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Message-ID: <CALrw=nE9jYhHZnix8RV9UHApOZaF7otRLPHn3cmvOPaqQLzrnw@mail.gmail.com>
Date: Tue, 16 Sep 2025 11:28:47 +0100
From: Ignat Korchagin <ignat@...udflare.com>
To: Ethan Graham <ethan.w.s.graham@...il.com>
Cc: ethangraham@...gle.com, glider@...gle.com, andreyknvl@...il.com,
andy@...nel.org, brauner@...nel.org, brendan.higgins@...ux.dev,
davem@...emloft.net, davidgow@...gle.com, dhowells@...hat.com,
dvyukov@...gle.com, elver@...gle.com, herbert@...dor.apana.org.au,
jack@...e.cz, jannh@...gle.com, johannes@...solutions.net,
kasan-dev@...glegroups.com, kees@...nel.org, kunit-dev@...glegroups.com,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, lukas@...ner.de, rmoar@...gle.com, shuah@...nel.org,
tarasmadan@...gle.com
Subject: Re: [PATCH v1 07/10] crypto: implement KFuzzTest targets for PKCS7
and RSA parsing
On Tue, Sep 16, 2025 at 10:01 AM Ethan Graham
<ethan.w.s.graham@...il.com> wrote:
>
> From: Ethan Graham <ethangraham@...gle.com>
>
> Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
> rsa_parse_priv_key to serve as real-world examples of how the framework
> is used.
>
> These functions are ideal candidates for KFuzzTest as they perform
> complex parsing of user-controlled data but are not directly exposed at
> the syscall boundary. This makes them difficult to exercise with
> traditional fuzzing tools and showcases the primary strength of the
> KFuzzTest framework: providing an interface to fuzz internal functions.
>
> To validate the effectiveness of the framework on these new targets, we
> injected two artificial bugs and let syzkaller fuzz the targets in an
> attempt to catch them.
>
> The first of these was calling the asn1 decoder with an incorrect input
> from pkcs7_parse_message, like so:
>
> - ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen);
> + ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen + 1);
>
> The second was bug deeper inside of asn1_ber_decoder itself, like so:
>
> - for (len = 0; n > 0; n--)
> + for (len = 0; n >= 0; n--)
>
> syzkaller was able to trigger these bugs, and the associated KASAN
> slab-out-of-bounds reports, within seconds.
>
> The targets are defined within /lib/tests, alongside existing KUnit
> tests.
>
> Signed-off-by: Ethan Graham <ethangraham@...gle.com>
Reviewed-by: Ignat Korchagin <ignat@...udflare.com>
> ---
> v3:
> - Change the fuzz target build to depend on CONFIG_KFUZZTEST=y,
> eliminating the need for a separate config option for each individual
> file as suggested by Ignat Korchagin.
> - Remove KFUZZTEST_EXPECT_LE on the length of the `key` field inside of
> the fuzz targets. A maximum length is now set inside of the core input
> parsing logic.
> v2:
> - Move KFuzzTest targets outside of the source files into dedicated
> _kfuzz.c files under /crypto/asymmetric_keys/tests/ as suggested by
> Ignat Korchagin and Eric Biggers.
> ---
> ---
> crypto/asymmetric_keys/Makefile | 2 +
> crypto/asymmetric_keys/tests/Makefile | 2 +
> crypto/asymmetric_keys/tests/pkcs7_kfuzz.c | 22 +++++++++++
> .../asymmetric_keys/tests/rsa_helper_kfuzz.c | 38 +++++++++++++++++++
> 4 files changed, 64 insertions(+)
> create mode 100644 crypto/asymmetric_keys/tests/Makefile
> create mode 100644 crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> create mode 100644 crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
>
> diff --git a/crypto/asymmetric_keys/Makefile b/crypto/asymmetric_keys/Makefile
> index bc65d3b98dcb..77b825aee6b2 100644
> --- a/crypto/asymmetric_keys/Makefile
> +++ b/crypto/asymmetric_keys/Makefile
> @@ -67,6 +67,8 @@ obj-$(CONFIG_PKCS7_TEST_KEY) += pkcs7_test_key.o
> pkcs7_test_key-y := \
> pkcs7_key_type.o
>
> +obj-y += tests/
> +
> #
> # Signed PE binary-wrapped key handling
> #
> diff --git a/crypto/asymmetric_keys/tests/Makefile b/crypto/asymmetric_keys/tests/Makefile
> new file mode 100644
> index 000000000000..4ffe0bbe9530
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/Makefile
> @@ -0,0 +1,2 @@
> +obj-$(CONFIG_KFUZZTEST) += pkcs7_kfuzz.o
> +obj-$(CONFIG_KFUZZTEST) += rsa_helper_kfuzz.o
> diff --git a/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> new file mode 100644
> index 000000000000..37e02ba517d8
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/pkcs7_kfuzz.c
> @@ -0,0 +1,22 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * PKCS#7 parser KFuzzTest target
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <crypto/pkcs7.h>
> +#include <linux/kfuzztest.h>
> +
> +struct pkcs7_parse_message_arg {
> + const void *data;
> + size_t datalen;
> +};
> +
> +FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
> + KFUZZTEST_ANNOTATE_ARRAY(pkcs7_parse_message_arg, data);
> + KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
> +
> + pkcs7_parse_message(arg->data, arg->datalen);
> +}
> diff --git a/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
> new file mode 100644
> index 000000000000..bd29ed5e8c82
> --- /dev/null
> +++ b/crypto/asymmetric_keys/tests/rsa_helper_kfuzz.c
> @@ -0,0 +1,38 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * RSA key extract helper KFuzzTest targets
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <linux/kfuzztest.h>
> +#include <crypto/internal/rsa.h>
> +
> +struct rsa_parse_pub_key_arg {
> + const void *key;
> + size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
> + KFUZZTEST_ANNOTATE_ARRAY(rsa_parse_pub_key_arg, key);
> + KFUZZTEST_ANNOTATE_LEN(rsa_parse_pub_key_arg, key_len, key);
> +
> + struct rsa_key out;
> + rsa_parse_pub_key(&out, arg->key, arg->key_len);
> +}
> +
> +struct rsa_parse_priv_key_arg {
> + const void *key;
> + size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
> +{
> + KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
> + KFUZZTEST_ANNOTATE_ARRAY(rsa_parse_priv_key_arg, key);
> + KFUZZTEST_ANNOTATE_LEN(rsa_parse_priv_key_arg, key_len, key);
> +
> + struct rsa_key out;
> + rsa_parse_priv_key(&out, arg->key, arg->key_len);
> +}
> --
> 2.51.0.384.g4c02a37b29-goog
>
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