linux-kernel - Re: [PATCH v3] lib/string.c: implement stpcpy

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Date:   Thu, 27 Aug 2020 11:26:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        stable <stable@...r.kernel.org>, Andy Lavr <andy.lavr@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] lib/string.c: implement stpcpy

On Wed, Aug 26, 2020 at 07:42:17PM -0700, Joe Perches wrote:
> On Wed, 2020-08-26 at 19:33 -0700, Kees Cook wrote:
> > On Wed, Aug 26, 2020 at 04:57:41PM -0700, Joe Perches wrote:
> > > On Wed, 2020-08-26 at 16:38 -0700, Kees Cook wrote:
> > > > On Thu, Aug 27, 2020 at 07:59:45AM +0900, Masahiro Yamada wrote:
> > > []
> > > > > OK, then stpcpy(), strcpy() and sprintf()
> > > > > have the same level of unsafety.
> > > > 
> > > > Yes. And even snprintf() is dangerous because its return value is how
> > > > much it WOULD have written, which when (commonly) used as an offset for
> > > > further pointer writes, causes OOB writes too. :(
> > > > https://github.com/KSPP/linux/issues/105
> > > > 
> > > > > strcpy() is used everywhere.
> > > > 
> > > > Yes. It's very frustrating, but it's not an excuse to continue
> > > > using it nor introducing more bad APIs.
> > > > 
> > > > $ git grep '\bstrcpy\b' | wc -l
> > > > 2212
> > > > $ git grep '\bstrncpy\b' | wc -l
> > > > 751
> > > > $ git grep '\bstrlcpy\b' | wc -l
> > > > 1712
> > > > 
> > > > $ git grep '\bstrscpy\b' | wc -l
> > > > 1066
> > > > 
> > > > https://www.kernel.org/doc/html/latest/process/deprecated.html#strcpy
> > > > https://github.com/KSPP/linux/issues/88
> > > > 
> > > > https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
> > > > https://github.com/KSPP/linux/issues/89
> > > > 
> > > > https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > > > https://github.com/KSPP/linux/issues/90
> > > > 
> > > > We have no way right now to block the addition of deprecated API usage,
> > > > which makes ever catching up on this replacement very challenging.
> > > 
> > > These could be added to checkpatch's deprecated_api test.
> > > ---
> > >  scripts/checkpatch.pl | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> > > index 149518d2a6a7..f9ccb2a63a95 100755
> > > --- a/scripts/checkpatch.pl
> > > +++ b/scripts/checkpatch.pl
> > > @@ -605,6 +605,9 @@ foreach my $entry (@mode_permission_funcs) {
> > >  $mode_perms_search = "(?:${mode_perms_search})";
> > >  
> > >  our %deprecated_apis = (
> > > +	"strcpy"				=> "strscpy",
> > > +	"strncpy"				=> "strscpy",
> > > +	"strlcpy"				=> "strscpy",
> > >  	"synchronize_rcu_bh"			=> "synchronize_rcu",
> > >  	"synchronize_rcu_bh_expedited"		=> "synchronize_rcu_expedited",
> > >  	"call_rcu_bh"				=> "call_rcu",
> > > 
> > > 
> > 
> > Good idea, yeah. We, unfortunately, need to leave strncpy() off this
> > list for now because it's not *strictly* deprecated (see the notes in
> > bug report[1]), but the others can be.
> 
> OK, but it is in Documentation/process/deprecated.rst
> 
> strncpy() on NUL-terminated strings

"... on NUL-terminated strings". It's "valid" to use it on known-size
(either external or by definition) NUL-padded buffers (e.g. NLA_STRING).

-- 
Kees Cook