From mboxrd@z Thu Jan 1 00:00:00 1970 MIME-Version: 1.0 References: <20190218232308.11241-1-tobin@kernel.org> <20190218232308.11241-5-tobin@kernel.org> In-Reply-To: <20190218232308.11241-5-tobin@kernel.org> From: Kees Cook Date: Wed, 20 Feb 2019 16:48:18 -0800 Message-ID: Subject: Re: [PATCH 4/6] lib/string: Add string copy/zero function Content-Type: text/plain; charset="UTF-8" To: "Tobin C. Harding" Cc: Shuah Khan , Alexander Shishkin , Greg Kroah-Hartman , Andy Shevchenko , Kernel Hardening , LKML , Matthew Wilcox , Rasmus Villemoes , Daniel Micay List-ID: On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding 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 > --- > 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