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SubjectRe: [PATCH] selinux: refactor mls_context_to_sid() and make it stricter
On Wed, Aug 8, 2018 at 9:56 PM Paul Moore <paul@paul-moore.com> wrote:
>
> On Mon, Aug 6, 2018 at 5:19 PM Jann Horn <jannh@google.com> wrote:
> >
> > The intended behavior change for this patch is to reject any MLS strings
> > that contain (trailing) garbage if p->mls_enabled is true.
> >
> > As suggested by Paul Moore, change mls_context_to_sid() so that the two
> > parts of the range are extracted before the rest of the parsing. Because
> > now we don't have to scan for two different separators simultaneously
> > everywhere, we can actually switch to strchr() everywhere instead of the
> > open-coded loops that scan for two separators at once.
> >
> > mls_context_to_sid() used to signal how much of the input string was parsed
> > by updating `*scontext`. However, there is actually no case in which
> > mls_context_to_sid() only parses a subset of the input and still returns
> > a success (other than the buggy case with a second '-' in which it
> > incorrectly claims to have consumed the entire string). Turn `scontext`
> > into a simple pointer argument and stop redundantly checking whether the
> > entire input was consumed in string_to_context_struct(). This also lets us
> > remove the `scontext_len` argument from `string_to_context_struct()`.
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> > Refactored version of
> > "[PATCH] selinux: stricter parsing in mls_context_to_sid()" based on
> > Paul's comments. WDYT?
> > I've thrown some inputs at it, and it seems to work.
> >
> > security/selinux/ss/mls.c | 178 ++++++++++++++-------------------
> > security/selinux/ss/mls.h | 2 +-
> > security/selinux/ss/services.c | 12 +--
> > 3 files changed, 82 insertions(+), 110 deletions(-)
>
> Thanks for the rework, this looks much better than what we currently
> have. I have some comments/questions below ...
>
> > diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
> > index 39475fb455bc..2fe459df3c85 100644
> > --- a/security/selinux/ss/mls.c
> > +++ b/security/selinux/ss/mls.c
> > @@ -218,9 +218,7 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
> > /*
> > * Set the MLS fields in the security context structure
> > * `context' based on the string representation in
> > - * the string `*scontext'. Update `*scontext' to
> > - * point to the end of the string representation of
> > - * the MLS fields.
> > + * the string `scontext'.
> > *
> > * This function modifies the string in place, inserting
> > * NULL characters to terminate the MLS fields.
> > @@ -235,22 +233,21 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
> > */
> > int mls_context_to_sid(struct policydb *pol,
> > char oldc,
> > - char **scontext,
> > + char *scontext,
> > struct context *context,
> > struct sidtab *s,
> > u32 def_sid)
> > {
> > -
> > - char delim;
> > - char *scontextp, *p, *rngptr;
> > + char *sensitivity, *cur_cat, *next_cat, *rngptr;
> > struct level_datum *levdatum;
> > struct cat_datum *catdatum, *rngdatum;
> > - int l, rc = -EINVAL;
> > + int l, rc, i;
> > + char *rangep[2];
> >
> > if (!pol->mls_enabled) {
> > - if (def_sid != SECSID_NULL && oldc)
> > - *scontext += strlen(*scontext) + 1;
> > - return 0;
> > + if ((def_sid != SECSID_NULL && oldc) || (*scontext) == '\0')
> > + return 0;
> > + return -EINVAL;
>
> Why are we simply not always return 0 in the case where MLS is not
> enabled in the policy? The mls_context_to_sid() is pretty much a
> no-op in this case (even more so with your pat regardless of input and
> I worry that returning EINVAL here is a deviation from current
> behavior which could cause problems.

Sorry, I was rephrasing the text above when I accidentally hit send.
While my emails are generally a good source of typos, the above is
pretty bad, so let me try again ...

Why are we simply not always returning 0 in the case where MLS is not
enabled in the policy? The mls_context_to_sid() function is pretty
much a no-op in this case regardless of input and I worry that
returning EINVAL here is a deviation from current behavior which could
cause problems.

> > }
> >
> > /*
> > @@ -261,113 +258,94 @@ int mls_context_to_sid(struct policydb *pol,
> > struct context *defcon;
> >
> > if (def_sid == SECSID_NULL)
> > - goto out;
> > + return -EINVAL;
> >
> > defcon = sidtab_search(s, def_sid);
> > if (!defcon)
> > - goto out;
> > + return -EINVAL;
> >
> > - rc = mls_context_cpy(context, defcon);
> > - goto out;
> > + return mls_context_cpy(context, defcon);
> > }
> >
> > - /* Extract low sensitivity. */
> > - scontextp = p = *scontext;
> > - while (*p && *p != ':' && *p != '-')
> > - p++;
> > -
> > - delim = *p;
> > - if (delim != '\0')
> > - *p++ = '\0';
> > + /*
> > + * If we're dealing with a range, figure out where the two parts
> > + * of the range begin.
> > + */
> > + rangep[0] = scontext;
> > + rangep[1] = strchr(scontext, '-');
> > + if (rangep[1]) {
> > + rangep[1][0] = '\0';
> > + rangep[1]++;
> > + }
> >
> > + /* For each part of the range: */
> > for (l = 0; l < 2; l++) {
> > - levdatum = hashtab_search(pol->p_levels.table, scontextp);
> > - if (!levdatum) {
> > - rc = -EINVAL;
> > - goto out;
> > - }
> > + /* Split sensitivity and category set. */
> > + sensitivity = rangep[l];
> > + if (sensitivity == NULL)
> > + break;
> > + next_cat = strchr(sensitivity, ':');
> > + if (next_cat)
> > + *(next_cat++) = '\0';
> >
> > + /* Parse sensitivity. */
> > + levdatum = hashtab_search(pol->p_levels.table, sensitivity);
> > + if (!levdatum)
> > + return -EINVAL;
> > context->range.level[l].sens = levdatum->level->sens;
> >
> > - if (delim == ':') {
> > - /* Extract category set. */
> > - while (1) {
> > - scontextp = p;
> > - while (*p && *p != ',' && *p != '-')
> > - p++;
> > - delim = *p;
> > - if (delim != '\0')
> > - *p++ = '\0';
> > -
> > - /* Separate into range if exists */
> > - rngptr = strchr(scontextp, '.');
> > - if (rngptr != NULL) {
> > - /* Remove '.' */
> > - *rngptr++ = '\0';
> > - }
> > + /* Extract category set. */
> > + while (next_cat != NULL) {
> > + cur_cat = next_cat;
> > + next_cat = strchr(next_cat, ',');
> > + if (next_cat != NULL)
> > + *(next_cat++) = '\0';
> > +
> > + /* Separate into range if exists */
> > + rngptr = strchr(cur_cat, '.');
> > + if (rngptr != NULL) {
> > + /* Remove '.' */
>
> On the chance you need to respin this patch, you can probably get rid
> of the above comment and the if-body braces; we don't have "Remove X"
> comments in other similar places in this function.
>
> > + *rngptr++ = '\0';
> > + }
> >
> > - catdatum = hashtab_search(pol->p_cats.table,
> > - scontextp);
> > - if (!catdatum) {
> > - rc = -EINVAL;
> > - goto out;
> > - }
> > + catdatum = hashtab_search(pol->p_cats.table, cur_cat);
> > + if (!catdatum)
> > + return -EINVAL;
> >
> > - rc = ebitmap_set_bit(&context->range.level[l].cat,
> > - catdatum->value - 1, 1);
> > - if (rc)
> > - goto out;
> > -
> > - /* If range, set all categories in range */
> > - if (rngptr) {
> > - int i;
> > -
> > - rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> > - if (!rngdatum) {
> > - rc = -EINVAL;
> > - goto out;
> > - }
> > -
> > - if (catdatum->value >= rngdatum->value) {
> > - rc = -EINVAL;
> > - goto out;
> > - }
> > -
> > - for (i = catdatum->value; i < rngdatum->value; i++) {
> > - rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> > - if (rc)
> > - goto out;
> > - }
> > - }
> > + rc = ebitmap_set_bit(&context->range.level[l].cat,
> > + catdatum->value - 1, 1);
> > + if (rc)
> > + return rc;
> > +
> > + /* If range, set all categories in range */
> > + if (rngptr == NULL)
> > + continue;
> > +
> > + rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> > + if (!rngdatum)
> > + return -EINVAL;
> > +
> > + if (catdatum->value >= rngdatum->value)
> > + return -EINVAL;
> >
> > - if (delim != ',')
> > - break;
> > + for (i = catdatum->value; i < rngdatum->value; i++) {
> > + rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> > + if (rc)
> > + return rc;
> > }
> > }
> > - if (delim == '-') {
> > - /* Extract high sensitivity. */
> > - scontextp = p;
> > - while (*p && *p != ':')
> > - p++;
> > -
> > - delim = *p;
> > - if (delim != '\0')
> > - *p++ = '\0';
> > - } else
> > - break;
> > }
> >
> > - if (l == 0) {
> > + /* If we didn't see a '-', the range start is also the range end. */
> > + if (rangep[1] == NULL) {
> > context->range.level[1].sens = context->range.level[0].sens;
> > rc = ebitmap_cpy(&context->range.level[1].cat,
> > &context->range.level[0].cat);
> > if (rc)
> > - goto out;
> > + return rc;
> > }
> > - *scontext = ++p;
> > - rc = 0;
> > -out:
> > - return rc;
> > +
> > + return 0;
>
> In the case where we have a MLS policy loaded (pol->mls_enabled != 0)
> and scontext is empty (scontext[0] = '\0'), we could end up returning
> 0 couldn't we? It seems like we might want a quick check for this
> before we parse the low/high portions of the field into the rangep
> array.
>
> > }
> >
> > /*
> > @@ -379,21 +357,19 @@ int mls_context_to_sid(struct policydb *pol,
> > int mls_from_string(struct policydb *p, char *str, struct context *context,
> > gfp_t gfp_mask)
> > {
> > - char *tmpstr, *freestr;
> > + char *tmpstr;
> > int rc;
> >
> > if (!p->mls_enabled)
> > return -EINVAL;
> >
> > - /* we need freestr because mls_context_to_sid will change
> > - the value of tmpstr */
> > - tmpstr = freestr = kstrdup(str, gfp_mask);
> > + tmpstr = kstrdup(str, gfp_mask);
> > if (!tmpstr) {
> > rc = -ENOMEM;
> > } else {
> > - rc = mls_context_to_sid(p, ':', &tmpstr, context,
> > + rc = mls_context_to_sid(p, ':', tmpstr, context,
> > NULL, SECSID_NULL);
> > - kfree(freestr);
> > + kfree(tmpstr);
> > }
> >
> > return rc;
> > diff --git a/security/selinux/ss/mls.h b/security/selinux/ss/mls.h
> > index 9a3ff7af70ad..67093647576d 100644
> > --- a/security/selinux/ss/mls.h
> > +++ b/security/selinux/ss/mls.h
> > @@ -34,7 +34,7 @@ int mls_level_isvalid(struct policydb *p, struct mls_level *l);
> >
> > int mls_context_to_sid(struct policydb *p,
> > char oldc,
> > - char **scontext,
> > + char *scontext,
> > struct context *context,
> > struct sidtab *s,
> > u32 def_sid);
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > index dd2ceec06fef..9212d4dd817a 100644
> > --- a/security/selinux/ss/services.c
> > +++ b/security/selinux/ss/services.c
> > @@ -1367,7 +1367,6 @@ int security_sid_to_context_force(struct selinux_state *state, u32 sid,
> > static int string_to_context_struct(struct policydb *pol,
> > struct sidtab *sidtabp,
> > char *scontext,
> > - u32 scontext_len,
> > struct context *ctx,
> > u32 def_sid)
> > {
> > @@ -1428,15 +1427,12 @@ static int string_to_context_struct(struct policydb *pol,
> >
> > ctx->type = typdatum->value;
> >
> > - rc = mls_context_to_sid(pol, oldc, &p, ctx, sidtabp, def_sid);
> > + rc = mls_context_to_sid(pol, oldc, p, ctx, sidtabp, def_sid);
> > if (rc)
> > goto out;
> >
> > - rc = -EINVAL;
> > - if ((p - scontext) < scontext_len)
> > - goto out;
> > -
> > /* Check the validity of the new context. */
> > + rc = -EINVAL;
> > if (!policydb_context_isvalid(pol, ctx))
> > goto out;
> > rc = 0;
> > @@ -1491,7 +1487,7 @@ static int security_context_to_sid_core(struct selinux_state *state,
> > policydb = &state->ss->policydb;
> > sidtab = &state->ss->sidtab;
> > rc = string_to_context_struct(policydb, sidtab, scontext2,
> > - scontext_len, &context, def_sid);
> > + &context, def_sid);
> > if (rc == -EINVAL && force) {
> > context.str = str;
> > context.len = strlen(str) + 1;
> > @@ -1959,7 +1955,7 @@ static int convert_context(u32 key,
> > goto out;
> >
> > rc = string_to_context_struct(args->newp, NULL, s,
> > - c->len, &ctx, SECSID_NULL);
> > + &ctx, SECSID_NULL);
> > kfree(s);
> > if (!rc) {
> > printk(KERN_INFO "SELinux: Context %s became valid (mapped).\n",
> > --
> > 2.18.0.597.ga71716f1ad-goog
>
> --
> paul moore
> www.paul-moore.com
> --
> To unsubscribe from this list: send the line "unsubscribe linux-security-module" in
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--
paul moore
security @ redhat

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