On 03/19/2018 10:29 PM, Joe Kirwin wrote:
> *_Empirical Observations _*
> *
> *
> If I was to create an SELinux policy containing the following file_contexts (fruits.fc)
> ```
> /apple/orange/.* -- gen_context(system_u:object_r:atype_t,s0)
> /banana/.* -- gen_context(system_u:object_r:btype_t,s0)
> ```
>
> If I then take the file
> /etc/selinux/default/contexts/files/file_contexts.subs_dist and append to it the alias
> ```
> /apple /banana
> ```
>
> The resulting behavior is that when running:
> ```
> $ ./libselinux/utils/selabel_lookup_best_match -p /apple/orange/foo
> Best match context: system_u:object_r:btype_t:s0
>
> But the expected behavior is to match `atype_t` as that is a "more-specific" match pattern
I don't think this is a bug based on the documented behavior for file_contexts.subs. That said,
that support was added by Red Hat so I'll let them speak to it.
>
> *_Looking into why_*
>
> From the method in `libselinux/src/label_file.c` :
> lookup_common(struct selabel_handle *rec, const char *key, int type, bool partial)
>
> we encounter a call to :
> selabel_sub_key(struct saved_data *data, const char *key)
>
> In the example above the candidate path we're trying to match (referred to as the key in the code) is "canonicalized" to the /banana alias but the regex being evaluated is not
>
> *_A proposed fix_*
> *
> *
> /Also attached (label_file.patch), if the patch formatting is off on this thread, apologies./
> *
> *
> diff --git a/libselinux/src/label_file.c b/libselinux/src/label_file.c
> index 560d8c3..98a8d1b 100644
> --- a/libselinux/src/label_file.c
> +++ b/libselinux/src/label_file.c
> @@ -848,7 +848,7 @@ static struct spec *lookup_common(struct selabel_handle *rec,
> {
> struct saved_data *data = (struct saved_data *)rec->data;
> struct spec *spec_arr = data->spec_arr;
> - int i, rc, file_stem;
> + int i, rc, file_stem, orig_file_stem;
> mode_t mode = (mode_t)type;
> const char *buf;
> struct spec *ret = NULL;
> @@ -879,8 +879,12 @@ static struct spec *lookup_common(struct selabel_handle *rec,
> }
>
> sub = selabel_sub_key(data, key);
> - if (sub)
> + orig_file_stem = -1;
> + if (sub) {
> + orig_file_stem = find_stem_from_file(data, &key);
> key = sub;
> + }
>
> buf = key;
> file_stem = find_stem_from_file(data, &buf);
> @@ -896,7 +900,8 @@ static struct spec *lookup_common(struct selabel_handle *rec,
> * stem as the file AND if the spec in question has no mode
> * specified or if the mode matches the file mode then we do
> * a regex check */
> - if ((spec->stem_id == -1 || spec->stem_id == file_stem) &&
> + if ((spec->stem_id == -1 || spec->stem_id == file_stem ||
> + spec->stem_id == orig_file_stem) &&
> (!mode || !spec->mode || mode == spec->mode)) {
> if (compile_regex(data, spec, NULL) < 0)
> goto finish;
>
>
>
> I think there is still some simplification that could be done with aliases, in that they really shouldn't have a direction (e.g. alias -> original) instead they should go both ways and if there is a tie it should go by the ordering of the specs.
> Reason for this is that a developer of an SELinux policy, may not know the contents or directionality of file_contexts.subs_dist ahead of time or when it might change.
>
> Thanks,
> Joe Kirwin and Travis Szucs
>
