[PATCH bpf v4 0/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[PATCH bpf v4 0/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[Qi Tang]

check_mem_access() allows direct dereference of PTR_TO_BUF |
PTR_MAYBE_NULL without a null check, causing kernel NULL dereference
on map iterator stop callbacks.

Patch 1 adds the missing type_may_be_null() guard.
Patch 2 adds a selftest with a test runner.

Changes in v4:
  - Add test runner in prog_tests/ so the test is actually
    invoked by test_progs (Kumar)

Changes in v3:
  - Drop stack trace from commit log
  - Split kernel fix and selftest into separate patches
  - Use [PATCH bpf] prefix

Changes in v2:
  - Add selftest

Link: https://lore.kernel.org/bpf/20260402092923.38357-1-tpluszz77@gmail.com/ (v3)
Link: https://lore.kernel.org/all/20260401142316.2452-1-tpluszz77@gmail.com/ (v2)
Link: https://lore.kernel.org/all/20260331172720.29938-1-tpluszz77@gmail.com/ (v1)

Qi Tang (2):
  bpf: reject direct access to nullable PTR_TO_BUF pointers
  selftests/bpf: add test for nullable PTR_TO_BUF access

 kernel/bpf/verifier.c                                       |  3 ++-
 tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c  |  9 +++++++++
 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c       | 51 ++++++++++++
 3 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
 create mode 100644 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c

[PATCH bpf v4 1/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[Qi Tang]

check_mem_access() matches PTR_TO_BUF via base_type() which strips
PTR_MAYBE_NULL, allowing direct dereference without a null check.

Map iterator ctx->key and ctx->value are PTR_TO_BUF | PTR_MAYBE_NULL.
On stop callbacks these are NULL, causing a kernel NULL dereference.

Add a type_may_be_null() guard to the PTR_TO_BUF branch, matching the
existing PTR_TO_BTF_ID pattern.

Fixes: 20b2aff4bc15 ("bpf: Introduce MEM_RDONLY flag")
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 937c9adf1b3d..17850836943b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7905,7 +7905,8 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 	} else if (reg->type == CONST_PTR_TO_MAP) {
 		err = check_ptr_to_map_access(env, regs, regno, off, size, t,
 					      value_regno);
-	} else if (base_type(reg->type) == PTR_TO_BUF) {
+	} else if (base_type(reg->type) == PTR_TO_BUF &&
+		   !type_may_be_null(reg->type)) {
 		bool rdonly_mem = type_is_rdonly_mem(reg->type);
 		u32 *max_access;
 
-- 
2.43.0

Re: [PATCH bpf v4 1/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[Amery Hung]

On Thu, Apr 2, 2026 at 8:38 PM Qi Tang <tpluszz77@gmail.com> wrote:
>
> check_mem_access() matches PTR_TO_BUF via base_type() which strips
> PTR_MAYBE_NULL, allowing direct dereference without a null check.
>
> Map iterator ctx->key and ctx->value are PTR_TO_BUF | PTR_MAYBE_NULL.
> On stop callbacks these are NULL, causing a kernel NULL dereference.
>
> Add a type_may_be_null() guard to the PTR_TO_BUF branch, matching the
> existing PTR_TO_BTF_ID pattern.
>
> Fixes: 20b2aff4bc15 ("bpf: Introduce MEM_RDONLY flag")
> Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Signed-off-by: Qi Tang <tpluszz77@gmail.com>

Reviewed-by: Amery Hung <ameryhung@gmail.com>

[PATCH bpf v4 2/2] selftests/bpf: add test for nullable PTR_TO_BUF access

[Qi Tang]

Add iter_buf_null_fail with two tests and a test runner:
  - iter_buf_null_deref: verifier must reject direct dereference of
    ctx->key (PTR_TO_BUF | PTR_MAYBE_NULL) without a null check
  - iter_buf_null_check_ok: verifier must accept dereference after
    an explicit null check

Signed-off-by: Qi Tang <tpluszz77@gmail.com>
---
 .../bpf/prog_tests/iter_buf_null_fail.c       |  9 ++++
 .../selftests/bpf/progs/iter_buf_null_fail.c  | 51 +++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
 create mode 100644 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c

diff --git a/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
new file mode 100644
index 000000000000..ea97787b870d
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "iter_buf_null_fail.skel.h"
+
+void test_iter_buf_null_fail(void)
+{
+	RUN_TESTS(iter_buf_null_fail);
+}
diff --git a/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
new file mode 100644
index 000000000000..963a4cfc33f3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Qi Tang */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u64);
+} hashmap SEC(".maps");
+
+/*
+ * Verify that the verifier rejects direct access to nullable PTR_TO_BUF
+ * (ctx->key) without a null check.  On the iterator stop callback,
+ * ctx->key is NULL, so unconditional access would be a NULL deref.
+ */
+SEC("iter/bpf_map_elem")
+__failure __msg("invalid mem access")
+int iter_buf_null_deref(struct bpf_iter__bpf_map_elem *ctx)
+{
+	/* ctx->key is PTR_TO_BUF | PTR_MAYBE_NULL | MEM_RDONLY.
+	 * Direct access without null check must be rejected.
+	 */
+	volatile __u32 v = *(__u32 *)ctx->key;
+
+	(void)v;
+	return 0;
+}
+
+/*
+ * Verify that access after a null check is still accepted.
+ */
+SEC("iter/bpf_map_elem")
+__success
+int iter_buf_null_check_ok(struct bpf_iter__bpf_map_elem *ctx)
+{
+	__u32 *key = ctx->key;
+
+	if (!key)
+		return 0;
+
+	volatile __u32 v = *key;
+
+	(void)v;
+	return 0;
+}
-- 
2.43.0

Re: [PATCH bpf v4 2/2] selftests/bpf: add test for nullable PTR_TO_BUF access

[Amery Hung]

On Thu, Apr 2, 2026 at 8:39 PM Qi Tang <tpluszz77@gmail.com> wrote:
>
> Add iter_buf_null_fail with two tests and a test runner:
>   - iter_buf_null_deref: verifier must reject direct dereference of
>     ctx->key (PTR_TO_BUF | PTR_MAYBE_NULL) without a null check
>   - iter_buf_null_check_ok: verifier must accept dereference after
>     an explicit null check
>
> Signed-off-by: Qi Tang <tpluszz77@gmail.com>
> ---
>  .../bpf/prog_tests/iter_buf_null_fail.c       |  9 ++++
>  .../selftests/bpf/progs/iter_buf_null_fail.c  | 51 +++++++++++++++++++
>  2 files changed, 60 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
>  create mode 100644 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
> new file mode 100644
> index 000000000000..ea97787b870d
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
> @@ -0,0 +1,9 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <test_progs.h>
> +#include "iter_buf_null_fail.skel.h"
> +
> +void test_iter_buf_null_fail(void)
> +{
> +       RUN_TESTS(iter_buf_null_fail);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
> new file mode 100644
> index 000000000000..963a4cfc33f3
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
> @@ -0,0 +1,51 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 Qi Tang */
> +
> +#include <vmlinux.h>
> +#include <bpf/bpf_helpers.h>
> +#include "bpf_misc.h"
> +
> +char _license[] SEC("license") = "GPL";
> +
> +struct {
> +       __uint(type, BPF_MAP_TYPE_HASH);
> +       __uint(max_entries, 1);
> +       __type(key, __u32);
> +       __type(value, __u64);
> +} hashmap SEC(".maps");
> +

hashmap is not used and can be removed.

> +/*
> + * Verify that the verifier rejects direct access to nullable PTR_TO_BUF
> + * (ctx->key) without a null check.  On the iterator stop callback,
> + * ctx->key is NULL, so unconditional access would be a NULL deref.
> + */
> +SEC("iter/bpf_map_elem")
> +__failure __msg("invalid mem access")
> +int iter_buf_null_deref(struct bpf_iter__bpf_map_elem *ctx)
> +{
> +       /* ctx->key is PTR_TO_BUF | PTR_MAYBE_NULL | MEM_RDONLY.
> +        * Direct access without null check must be rejected.
> +        */

nit: consider kernel-style comment instead of netdev

> +       volatile __u32 v = *(__u32 *)ctx->key;
> +
> +       (void)v;
> +       return 0;
> +}
> +
> +/*
> + * Verify that access after a null check is still accepted.
> + */

nit: maybe just one line /* ... */ ?

Reviewed-by: Amery Hung <ameryhung@gmail.com>

> +SEC("iter/bpf_map_elem")
> +__success
> +int iter_buf_null_check_ok(struct bpf_iter__bpf_map_elem *ctx)
> +{
> +       __u32 *key = ctx->key;
> +
> +       if (!key)
> +               return 0;
> +
> +       volatile __u32 v = *key;
> +
> +       (void)v;
> +       return 0;
> +}
> --
> 2.43.0
>
>

Re: [PATCH bpf v4 2/2] selftests/bpf: add test for nullable PTR_TO_BUF access

[Kumar Kartikeya Dwivedi]

On Fri, 3 Apr 2026 at 08:16, Amery Hung <ameryhung@gmail.com> wrote:
>
> On Thu, Apr 2, 2026 at 8:39 PM Qi Tang <tpluszz77@gmail.com> wrote:
> >
> > Add iter_buf_null_fail with two tests and a test runner:
> >   - iter_buf_null_deref: verifier must reject direct dereference of
> >     ctx->key (PTR_TO_BUF | PTR_MAYBE_NULL) without a null check
> >   - iter_buf_null_check_ok: verifier must accept dereference after
> >     an explicit null check
> >
> > Signed-off-by: Qi Tang <tpluszz77@gmail.com>
> > ---
> >  .../bpf/prog_tests/iter_buf_null_fail.c       |  9 ++++
> >  .../selftests/bpf/progs/iter_buf_null_fail.c  | 51 +++++++++++++++++++
> >  2 files changed, 60 insertions(+)
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
> > new file mode 100644
> > index 000000000000..ea97787b870d
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/iter_buf_null_fail.c
> > @@ -0,0 +1,9 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <test_progs.h>
> > +#include "iter_buf_null_fail.skel.h"
> > +
> > +void test_iter_buf_null_fail(void)
> > +{
> > +       RUN_TESTS(iter_buf_null_fail);
> > +}
> > diff --git a/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
> > new file mode 100644
> > index 000000000000..963a4cfc33f3
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
> > @@ -0,0 +1,51 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2026 Qi Tang */
> > +
> > +#include <vmlinux.h>
> > +#include <bpf/bpf_helpers.h>
> > +#include "bpf_misc.h"
> > +
> > +char _license[] SEC("license") = "GPL";
> > +
> > +struct {
> > +       __uint(type, BPF_MAP_TYPE_HASH);
> > +       __uint(max_entries, 1);
> > +       __type(key, __u32);
> > +       __type(value, __u64);
> > +} hashmap SEC(".maps");
> > +
>
> hashmap is not used and can be removed.
>
> > +/*
> > + * Verify that the verifier rejects direct access to nullable PTR_TO_BUF
> > + * (ctx->key) without a null check.  On the iterator stop callback,
> > + * ctx->key is NULL, so unconditional access would be a NULL deref.
> > + */
> > +SEC("iter/bpf_map_elem")
> > +__failure __msg("invalid mem access")
> > +int iter_buf_null_deref(struct bpf_iter__bpf_map_elem *ctx)
> > +{
> > +       /* ctx->key is PTR_TO_BUF | PTR_MAYBE_NULL | MEM_RDONLY.
> > +        * Direct access without null check must be rejected.
> > +        */
>
> nit: consider kernel-style comment instead of netdev
>
> > +       volatile __u32 v = *(__u32 *)ctx->key;
> > +
> > +       (void)v;
> > +       return 0;
> > +}
> > +
> > +/*
> > + * Verify that access after a null check is still accepted.
> > + */
>
> nit: maybe just one line /* ... */ ?
>
> Reviewed-by: Amery Hung <ameryhung@gmail.com>

Qi, only the fix was applied, the test still needs to be resent.
Please respin the selftest alone and send it against bpf-next.
Thanks.

Re: [PATCH bpf v4 2/2] selftests/bpf: add test for nullable PTR_TO_BUF access

[Qi Tang]

On Mon, Apr 6, 2026, Kumar Kartikeya Dwivedi wrote:
> Qi, only the fix was applied, the test still needs to be resent.
> Please respin the selftest alone and send it against bpf-next.

Got it. Thanks for the ack and for guiding me through the
review rounds on this one. Will respin the selftest with
Amery's nits fixed and send against bpf-next.

Qi Tang

Re: [PATCH bpf v4 0/2] bpf: reject direct access to nullable PTR_TO_BUF pointers

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Fri,  3 Apr 2026 11:38:26 +0800 you wrote:
> check_mem_access() allows direct dereference of PTR_TO_BUF |
> PTR_MAYBE_NULL without a null check, causing kernel NULL dereference
> on map iterator stop callbacks.
> 
> Patch 1 adds the missing type_may_be_null() guard.
> Patch 2 adds a selftest with a test runner.
> 
> [...]

Here is the summary with links:
  - [bpf,v4,1/2] bpf: reject direct access to nullable PTR_TO_BUF pointers
    https://git.kernel.org/bpf/bpf-next/c/b0db1accbc73
  - [bpf,v4,2/2] selftests/bpf: add test for nullable PTR_TO_BUF access
    (no matching commit)

You are awesome, thank you!
-- 
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