From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 177D52DCBEC; Sat, 19 Sep 2026 00:27:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789777640; cv=none; b=pEDkvupt/sK2WtPwB7jlMypcd2165fgM2vR0d/X2TRC5lTbh4IpmoButFBusy5ilJf2V0nKRpxBmNtZqe5nw82Jnaq7AaWvoybmntirrYnNtieT98OuNxmlCsaoAiW8hxAXOaznnMDwhWtMm3m5+PWtgg5Ssepwjvzf86MoPHsE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789777640; c=relaxed/simple; bh=fYjJCoMwHmQye7/VloP5QI8g55DMF1gGv8QsEafA+tA=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=tq93VuNt78Z6g4HwwNHIXW7l/EghGEMolfkgFjUXbHrDPPzLHM6o9Pl5oD0CMzTvmakwiGZzWhbgcwaDEbWrMaorKEZs6MsLsZZaJsJZih/xRC1byuurgreKYZQbAyZTNEdAZlXYJqz4JyBln/4g6r8QVez/zX3Xk0W5qg0LOY0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JuV/wG2s; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JuV/wG2s" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78EE41F000FF; Sat, 19 Sep 2026 00:27:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789777634; bh=OZopYLFTJlf3PkMhPLbAyetqyxTv3lYaNJu5W/0y9dk=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=JuV/wG2sz0qCIQSjQxsrReDBEzPo7huMI8B/ftpiCGhQ3UIfd7h8V5pbJkl3Jtn9N t6I+HuVuH47jco2b0SNsuD7buEfsry8aR9XZU6p3VJn3wq+SKa5nVxpu8O4Nrdhiod 9oiaXsEGWCTtfPW0D9G3gaDarIGcuEk4EdUVM32MvcllbccEAZs7gnWH3eruoVfYk7 2MQVk3fXUiV5KYdrrPzZNZqtbWXsMny/KgpmIkW0wYYHQXxrCtypVFrsrdW5pFfaX9 obyph5+lgJaXEUYndIU2dB2kzbAuxmJEZlCXkbAYI9sBfPlIfqW3L/DSelB88Qy5Cf hgCGj2lW0NuAg== From: Kees Cook To: Bill Wendling Cc: Kees Cook , Andy Shevchenko , =?UTF-8?q?G=C3=BCnther=20Noack?= , "Matthew Wilcox (Oracle)" , =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= , bpf@vger.kernel.org, linux-security-module@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Andrew Morton , David Gow , Masami Hiramatsu , Mathieu Desnoyers , Petr Mladek , Shuvam Pandey , Steven Rostedt , nikitash.mariiaw@gmail.com, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH v2 2/9] seq_buf: Do not pop from an overflowed seq_buf Date: Fri, 18 Sep 2026 17:27:00 -0700 Message-Id: <20260919002714.4060307-2-kees@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260919002658.stay.929-kees@kernel.org> References: <20260919002658.stay.929-kees@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Developer-Signature: v=1; a=openpgp-sha256; l=5577; i=kees@kernel.org; h=from:subject; bh=fYjJCoMwHmQye7/VloP5QI8g55DMF1gGv8QsEafA+tA=; b=owGbwMvMwCVmps19z/KJym7G02pJDFlrr93IXbp7Ld+m2KjGktU8+ry/9k2bf1eww21LT8Q0l s3KGzdnd5SyMIhxMciKKbIE2bnHuXi8bQ93n6sIM4eVCWQIAxenAExkjxHD/3TRVR4XOIL2fMwO FDufcllw9wzWJoP7lUnPVl3wyTkv1szwv1z6NO/BO/cY7fcISvstXznZQP6X1MKCUwuTlG/s1qs 6zgsA X-Developer-Key: i=kees@kernel.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 Content-Transfer-Encoding: 8bit When a seq_buf has overflowed, its len is size + 1, so seq_buf_pop() decrements len to size and reads buffer[size], one byte past the end of the buffer. It also leaves len equal to size, which no longer counts as overflowed, so a truncated seq_buf then looks like a complete, full one. An overflowed seq_buf logically has no last character to pop: the length of what was written has been lost, and the last byte of the buffer may be the NUL written by vsnprintf() or bytes that were never committed. Return -1 for an overflowed seq_buf, as for an empty one, and leave it overflowed, as the rest of the seq_buf API does until seq_buf_clear() or seq_buf_init(). The current callers do not reach this, e.g. trace_syscalls only calls trace_seq_pop() when the trace_seq it pops from has not overflowed, and kernel/bpf/diagnostics.c sets the length from strnlen() before popping. Add tests for the pop corner cases. Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y, and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0. Fixes: 32e0f607ac6a2 ("tracing: Add trace_seq_pop() and seq_buf_pop()") Assisted-by: LLM Reviewed-by: Andy Shevchenko Signed-off-by: Kees Cook --- Cc: "Günther Noack" Cc: "Matthew Wilcox (Oracle)" Cc: "Mickaël Salaün" Cc: Cc: Cc: Cc: Andrew Morton Cc: Andy Shevchenko Cc: David Gow Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Petr Mladek Cc: Shuvam Pandey Cc: Steven Rostedt --- include/linux/seq_buf.h | 4 ++-- include/linux/trace_seq.h | 5 ++++- lib/tests/seq_buf_kunit.c | 42 +++++++++++++++++++++++++++++++++++++++ 3 files changed, 48 insertions(+), 3 deletions(-) diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h index 9f2839e73f8a..f5a350347bc5 100644 --- a/include/linux/seq_buf.h +++ b/include/linux/seq_buf.h @@ -155,11 +155,11 @@ static inline void seq_buf_commit(struct seq_buf *s, int num) * * Removes the last written character to the seq_buf @s. * - * Returns the last character or -1 if it is empty. + * Returns the last character, or -1 if @s is empty or has overflowed. */ static inline int seq_buf_pop(struct seq_buf *s) { - if (!s->len) + if (!s->len || seq_buf_has_overflowed(s)) return -1; s->len--; diff --git a/include/linux/trace_seq.h b/include/linux/trace_seq.h index 697d619aafdc..7174ebf3f015 100644 --- a/include/linux/trace_seq.h +++ b/include/linux/trace_seq.h @@ -86,7 +86,10 @@ static inline bool trace_seq_has_overflowed(struct trace_seq *s) * * Removes the last written character to the trace_seq @s. * - * Returns the last character or -1 if it is empty. + * Returns the last character, or -1 if the underlying seq_buf is empty or + * has overflowed. Note that only that buffer is consulted: a @s marked + * full by a write that did not fit, which trace_seq_has_overflowed() + * reports as overflowed, still pops the last character written. */ static inline int trace_seq_pop(struct trace_seq *s) { diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c index 9ceccdc3029f..d5a0c618b880 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -115,6 +115,47 @@ static void seq_buf_putc_test(struct kunit *test) KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), ""); } +static void seq_buf_pop_test(struct kunit *test) +{ + DECLARE_SEQ_BUF(s, 8); + struct seq_buf t; + char *buf; + + /* Nothing to pop. */ + KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), -1); + KUNIT_EXPECT_EQ(test, s.len, 0); + + seq_buf_puts(&s, "hello"); + KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'o'); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hell"); + + /* A 0xff byte must not be mistaken for an empty buffer. */ + seq_buf_putc(&s, 0xff); + KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 0xff); + + /* A full buffer pops its last byte. */ + seq_buf_puts(&s, "abc"); + seq_buf_putc(&s, 'd'); + KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s)); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 8); + KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), 'd'); + KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 7); + KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "hellabc"); + + /* + * An overflowed buffer has nothing to pop, and stays overflowed. Use + * a buffer allocated at its exact size, so that KASAN reports any + * read past its end. + */ + buf = kunit_kmalloc(test, 16, GFP_KERNEL); + KUNIT_ASSERT_NOT_NULL(test, buf); + seq_buf_init(&t, buf, 16); + KUNIT_EXPECT_EQ(test, seq_buf_printf(&t, "%s", "longer than sixteen"), -1); + KUNIT_EXPECT_EQ(test, seq_buf_pop(&t), -1); + KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t)); +} + static void seq_buf_printf_test(struct kunit *test) { DECLARE_SEQ_BUF(s, 32); @@ -355,6 +396,7 @@ static struct kunit_case seq_buf_test_cases[] = { KUNIT_CASE(seq_buf_puts_test), KUNIT_CASE(seq_buf_puts_overflow_test), KUNIT_CASE(seq_buf_putc_test), + KUNIT_CASE(seq_buf_pop_test), KUNIT_CASE(seq_buf_printf_test), KUNIT_CASE(seq_buf_printf_overflow_test), KUNIT_CASE(seq_buf_get_buf_commit_test), -- 2.34.1