From mboxrd@z Thu Jan  1 00:00:00 1970
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References: <20190218232308.11241-1-tobin@kernel.org> <20190218232308.11241-6-tobin@kernel.org>
In-Reply-To: <20190218232308.11241-6-tobin@kernel.org>
From: Jann Horn <jannh@google.com>
Date: Tue, 19 Feb 2019 01:51:45 +0100
Message-ID: <CAG48ez0utQJjoJW69OwGiuH1EQtcD939-5u71ruD_1nr0eJK0g@mail.gmail.com>
Subject: Re: [PATCH 5/6] lib: Fix function documentation for strncpy_from_user
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To: "Tobin C. Harding" <tobin@kernel.org>
Cc: Kees Cook <keescook@chromium.org>, Shuah Khan <shuah@kernel.org>, Alexander Shishkin <alexander.shishkin@linux.intel.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Andy Shevchenko <andriy.shevchenko@linux.intel.com>, Kernel Hardening <kernel-hardening@lists.openwall.com>, kernel list <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>
List-ID: <kernel-hardening.lists.openwall.com>

+cc Andy because he's also preparing a patch for this function

On Tue, Feb 19, 2019 at 12:25 AM Tobin C. Harding <tobin@kernel.org> wrote:
> Current function documentation for strncpy_from_user() is incorrect.  If
> @count (size of destination buffer) is less than the length of the user
> string the function does _not_ return @count but rather returns -EFAULT.
>
> Document correctly the function return value, also add note that string
> may be left non-null terminated.
>
> Signed-off-by: Tobin C. Harding <tobin@kernel.org>
> ---
>  lib/strncpy_from_user.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 58eacd41526c..11fe9a4a00fd 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -82,22 +82,19 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
>  }
>
>  /**
> - * strncpy_from_user: - Copy a NUL terminated string from userspace.
> + * strncpy_from_user() - Copy a NUL terminated string from userspace.
>   * @dst:   Destination address, in kernel space.  This buffer must be at
>   *         least @count bytes long.
>   * @src:   Source address, in user space.
> - * @count: Maximum number of bytes to copy, including the trailing NUL.
> + * @count: Maximum number of bytes to copy, including the trailing %NUL.
>   *
>   * Copies a NUL-terminated string from userspace to kernel space.
>   *
> - * On success, returns the length of the string (not including the trailing
> - * NUL).
> - *
> - * If access to userspace fails, returns -EFAULT (some data may have been
> - * copied).
> - *
> - * If @count is smaller than the length of the string, copies @count bytes
> - * and returns @count.
> + * Return: If access to userspace fails, returns -EFAULT.  Otherwise,
> + *         return the number of characters copied excluding the trailing
> + *         %NUL, if the length of the user string exceeds @count return
> + *         -EFAULT (in which case @dst will be left without a %NUL
> + *         terminator).
>   */

AFAICS the byte_at_a_time loop exits when max==0 is reached, and then
if `res >= count` (in other words, if we've copied as many bytes as
requested, haven't encountered a null byte so far, and haven't reached
the end of the address space), we return `res`, which is the same as
`count`. Are you sure?