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Message-ID: <20190221052034.GE11758@eros.localdomain>
Date: Thu, 21 Feb 2019 16:20:34 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: Kees Cook <keescook@...omium.org>
Cc: "Tobin C. Harding" <tobin@...nel.org>,
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 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@...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)
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
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