Re: [PATCH 5/5] selinux: drop unnecessary NULL check

[Ondrej Mosnacek <omosnace@redhat.com>]

On Mon, May 2, 2022 at 3:43 PM Christian Göttsche
<cgzones@googlemail.com> wrote:
>
> On Tue, 8 Mar 2022 at 17:09, Christian Göttsche <cgzones@googlemail.com> wrote:
> >
> > On Fri, 18 Feb 2022 at 18:31, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > On Thu, Feb 17, 2022 at 6:22 AM Christian Göttsche
> > > <cgzones@googlemail.com> wrote:
> > > >
> > > > Commit e3489f8974e1 ("selinux: kill selinux_sb_get_mnt_opts()")
> > > > introduced a NULL check on the context after a successful call to
> > > > security_sid_to_context().  This is on the one hand redundant after
> > > > checking for success and on the other hand insufficient on an actual
> > > > NULL pointer, since the context is passed to seq_escape() leading to a
> > > > call of strlen() on it.
> > > >
> > > > Reported by Clang analyzer:
> > > >
> > > >     In file included from security/selinux/hooks.c:28:
> > > >     In file included from ./include/linux/tracehook.h:50:
> > > >     In file included from ./include/linux/memcontrol.h:13:
> > > >     In file included from ./include/linux/cgroup.h:18:
> > > >     ./include/linux/seq_file.h:136:25: warning: Null pointer passed as 1st argument to string length function [unix.cstring.NullArg]
> > > >             seq_escape_mem(m, src, strlen(src), flags, esc);
> > > >                                    ^~~~~~~~~~~
> > >
> > > I'm guessing there was more to this trace for this instance of this warning?
> >
> > Yes, complete output appended at the end.
> >
> > >
> > > >
> > > > Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> > > > ---
> > > >  security/selinux/hooks.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > > index 1e69f88eb326..ac802b99d36c 100644
> > > > --- a/security/selinux/hooks.c
> > > > +++ b/security/selinux/hooks.c
> > > > @@ -1020,7 +1020,7 @@ static int show_sid(struct seq_file *m, u32 sid)
> > > >         rc = security_sid_to_context(&selinux_state, sid,
> > > >                                              &context, &len);
> > > >         if (!rc) {
> > >
> > > ^ perhaps changing this condition to:
> > >
> > > if (!rc && context) {
> > >
> > > It might be nice to retain the null ptr check should the semantics of
> > > security_sid_to_context ever change.
> >
> > If I read the implementation of security_sid_to_context() and its callees
> > correctly it should never return 0 (success) and not have populated its 3
> > argument, unless the passed pointer was zero, which by passing the address
> > of a stack variable - &context - is not the case).
> >
>
> Kindly ping;
> is my analysis correct that after
>
>     rc = security_sid_to_context(&selinux_state, sid,
>                                                   &context, &len);
>
> there is no possibility that rc is 0 AND context is NULL?

Yes, I think this patch is good. rc == 0 means success, which in this
case means that a valid context string has been returned. Thus, there
is no point in checking for NULL, other than being super-defensive
about future changes to security_sid_to_context() messing something up
(if we did this everywhere, then there would be NULL checks all over
the place...).

>
> > >
> > > > -               bool has_comma = context && strchr(context, ',');
> > > > +               bool has_comma = strchr(context, ',');
> > > >
> > > >                 seq_putc(m, '=');
> > > >                 if (has_comma)
> > > > --
> > > > 2.35.1
> > > >
> > >
> > >
> > > --
> > > Thanks,
> > > ~Nick Desaulniers
> >
> >
> > clang-tidy report:
> >
> > ./include/linux/seq_file.h:136:25: warning: Null pointer passed as 1st
> > argument to string length function
> > [clang-analyzer-unix.cstring.NullArg]
> >         seq_escape_mem(m, src, strlen(src), flags, esc);
> >                                ^
> > ./security/selinux/hooks.c:1041:6: note: Assuming the condition is false
> >         if (!(sbsec->flags & SE_SBINITIALIZED))
> >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ./security/selinux/hooks.c:1041:2: note: Taking false branch
> >         if (!(sbsec->flags & SE_SBINITIALIZED))
> >         ^
> > ./security/selinux/hooks.c:1044:6: note: Assuming the condition is false
> >         if (!selinux_initialized(&selinux_state))
> >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ./security/selinux/hooks.c:1044:2: note: Taking false branch
> >         if (!selinux_initialized(&selinux_state))
> >         ^
> > ./security/selinux/hooks.c:1047:6: note: Assuming the condition is true
> >         if (sbsec->flags & FSCONTEXT_MNT) {
> >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ./security/selinux/hooks.c:1047:2: note: Taking true branch
> >         if (sbsec->flags & FSCONTEXT_MNT) {
> >         ^
> > ./security/selinux/hooks.c:1050:8: note: Calling 'show_sid'
> >                 rc = show_sid(m, sbsec->sid);
> >                      ^~~~~~~~~~~~~~~~~~~~~~~
> > ./security/selinux/hooks.c:1020:7: note: Value assigned to 'context'
> >         rc = security_sid_to_context(&selinux_state, sid,
> >              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ./security/selinux/hooks.c:1022:6: note: Assuming 'rc' is 0
> >         if (!rc) {
> >             ^~~
> > ./security/selinux/hooks.c:1022:2: note: Taking true branch
> >         if (!rc) {
> >         ^
> > ./security/selinux/hooks.c:1023:20: note: Assuming 'context' is null
> >                 bool has_comma = context && strchr(context, ',');
> >                                  ^~~~~~~
> > ./security/selinux/hooks.c:1023:28: note: Left side of '&&' is false
> >                 bool has_comma = context && strchr(context, ',');
> >                                          ^
> > ./security/selinux/hooks.c:1026:7: note: 'has_comma' is false
> >                 if (has_comma)
> >                     ^~~~~~~~~
> > ./security/selinux/hooks.c:1026:3: note: Taking false branch
> >                 if (has_comma)
> >                 ^
> > ./security/selinux/hooks.c:1028:17: note: Passing null pointer value
> > via 2nd parameter 's'
> >                 seq_escape(m, context, "\"\n\\");
> >                               ^~~~~~~
> > ./security/selinux/hooks.c:1028:3: note: Calling 'seq_escape'
> >                 seq_escape(m, context, "\"\n\\");
> >                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ././include/linux/seq_file.h:152:20: note: Passing null pointer value
> > via 2nd parameter 'src'
> >         seq_escape_str(m, s, ESCAPE_OCTAL, esc);
> >                           ^
> > ././include/linux/seq_file.h:152:2: note: Calling 'seq_escape_str'
> >         seq_escape_str(m, s, ESCAPE_OCTAL, esc);
> >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ././include/linux/seq_file.h:136:25: note: Null pointer passed as 1st
> > argument to string length function
> >         seq_escape_mem(m, src, strlen(src), flags, esc);
> >                                ^      ~~~
>

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.