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Date: Wed, 20 Feb 2019 16:48:18 -0800
From: Kees Cook <keescook@...omium.org>
To: "Tobin C. Harding" <tobin@...nel.org>
Cc: Shuah Khan <shuah@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>,
Matthew Wilcox <willy@...radead.org>, Rasmus Villemoes <linux@...musvillemoes.dk>,
Daniel Micay <danielmicay@...il.com>
Subject: Re: [PATCH 4/6] lib/string: Add string copy/zero function
On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding <tobin@...nel.org> wrote:
>
> We have a function to copy strings safely and we have a function to copy
> strings _and_ zero the tail of the destination (if source string is
> shorter than destination buffer) but we do not have a function to do
> both at once. This means developers must write this themselves if they
> desire this functionality. This is a chore, and also leaves us open to
> off by one errors unnecessarily.
>
> Add a function that calls strscpy() then memset()s the tail to zero if
> the source string is shorter than the destination buffer.
>
> Add testing via kselftest.
>
> Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> ---
> include/linux/string.h | 4 ++++
> lib/Kconfig.debug | 2 +-
> lib/string.c | 30 ++++++++++++++++++++++++++++--
> lib/test_string.c | 31 +++++++++++++++++++++++++++++++
> 4 files changed, 64 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/string.h b/include/linux/string.h
> index 7927b875f80c..695a5e6a31e3 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -31,6 +31,10 @@ size_t strlcpy(char *, const char *, size_t);
> #ifndef __HAVE_ARCH_STRSCPY
> ssize_t strscpy(char *, const char *, size_t);
> #endif
> +
> +/* Wrapper function, no arch specific code required */
> +ssize_t strscpy_zeroed(char *dest, const char *src, size_t count);
bikeshed: I think "pad" is shorter and more descriptive. How about
something like strspad() strscpy_pad() or strscpy_zero()? (just to
shorten it slightly)
Not a blocker, just a TODO: we need a wrapper to do
CONFIG_FORTIFY_SOURCE checking for strscpy() (and strscpy_zeroed()) to
check for __builtin_object_size() vs the "size" argument, as done in
strlcpy() in include/linux/string.h
> @@ -238,6 +237,33 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
> EXPORT_SYMBOL(strscpy);
> #endif
>
> +/**
> + * strscopy_zeroed() - Copy a C-string into a sized buffer
> + * @dest: Where to copy the string to
> + * @src: Where to copy the string from
> + * @count: Size of destination buffer
> + *
> + * If the source string is shorter than the destination buffer, zeros
> + * the tail of the destination buffer.
> + *
> + * Return: The number of characters copied (not including the trailing
> + * NUL) or -E2BIG if the destination buffer wasn't big enough.
> + */
> +ssize_t strscpy_zeroed(char *dest, const char *src, size_t count)
> +{
> + ssize_t written;
> +
> + written = strscpy(dest, src, count);
> + if (written < 0)
> + return written;
If written < 0 we filled everything (i.e. we wrote "count - 1" bytes).
If we also exactly wrote "count - 1", then we also don't need the zero
padding either, since strscpy wrote the trailing NUL.
so:
if (written < 0 || (count && written == count - 1))
return written;
> +
> + if (written < count)
> + memset(dest + written, 0, count - written);
Now we know written must be [0, count - 2], so we can just:
memset(dest + written + 1, 0, count - written - 1);
The pattern (which should be added to the seltest) is:
count source written pad@
0 * -E2BIG (0 char, 0 NUL, 0 to zero)
1 "a" -E2BIG (0 char, 1 NUL, 0 to zero)
1 "" 0 (0 char, 1 NUL, 0 to zero)
2 "ab" -E2BIG (1 char, 1 NUL, 0 to zero)
2 "a" 1 (1 char, 1 NUL, 0 to zero)
2 "" 0 (0 char, 1 NUL, 1 to zero) dest + 1
3 "abc" -E2BIG (2 char, 1 NUL, 0 to zero)
3 "ab" 2 (2 char, 1 NUL, 0 to zero)
3 "a" 1 (1 char, 1 NUL, 1 to zero) dest + 2
3 "" 0 (0 char, 1 NUL, 2 to zero) dest + 1
4 "abcd" -E2BIG (3 char, 1 NUL, 0 to zero)
4 "abc" 3 (3 char, 1 NUL, 0 to zero)
4 "ab" 2 (2 char, 1 NUL, 1 to zero) dest + 3
4 "a" 1 (1 char, 1 NUL, 2 to zero) dest + 2
4 "" 0 (0 char, 1 NUL, 3 to zero) dest + 1
> +
> + return written;
> +}
> +EXPORT_SYMBOL(strscpy_zeroed);
> +
> #ifndef __HAVE_ARCH_STRCAT
> /**
> * strcat - Append one %NUL-terminated string to another
> diff --git a/lib/test_string.c b/lib/test_string.c
> index a9cba442389a..cc4eef51a395 100644
> --- a/lib/test_string.c
> +++ b/lib/test_string.c
> @@ -111,6 +111,32 @@ static __init int memset64_selftest(void)
> return 0;
> }
>
> +static __init int strscpy_zeroed_selftest(void)
> +{
> + char buf[6];
> + int written;
> +
> + memset(buf, 'a', sizeof(buf));
> +
> + written = strscpy_zeroed(buf, "bb", 4);
> + if (written != 2)
> + return 1;
> +
> + /* Copied correctly */
> + if (buf[0] != 'b' || buf[1] != 'b')
> + return 2;
> +
> + /* Zeroed correctly */
> + if (buf[2] != '\0' || buf[3] != '\0')
> + return 3;
> +
> + /* Only touched what it was supposed to */
> + if (buf[4] != 'a' || buf[5] != 'a')
> + return 4;
> +
> + return 0;
> +}
Cool, I like both the positive and negative tests. :) Can you add all
the cases above, too, which should validate the various corners?
> +
> static __init int test_string_init(void)
> {
> int test, subtest;
> @@ -130,6 +156,11 @@ static __init int test_string_init(void)
> if (subtest)
> goto fail;
>
> + test = 4;
> + subtest = strscpy_zeroed_selftest();
> + if (subtest)
> + goto fail;
> +
> pr_info("String selftests succeeded\n");
> return 0;
> fail:
> --
> 2.20.1
>
Nice! :)
--
Kees Cook
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