Re: [PATCH] libselinux: prioritize local literal fcontext definitions

[Petr Lautrbach <lautrbach@redhat.com>]

James Carter <jwcart2@gmail.com> writes:

> On Fri, Apr 25, 2025 at 1:19 PM Christian Göttsche
> <cgzones@googlemail.com> wrote:
>>
>> On Fri, 18 Apr 2025 at 21:13, James Carter <jwcart2@gmail.com> wrote:
>> >
>> > On Thu, Apr 17, 2025 at 3:18 PM Christian Göttsche
>> > <cgoettsche@seltendoof.de> wrote:
>> > >
>> > > From: Christian Göttsche <cgzones@googlemail.com>
>> > >
>> > > For literal file context definitions respect overrides from homedirs or
>> > > local configurations by ordering them first.
>> > >
>> > > Fixes: 92306daf ("libselinux: rework selabel_file(5) database")
>> > > Reported-by: Paul Holzinger
>> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2360183
>> > > Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
>> > > ---
>> > >  libselinux/src/label_file.c       |  5 +++--
>> > >  libselinux/src/label_file.h       | 10 +++++++++-
>> > >  libselinux/src/selinux_internal.h |  2 ++
>> > >  3 files changed, 14 insertions(+), 3 deletions(-)
>> > >
>> > > diff --git a/libselinux/src/label_file.c b/libselinux/src/label_file.c
>> > > index 5d73fb84..61a9ddb6 100644
>> > > --- a/libselinux/src/label_file.c
>> > > +++ b/libselinux/src/label_file.c
>> > > @@ -480,7 +480,7 @@ static int load_mmap_ctxarray(struct mmap_area *mmap_area, const char *path, str
>> > >         return 0;
>> > >  }
>> > >
>> > > -static int load_mmap_literal_spec(struct mmap_area *mmap_area, bool validating,
>> > > +static int load_mmap_literal_spec(struct mmap_area *mmap_area, bool validating, uint8_t inputno,
>> > >                                   struct literal_spec *lspec, const struct context_array *ctx_array)
>> > >  {
>> > >         uint32_t data_u32, ctx_id;
>> > > @@ -489,6 +489,7 @@ static int load_mmap_literal_spec(struct mmap_area *mmap_area, bool validating,
>> > >         int rc;
>> > >
>> > >         lspec->from_mmap = true;
>> > > +       lspec->inputno = inputno;
>> > >
>> > >
>> > >         /*
>> > > @@ -742,7 +743,7 @@ static int load_mmap_spec_node(struct mmap_area *mmap_area, const char *path, bo
>> > >                 node->literal_specs_alloc = lspec_num;
>> > >
>> > >                 for (uint32_t i = 0; i < lspec_num; i++) {
>> > > -                       rc = load_mmap_literal_spec(mmap_area, validating, &node->literal_specs[i], ctx_array);
>> > > +                       rc = load_mmap_literal_spec(mmap_area, validating, inputno, &node->literal_specs[i], ctx_array);
>> > >                         if (rc)
>> > >                                 return -1;
>> > >                 }
>> > > diff --git a/libselinux/src/label_file.h b/libselinux/src/label_file.h
>> > > index 67db78e5..b7aed0bc 100644
>> > > --- a/libselinux/src/label_file.h
>> > > +++ b/libselinux/src/label_file.h
>> > > @@ -96,6 +96,7 @@ struct literal_spec {
>> > >         char *regex_str;                        /* original regular expression string for diagnostics */
>> > >         char *literal_match;                    /* simplified string from regular expression */
>> > >         uint16_t prefix_len;                    /* length of fixed path prefix, i.e. length of the literal match */
>> > > +       uint8_t inputno;                        /* Input number of source file */
>> > >         uint8_t file_kind;                      /* file type */
>> > >         bool any_matches;                       /* whether any pathname match */
>> > >         bool from_mmap;                         /* whether this spec is from an mmap of the data */
>> > > @@ -368,7 +369,13 @@ static inline int compare_literal_spec(const void *p1, const void *p2)
>> > >                 return ret;
>> > >
>> > >         /* Order wildcard mode (0) last */
>> > > -       return (l1->file_kind < l2->file_kind) - (l1->file_kind > l2->file_kind);
>> > > +       ret = spaceship_cmp(l1->file_kind, l2->file_kind);
>> > > +       if (ret)
>> > > +               return -ret;
>> > > +
>> > These four lines should be removed. It makes sense to consider the
>> > wildcard mode as less specific and give priority to a rule that is not
>> > using a wildcard, but that is not how it was done in the past and that
>> > is not (from my testing) what is being done if a regex is involved. So
>> > for both consistency and in keeping with past practice, we should not
>> > use the file kind to sort here.
>> >
>> > By my testing, everything works as expected with those lines removed.
>>
>> The only difference I am seeing is when omitting specifying the file
>> type to lookup (-t for selabel_lookup, -m for matchpathcon).
>> Previously the last defined definition was returned, now the one with
>> the highest LABEL_FILE_KIND_* value (e.g. regular has currently the
>> highest value of 7).
>
> That is the difference.
>
>> But in practice that should not matter since for corretly labeling
>> files the type is important, and one could argue it is unspecified
>> what definition is returned if the type is omitted.
>>
>
> It is not unspecified. It means "all file types"
>
>> If the order should not depend on the mode one would need to store the
>> file linenumber in the compiled format (i.e. a binary format change),
>> since otherwise the order would be dependent on the used sorting
>> algorithm/implementation.
>>
>
> I am not following what you are saying here.
>
> In my testing, the file type only mattered when there was no regex in the path.
> If you specify a file context with a regex in an fc file, then
> regardless of whether the file type was a regular file or it was not
> specified, setting the file context with semanage would override it.
> But if you specify a file context that does not have a regex in an fc
> file, then setting the file context with semanage would override it
> only if you set the file type or if the file context in the fc file
> did not specify a file type.
>
> So it is more consistent to not sort based on the file type here.
> Also not sorting based on file type matches the old behavior.
>
> If this was new and we did not have to worry about regressions, then I
> would agree with you. I think that it would make sense to have a fc
> rule that specified a specific file type be selected over one that did
> not (everything else being equal).
>

I'd like to push this forward with the change Jim proposed as it follows
the original behavior.

The patch itself does not fix the regression:

    # semanage fcontext -a -t bin_t /usr/bin/sshd 
    # matchpathcon /usr/bin/sshd 
    /usr/bin/sshd   system_u:object_r:bin_t:s0

Without those 4 lines
-       /* Order wildcard mode (0) last */
-       ret = spaceship_cmp(l1->file_kind, l2->file_kind);
-       if (ret)
-               return -ret;
-

    # semanage fcontext -a -t bin_t /usr/bin/sshd 
    # matchpathcon /usr/bin/sshd 
    /usr/bin/sshd   system_u:object_r:bin_t:s0


Christian, are you ok to update your patch?


>
>> > The rest of the patch looks good to me.
>> >
>> > Thanks,
>> > Jim
>> >
>> > > +       /* Order by input number (higher number means added later, means higher priority) */
>> > > +       ret = spaceship_cmp(l1->inputno, l2->inputno);
>> > > +       return -ret;
>> > >  }
>> > >
>> > >  static inline int compare_spec_node(const void *p1, const void *p2)
>> > > @@ -754,6 +761,7 @@ static int insert_spec(const struct selabel_handle *rec, struct saved_data *data
>> > >                         .regex_str = regex,
>> > >                         .prefix_len = prefix_len,
>> > >                         .literal_match = literal_regex,
>> > > +                       .inputno = inputno,
>> > >                         .file_kind = file_kind,
>> > >                         .any_matches = false,
>> > >                         .lr.ctx_raw = context,
>> > > diff --git a/libselinux/src/selinux_internal.h b/libselinux/src/selinux_internal.h
>> > > index 964b8418..3fe7d4c3 100644
>> > > --- a/libselinux/src/selinux_internal.h
>> > > +++ b/libselinux/src/selinux_internal.h
>> > > @@ -150,4 +150,6 @@ static inline void fclose_errno_safe(FILE *stream)
>> > >  # define unlikely(x)                   (x)
>> > >  #endif /* __GNUC__ */
>> > >
>> > > +#define spaceship_cmp(a, b)            (((a) > (b)) - ((a) < (b)))
>> > > +
>> > >  #endif /* SELINUX_INTERNAL_H_ */
>> > > --
>> > > 2.49.0
>> > >
>> > >