->bss->out, "<unsupported kind:0>",
+				     "placeholder");
+		} else {
+			test__skip();
+		}
+	}
+
+	/* A BTF_KIND_VAR must resolve and render without error. */
+	if (test__start_subtest("var")) {
+		if (var)
+			ASSERT_GT(run(skel, var), 0, "ret");
+		else
+			test__skip();
+	}
+
+	snprintf_btf_void__destroy(skel);
+out_btf:
+	btf__free(btf);
+}

tools/testing/selftests/bpf/progs/snprintf_btf_void.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "btf_ptr.h"
+#include <bpf/bpf_helpers.h>
+
+__u32 type_id;
+/* A buffer we own to render the selected type from, kept in bounds. */
+char obj[256];
+char out[64];
+long ret;
+
+SEC("raw_tp/sys_enter")
+int dump_type(void *ctx)
+{
+	struct btf_ptr ptr = {
+		.ptr = obj,
+		.type_id = type_id,
+		.flags = 0,
+	};
+
+	ret = bpf_snprintf_btf(out, sizeof(out), &ptr, sizeof(ptr), 0);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";