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 98AE531328E; Thu, 17 Sep 2026 00:23:33 +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=1789604615; cv=none; b=blGYQ3ZusovvrV0fEaHSuKVs5LmuVV3JjdK58W52LVBMf1fYFVRYmDYmPXYAhGgin9D04CJz2r62FY7wku8MnJohkLDoAEsIMCgkC2BJqoyjzPcokaBeUyFptgTF3iTMXD6cEwxt1s0gUwStZr8JRq2z/C8hc2gK7MAD5AgOSNs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789604615; c=relaxed/simple; bh=2dWOoAScSeIiKgVRpgMZ6w+qzOKok7SkSwpny2uNySs=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=D8eWFuiYLx+Vfe+a9h/eTrCJhU3xDdR/7DrIuxqaNVCQWuuC0ngWAWDYTPasGwOpBc/VvZ6Fs52rFgrkISLtSUyvjTMIPLT/0JVvjOMhfybP3c/LJzNvNkslqfiDIuxweUmcKuH2VVYrKHOuKJ1cXrPq0aBHUQ2PIsE5HnRmgZo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MO9p4U1j; 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="MO9p4U1j" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5E88A1F00899; Thu, 17 Sep 2026 00:23:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789604613; bh=/6ZKzbTNPnpfoJwkQzuIsoomBE/MLjH9JGmNsSrwXUQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=MO9p4U1j3804lt0V0vU1eY1oxN7NftBFYSKVlTGtFiUUNf1Sb113cgypIoneghn6G 9IZ0gWOmAhOe+Esk58/UbAjWk7pSneRJUQwMoGZosBPzOwafU0ozTbYD9n3rWsUSAi q/kb6uDrvWSmxa/QJq+D5qV5HxMUsO1p1Y/uqSXWbGuJtmncdLXqtgLgjgsGD+sfaR y0PTxQVwKlpnzzSe3jUpvxoOY8gmTkb4Eh4Rp5R42uGse51UlXt8dX/cJWtmA3kRwI qwWEcIaoTMvjLyJ+axt38GH9Tcdna87VyahU6p7Cqt3+gv+iew1z/OUYT1zntVzYrt ZTrdUyl2HZcSg== From: Kees Cook To: Bill Wendling Cc: Kees Cook , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , =?UTF-8?q?Micka=C3=ABl=20Sala=C3=BCn?= , =?UTF-8?q?G=C3=BCnther=20Noack?= , Andy Shevchenko , Petr Mladek , "Matthew Wilcox (Oracle)" , Shuvam Pandey , David Gow , Andrew Morton , linux-trace-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, bpf@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH 1/7] seq_buf: Do not pop from an overflowed seq_buf Date: Wed, 16 Sep 2026 17:23:13 -0700 Message-Id: <20260917002333.2306346-1-kees@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260917002312.i.923-kees@kernel.org> References: <20260917002312.i.923-kees@kernel.org> Precedence: bulk X-Mailing-List: linux-hardening@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=5368; i=kees@kernel.org; h=from:subject; bh=2dWOoAScSeIiKgVRpgMZ6w+qzOKok7SkSwpny2uNySs=; b=owGbwMvMwCVmps19z/KJym7G02pJDFmrjb4c8Fvl7fgp+XHF6+6UIDHDzOTeb4v9Ik5KmarO1 Px+rqGho5SFQYyLQVZMkSXIzj3OxeNte7j7XEWYOaxMIEMYuDgFYCIK6Qx/xV1u8hXfnfguY3eK zeGQI0qMl/8ezGn/MvuoX0HQ/v7GWww/GYu6Xp6zbDgQamLDlDl1okLI0x0vitjUAnVfXtu1Yrk nGwA= 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 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 Signed-off-by: Kees Cook --- Cc: Steven Rostedt Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: "Mickaël Salaün" Cc: "Günther Noack" Cc: Andy Shevchenko Cc: Petr Mladek Cc: "Matthew Wilcox (Oracle)" Cc: Shuvam Pandey Cc: David Gow Cc: Andrew Morton Cc: Cc: Cc: --- include/linux/seq_buf.h | 4 ++-- include/linux/trace_seq.h | 4 +++- lib/tests/seq_buf_kunit.c | 42 +++++++++++++++++++++++++++++++++++++++ 3 files changed, 47 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..9aaa141d0965 100644 --- a/include/linux/trace_seq.h +++ b/include/linux/trace_seq.h @@ -86,7 +86,9 @@ 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 @s is empty or its buffer has + * overflowed. Note that a @s that is full, but has not overflowed, still + * pops. */ 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 eb466386bbef..9048037f6ba0 100644 --- a/lib/tests/seq_buf_kunit.c +++ b/lib/tests/seq_buf_kunit.c @@ -113,6 +113,47 @@ 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); @@ -223,6 +264,7 @@ 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