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([2620:10d:c090:500::3:d25]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3074dcad34esm25741417eec.11.2026.06.08.12.59.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Jun 2026 12:59:44 -0700 (PDT) Message-ID: Subject: Re: [PATCH bpf] bpf: Validate BTF repeated field counts before expansion From: Eduard Zingerman To: Paul Moses , Kumar Kartikeya Dwivedi Cc: martin.lau@linux.dev, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, bpf@vger.kernel.org, song@kernel.org, yonghong.song@linux.dev, jolsa@kernel.org, houtao1@huawei.com, linux-kernel@vger.kernel.org, stable@vger.kernel.org Date: Mon, 08 Jun 2026 12:59:41 -0700 In-Reply-To: <0_PQcsqBnb7dqgu9UPK6jIQvePSosttml5p2ZDoXAzy2AseVjvBu3ihswwZPWr5bZkOUCdH6HUvw3MRKJEwVYJAkT3j5gdNBHZp8l7_cP6Y=@1g4.org> References: <20260605234301.1109063-1-p@1g4.org> <0_PQcsqBnb7dqgu9UPK6jIQvePSosttml5p2ZDoXAzy2AseVjvBu3ihswwZPWr5bZkOUCdH6HUvw3MRKJEwVYJAkT3j5gdNBHZp8l7_cP6Y=@1g4.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sun, 2026-06-07 at 17:53 +0000, Paul Moses wrote: [...] The repro is legit. Here is a somewhat minimized version as a selftest: diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testi= ng/selftests/bpf/prog_tests/btf.c index a9de328a8697..212ca4472a89 100644 --- a/tools/testing/selftests/bpf/prog_tests/btf.c +++ b/tools/testing/selftests/bpf/prog_tests/btf.c @@ -4258,6 +4258,44 @@ static struct btf_raw_test raw_tests[] =3D { .max_entries =3D 1, }, =20 +{ +#define N 0x1999999aU + .descr =3D "repeat fields overflow", + .raw_types =3D { + /* int */ = /* [1] */ + BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4), + /* struct target {} */ = /* [2] */ + BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0,= 0), 1), + /* type_tag "kptr_untrusted" -> target */ = /* [3] */ + BTF_TYPE_TAG_ENC(NAME_TBD, 2), + /* target * (kptr) */ = /* [4] */ + BTF_PTR_ENC(3), + /* struct outer { target *kp; elem items[N]; } */ = /* [5] */ + BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0,= 2), (N * 8u + 8u)), + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* kp = */ + BTF_MEMBER_ENC(NAME_TBD, 6, 64), /* items = */ + /* elem[N] */ /* [6] = */ + BTF_TYPE_ARRAY_ENC(7, 1, N), + /* struct elem { target *f0..f9; } */ = /* [7] */ + BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0,= 10), 8), + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f0 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f1 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f2 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f3 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f4 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f5 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f6 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f7 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f8 = */ + BTF_MEMBER_ENC(NAME_TBD, 4, 0), /* f9 = */ + BTF_END_RAW, + }, + BTF_STR_SEC("\0target\0kptr_untrusted\0outer\0kp\0items\0elem" + "\0f0\0f1\0f2\0f3\0f4\0f5\0f6\0f7\0f8\0f9"), + .btf_load_err =3D true, +#undef N +}, + }; /* struct btf_raw_test raw_tests[] */ =20 static const char *get_next_str(const char *start, const char *end) However, as far as I understand the repro hits an overflow only because `BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 10), 8)` lies about `struct elem` size. It is specified as 8, while in reality it is= 80. The size of 80 would make `struct outer` unrepresentable in BTF, because (N * 80 + 8) exceeds u32 range, and that's what btf_type->size uses= . Given that btf_repeat_fields() only traverses structs/arrays but not unions= , I suspect that overflow won't happen in `field_cnt * (repeat_cnt + 1)` if proper size checks were implemented in btf_struct_check_meta() / btf_str= uct_resolve(). Even more, If I change "kptr_untrusted" to "kptr_untrusted11" to avoid fiel= ds parsing, the kernel accepts the bogus BTF. Paul, could you please investigate why is this happening?