| Message ID | 20190218232308.11241-5-tobin@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | lib: Add safe string funtions | expand |
On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding <tobin@kernel.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@kernel.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! :)
On Wed, Feb 20, 2019 at 04:48:18PM -0800, Kees Cook wrote: > On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding <tobin@kernel.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@kernel.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) I like strscpy_pad() > 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 I'll look into this for v2 > > @@ -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 So thorough, you're the man. > > + > > + 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? Sure thing. > > + > > 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! :) Cheers. And they said we don't test in kernel land :) Tobin
On Thu, Feb 21, 2019 at 2:49 AM Kees Cook <keescook@chromium.org> wrote: > On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding <tobin@kernel.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. > > +/* 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) zero / zeroed examples in the kernel have semantics of getting some area completely zeroed. OTOH pad means different and we have examples as well (see seq_pad() as one). So, I would definitely vote for _pad b/c of semantics.
On Wed, Feb 20, 2019 at 04:48:18PM -0800, Kees Cook wrote: > On Mon, Feb 18, 2019 at 3:24 PM Tobin C. Harding <tobin@kernel.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@kernel.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)) (I meant to reply yesterday before posting v2). At this stage we know count >= 0 otherwise written would be less than 0. So I removed the 'count' from the second part of this statement, leaving if (written < 0 || written == count - 1) > return written; > > > + > > + if (written < count) > > + memset(dest + written, 0, count - written); I used this :) thanks, Tobin.
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); + #ifndef __HAVE_ARCH_STRCAT extern char * strcat(char *, const char *); #endif diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 0dca64c1d8a4..faa15ff47c4f 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1807,7 +1807,7 @@ config TEST_STRING default n help Enable this option to test string manipulation functions. - Currently this only tests memset_{16,32,64}. + Currently this only tests memset_{16,32,64} and strscpy_zeroed(). If unsure, say N. diff --git a/lib/string.c b/lib/string.c index 65969cf32f5d..ff5106e8249f 100644 --- a/lib/string.c +++ b/lib/string.c @@ -171,8 +171,7 @@ EXPORT_SYMBOL(strlcpy); * * Preferred to strncpy() since it always returns a valid string, and * doesn't unnecessarily force the tail of the destination buffer to be - * zeroed. If the zeroing is desired, it's likely cleaner to use strscpy(), - * check the return size, then just memset() the tail of the dest buffer. + * zeroed. If the zeroing is desired use strscpy_zeroed(). * * Return: The number of characters copied (not including the trailing * NUL) or -E2BIG if the destination buffer wasn't big enough. @@ -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 < count) + memset(dest + written, 0, count - written); + + 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; +} + 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:
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@kernel.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(-)