Re: [PATCH] seq_buf: Mark a seq_buf overflowed when terminating it loses a byte

[Alejandro Colomar <alx+linux-hardening@kernel.org>]

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Hi Kees, Steve,

> Date: 2026-10-28 22:32:54-0700
> From: Kees Cook <kees@kernel.org>
>
> When there is no room after the data for a NUL, __seq_buf_terminate()
> writes it over the last byte of the buffer. An overflowed seq_buf has
> already lost data, but one that is exactly full, as seq_buf_putc(),
> seq_buf_putmem(), and seq_buf_commit() can leave it, has not: the NUL
> replaces the last byte written, and the seq_buf still does not report
> an overflow. seq_buf_str(), seq_buf_strlen(), and seq_buf_terminate()
> all go through it, so each of them can silently truncate what was
> written, and seq_buf_pop() then returns the NUL in place of the byte it
> replaced.
> 
> Mark the seq_buf overflowed in that case with seq_buf_set_overflow(),
> as suggested in review. Nothing is left between "len" and "size" for it
> to clear there, and seq_buf_used() still counts the lost byte.
> 
> No current caller checks for overflow after terminating a seq_buf that
> can be left exactly full, so none of them behaves differently.

This fix in theory solves the issue.  However, I wonder if this is maybe
too late to report the overflow.  Callers normally expect the overflow
to happen while they're performing their writes, and maybe they don't
expect the overflow to happen at this point, so they might forget to
test, and keep going with truncated data.

How about not allowing a full buffer without a NUL terminator in the
first place?  That is, we could report a _putmem or _putc operation that
writes a non-zero byte to the last element in the buffer as being
overflowed.

This would waste one byte, but would prevent users from having to expect
a possible overflow when reading a buffer.


Have a lovely day!
Alex

> 
> Add a test that fills a seq_buf exactly and checks that each of the
> three accessors marks it overflowed, that seq_buf_pop() then returns
> -1, and that a seq_buf with room for the NUL is not marked. Update the
> existing tests that terminate an exactly full seq_buf to expect it, and
> move the length check in the seq_buf_do_printk() test after the run,
> so that the buffer still reaches seq_buf_do_printk() exactly full.
> 
> 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 16.2.0, and the new
> test fails with the fix reverted.
> 
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> Closes: https://lore.kernel.org/all/20261016145928.16eddc9a@gandalf.local.home/
> Fixes: 8d1478bfd63be ("seq_buf: Add seq_buf_strlen()")
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
>  include/linux/seq_buf.h   | 19 ++++++++++-----
>  lib/tests/seq_buf_kunit.c | 49 +++++++++++++++++++++++++++++++++++++--
>  2 files changed, 60 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index f006f2654e4c..c38b5eccc212 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -128,8 +128,8 @@ static inline unsigned int seq_buf_used(struct seq_buf *s)
>  
>  /*
>   * NUL-terminate the buffer in @s: directly after the data when there is
> - * room for it, otherwise in the last byte of the buffer. @s->size must not
> - * be zero.
> + * room for it, otherwise in the last byte of the buffer, which marks @s as
> + * overflowed. @s->size must not be zero.
>   *
>   * Returns: the offset of the NUL.
>   */
> @@ -137,10 +137,16 @@ static inline size_t __seq_buf_terminate(struct seq_buf *s)
>  {
>  	size_t end;
>  
> -	if (seq_buf_buffer_left(s))
> +	if (seq_buf_buffer_left(s)) {
>  		end = s->len;
> -	else
> +	} else {
>  		end = s->size - 1;
> +		/*
> +		 * About to overwrite a written byte in the last position
> +		 * in the buffer, so it must be considered overflowed now.
> +		 */
> +		seq_buf_set_overflow(s);
> +	}
>  
>  	s->buffer[end] = 0;
>  
> @@ -158,7 +164,8 @@ static inline size_t __seq_buf_terminate(struct seq_buf *s)
>   * the last byte of the buffer is zeroed, and the len will still
>   * point passed it. The same happens when the buffer is exactly
>   * full: the NUL takes the place of the last byte written, which is
> - * lost, though seq_buf_used() still counts it.
> + * lost, so @s is marked as overflowed, though seq_buf_used() still
> + * counts that byte.
>   *
>   * A zero-sized seq_buf has nowhere to put a NUL, so the empty string
>   * is returned instead of writing to @s->buffer. Any other seq_buf
> @@ -189,7 +196,7 @@ static inline const char *seq_buf_str(struct seq_buf *s)
>   * without walking it. Unlike seq_buf_used(), this does not count the byte
>   * given up to the NUL when the buffer is full or has overflowed. When the
>   * buffer is exactly full, that byte is the last one written, and calling
> - * either function loses it.
> + * either function loses it and marks @s as overflowed.
>   *
>   * A zero-sized seq_buf holds no string, so 0 is returned without writing
>   * to @s->buffer, matching what seq_buf_str() returns for one.
> diff --git a/lib/tests/seq_buf_kunit.c b/lib/tests/seq_buf_kunit.c
> index fc53e9ef844c..a67ed6d7b517 100644
> --- a/lib/tests/seq_buf_kunit.c
> +++ b/lib/tests/seq_buf_kunit.c
> @@ -504,13 +504,13 @@ static void seq_buf_do_printk_test(struct kunit *test)
>  	seq_buf_putc(&s, '!');
>  	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&s));
>  	KUNIT_ASSERT_EQ(test, seq_buf_used(&s), 8);
> -	KUNIT_ASSERT_EQ(test, strlen(seq_buf_str(&s)), 7);
>  
>  	seq_buf_printk_run(&capture, &s);
>  
>  	/* The one line that was written, and nothing after it. */
>  	KUNIT_EXPECT_EQ(test, seq_buf_printk_marked, 1);
>  	KUNIT_EXPECT_EQ(test, seq_buf_printk_empty, 0);
> +	KUNIT_EXPECT_EQ(test, strlen(seq_buf_str(&s)), 7);
>  
>  	/* Check that lines without a trailing newline are shown. */
>  	seq_buf_puts(&t, SEQ_BUF_PRINTK_MARKER "\n" SEQ_BUF_PRINTK_MARKER);
> @@ -630,7 +630,8 @@ static void seq_buf_strlen_full_test(struct kunit *test)
>  	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&s));
>  	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4);
>  	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&s), 3);
> -	/* seq_buf_strlen() terminates the buffer by itself. */
> +	/* seq_buf_strlen() terminates the buffer by itself, losing the 'd'. */
> +	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
>  	KUNIT_EXPECT_EQ(test, s.buffer[3], '\0');
>  	KUNIT_EXPECT_EQ(test, strnlen(s.buffer, s.size), 3);
>  	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc");
> @@ -649,6 +650,7 @@ static void seq_buf_strlen_full_test(struct kunit *test)
>  	seq_buf_commit(&t, len);
>  	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&t));
>  	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 7);
> +	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
>  	KUNIT_EXPECT_EQ(test, t.buffer[7], '\0');
>  	KUNIT_EXPECT_EQ(test, strnlen(t.buffer, t.size), 7);
>  	KUNIT_EXPECT_STREQ(test, seq_buf_str(&t), "zzzzzzz");
> @@ -735,6 +737,48 @@ static void seq_buf_terminate_test(struct kunit *test)
>  	KUNIT_EXPECT_STREQ(test, buf, "untouched");
>  }
>  
> +static void seq_buf_terminate_full_test(struct kunit *test)
> +{
> +	DECLARE_SEQ_BUF(s, 4);
> +	DECLARE_SEQ_BUF(t, 4);
> +	DECLARE_SEQ_BUF(u, 4);
> +	DECLARE_SEQ_BUF(v, 4);
> +
> +	/*
> +	 * Terminating a buffer that is exactly full puts the NUL over the
> +	 * last byte written, so what it holds is truncated, and each of
> +	 * seq_buf_str(), seq_buf_strlen() and seq_buf_terminate() must mark
> +	 * it overflowed.
> +	 */
> +	seq_buf_putmem(&s, "abcd", 4);
> +	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&s));
> +	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc");
> +	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&s));
> +	KUNIT_EXPECT_EQ(test, seq_buf_used(&s), 4);
> +	/* The NUL must not be popped in place of the 'd' it replaced. */
> +	KUNIT_EXPECT_EQ(test, seq_buf_pop(&s), -1);
> +	KUNIT_EXPECT_STREQ(test, seq_buf_str(&s), "abc");
> +
> +	seq_buf_putmem(&t, "abcd", 4);
> +	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&t));
> +	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&t), 3);
> +	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&t));
> +
> +	seq_buf_putmem(&u, "abcd", 4);
> +	KUNIT_ASSERT_FALSE(test, seq_buf_has_overflowed(&u));
> +	seq_buf_terminate(&u);
> +	KUNIT_EXPECT_TRUE(test, seq_buf_has_overflowed(&u));
> +	KUNIT_EXPECT_STREQ(test, u.buffer, "abc");
> +
> +	/* With room left for the NUL, nothing is lost. */
> +	seq_buf_putmem(&v, "abc", 3);
> +	KUNIT_EXPECT_STREQ(test, seq_buf_str(&v), "abc");
> +	KUNIT_EXPECT_EQ(test, seq_buf_strlen(&v), 3);
> +	seq_buf_terminate(&v);
> +	KUNIT_EXPECT_FALSE(test, seq_buf_has_overflowed(&v));
> +	KUNIT_EXPECT_EQ(test, seq_buf_pop(&v), 'c');
> +}
> +
>  static struct kunit_case seq_buf_test_cases[] = {
>  	KUNIT_CASE(seq_buf_init_test),
>  	KUNIT_CASE(seq_buf_init_append_test),
> @@ -760,6 +804,7 @@ static struct kunit_case seq_buf_test_cases[] = {
>  	KUNIT_CASE(seq_buf_strlen_embedded_nul_test),
>  	KUNIT_CASE(seq_buf_strlen_zero_size_test),
>  	KUNIT_CASE(seq_buf_terminate_test),
> +	KUNIT_CASE(seq_buf_terminate_full_test),
>  	KUNIT_CASE(seq_buf_do_printk_test),
>  	{}
>  };
> -- 
> 2.55.0
> 
> 

-- 
<https://www.alejandro-colomar.es>

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