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Wed, 27 May 2026 23:22:17 -0700 (PDT) From: Taegu Ha To: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko Cc: Yonghong Song , Eduard Zingerman , Kumar Kartikeya Dwivedi , Shuah Khan , John Fastabend , Jiri Olsa , Stanislav Fomichev , Song Liu , Puranjay Mohan , Emil Tantilov , Martin KaFai Lau , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Taegu Ha Subject: [PATCH bpf-next v3] bpf: reject overlarge global subprog argument sizes Date: Thu, 28 May 2026 15:21:55 +0900 Message-ID: <20260528062155.3988156-1-hataegu0826@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260528052533.3940181-1-hataegu0826@gmail.com> References: <20260528052533.3940181-1-hataegu0826@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Global subprogram argument checking derives generic pointer sizes from BTF and passes the resolved size to check_mem_reg() as a u32. The access-size validation path then uses a signed int, and stack pointers negate the value before calling check_helper_mem_access(). This creates a wrap when BTF describes a pointee size larger than S32_MAX. For example, a global subprogram argument of type: int (*p)[0x3fffffff] has a BTF-resolved pointee size of 0xfffffffc bytes. At a call site the caller can pass a pointer to a 4-byte stack slot at fp-4. The current PTR_TO_STACK path computes: size = -(int)mem_size so 0xfffffffc becomes -4 as a signed int and the negation validates only a 4-byte stack range. That range is covered by the caller's stack slot, so the call is accepted. The callee is then verified independently with R1 as PTR_TO_MEM and mem_size 0xfffffffc. A small instruction such as: r0 = *(u32 *)(r1 + 4) is accepted as being inside that BTF-described memory region. At run time, however, the actual argument value is still fp-4, so r1 + 4 addresses fp+0, outside the 4-byte object that the caller provided. Reject sizes that cannot be represented by the verifier's signed access-size API before the stack-specific negation. Add a verifier regression test for the oversized BTF argument. Fixes: 2cb27158adb3 ("bpf: poison dead stack slots") Signed-off-by: Taegu Ha --- v3: - Fix bpf-next build by using reg_arg_name(env, argno) in check_mem_reg(); v2 referenced a stale regno parameter name. - Keep the existing known-NULL fast path before the overlarge-size guard. - Send as a single bpf-next patch without a cover letter. v2: - Expanded the commit message with the BTF-derived size wrap details. - Kept the verifier fix to a mem_size > S32_MAX guard. kernel/bpf/verifier.c | 6 ++++++ .../bpf/progs/verifier_global_subprogs.c | 17 +++++++++++++++++ 2 files changed, 23 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index c8d980fdd709..3a270bc485c2 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -6927,6 +6927,12 @@ static int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg if (bpf_register_is_null(reg)) return 0; + if (mem_size > S32_MAX) { + verbose(env, "%s memory size %u is too large\n", + reg_arg_name(env, argno), mem_size); + return -EACCES; + } + /* Assuming that the register contains a value check if the memory * access is safe. Temporarily save and restore the register's state as * the conversion shouldn't be visible to a caller. diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c index dc09d0e2d8ad..75a2e3f48d0f 100644 --- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c +++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c @@ -152,6 +152,23 @@ int anon_user_mem_valid(void *ctx) return subprog_user_anon_mem(&t); } +__noinline __weak int subprog_user_anon_mem_huge(int (*p)[0x3fffffff]) +{ + return p ? (*p)[1] : 0; +} + +SEC("?tracepoint") +__failure __log_level(2) +__msg("R1 memory size 4294967292 is too large") +int anon_user_mem_huge_size_invalid(void *ctx) +{ + int (*p)[0x3fffffff]; + int tiny = 42; + + p = (void *)&tiny; + return subprog_user_anon_mem_huge(p) + tiny; +} + __noinline __weak int subprog_nonnull_ptr_good(int *p1 __arg_nonnull, int *p2 __arg_nonnull) { return (*p1) * (*p2); /* good, no need for NULL checks */ -- 2.43.0