Re: [PATCH] selinux: optimize MLS context to string conversion

[Ondrej Mosnacek <omosnace@redhat.com>]

On Mon, Sep 16, 2019 at 7:30 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> On 9/16/19 10:43 AM, Stephen Smalley wrote:
> > On 7/30/19 8:48 AM, Ondrej Mosnacek wrote:
> >> When mls_compute_context_len() or mls_sid_to_context() encounters a
> >> context with large category ranges, it behaves suboptimally - it
> >> traverses each positive bit of the category bitmap, each time calling
> >> find_next_bit() again.
> >>
> >> This has a large performance impact on UNIX datagram sockets with
> >> SO_PASSSEC set, since here the peer context needs to be converted to
> >> string for each recieved datagram. See [1] for more information.
> >>
> >> This patch introduces a new helper for ebitmap traversal, which allows
> >> to efficiently iterate over positive ranges instead of bits -
> >> ebitmap_for_each_positive_range() - and converts the two mls_*()
> >> functions to leverage it.
> >>
> >> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1733259
> >>
> >> Reported-by: Michal Sekletar <msekleta@redhat.com>
> >> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> >> ---
> >>   security/selinux/ss/ebitmap.h | 46 +++++++++++++++++++++
> >>   security/selinux/ss/mls.c     | 76 +++++++++++++----------------------
> >>   2 files changed, 73 insertions(+), 49 deletions(-)
> >>
> >> diff --git a/security/selinux/ss/ebitmap.h
> >> b/security/selinux/ss/ebitmap.h
> >> index 6aa7cf6a2197..a415741cb206 100644
> >> --- a/security/selinux/ss/ebitmap.h
> >> +++ b/security/selinux/ss/ebitmap.h
> >> @@ -42,6 +42,10 @@ struct ebitmap {
> >>       u32 highbit;    /* highest position in the total bitmap */
> >>   };
> >> +struct ebitmap_range {
> >> +    unsigned int start, end;
> >> +};
> >> +
> >>   #define ebitmap_length(e) ((e)->highbit)
> >>   static inline unsigned int ebitmap_start_positive(struct ebitmap *e,
> >> @@ -80,6 +84,43 @@ static inline unsigned int
> >> ebitmap_next_positive(struct ebitmap *e,
> >>       return ebitmap_length(e);
> >>   }
> >> +static inline unsigned int ebitmap_next_negative(struct ebitmap *e,
> >> +                         struct ebitmap_node **n,
> >> +                         unsigned int bit)
> >> +{
> >> +    unsigned int ofs;
> >> +
> >> +    ofs = find_next_zero_bit((*n)->maps, EBITMAP_SIZE,
> >> +                 bit - (*n)->startbit + 1);
> >> +    if (ofs < EBITMAP_SIZE)
> >> +        return ofs + (*n)->startbit;
> >> +
> >> +    for (*n = (*n)->next; *n; *n = (*n)->next) {
> >> +        ofs = find_first_zero_bit((*n)->maps, EBITMAP_SIZE);
> >> +        if (ofs < EBITMAP_SIZE)
> >> +            return ofs + (*n)->startbit;
> >> +    }
> >> +    return ebitmap_length(e);
> >> +}
> >
> > This is likely moot given that the patch was deferred on exploring the
> > cache option, but if/when this patch is revisited, you don't seem to
> > account for the possibility that there could be a hole between the
> > bitmaps represented by (*n) and (*n)->next, and that might be where the
> > next negative/zero bit is.
>
> You can see this bug manifest by doing the following:
> runcon -l s0:c0.c383,c768 bash
> id -Z
>
> The correct output of course would be s0:c0.c383,c768, but your patched
> kernel will report s0:c0.c768.

Yes, good catch! This patch would really need some focused testing
(and fixing) before being committed... Some simple fuzz test over
/sys/fs/selinux/context should be enough, but I didn't get as far as
to write it. Probably should have initially sent the patch just as
RFC, though...

>
> >
> >> +
> >> +static inline void ebitmap_start_positive_range(struct ebitmap *e,
> >> +                        struct ebitmap_node **n,
> >> +                        struct ebitmap_range *range)
> >> +{
> >> +    range->end = range->start = ebitmap_start_positive(e, n);
> >> +    if (range->start < ebitmap_length(e))
> >> +        range->end = ebitmap_next_negative(e, n, range->start);
> >> +}
> >> +
> >> +static inline void ebitmap_next_positive_range(struct ebitmap *e,
> >> +                           struct ebitmap_node **n,
> >> +                           struct ebitmap_range *range)
> >> +{
> >> +    range->end = range->start = ebitmap_next_positive(e, n, range->end);
> >> +    if (range->start < ebitmap_length(e))
> >> +        range->end = ebitmap_next_negative(e, n, range->start);
> >> +}
> >> +
> >>   #define EBITMAP_NODE_INDEX(node, bit)    \
> >>       (((bit) - (node)->startbit) / EBITMAP_UNIT_SIZE)
> >>   #define EBITMAP_NODE_OFFSET(node, bit)    \
> >> @@ -122,6 +163,11 @@ static inline void ebitmap_node_clr_bit(struct
> >> ebitmap_node *n,
> >>            bit < ebitmap_length(e);            \
> >>            bit = ebitmap_next_positive(e, &n, bit))    \
> >> +#define ebitmap_for_each_positive_range(e, n, range)        \
> >> +    for (ebitmap_start_positive_range(e, &n, &range);    \
> >> +         range.start < ebitmap_length(e);            \
> >> +         ebitmap_next_positive_range(e, &n, &range))    \
> >> +
> >>   int ebitmap_cmp(struct ebitmap *e1, struct ebitmap *e2);
> >>   int ebitmap_cpy(struct ebitmap *dst, struct ebitmap *src);
> >>   int ebitmap_contains(struct ebitmap *e1, struct ebitmap *e2, u32
> >> last_e2bit);
> >> diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
> >> index 5e05f5b902d7..3abd6b950c66 100644
> >> --- a/security/selinux/ss/mls.c
> >> +++ b/security/selinux/ss/mls.c
> >> @@ -35,10 +35,12 @@
> >>    */
> >>   int mls_compute_context_len(struct policydb *p, struct context
> >> *context)
> >>   {
> >> -    int i, l, len, head, prev;
> >> +    int l, len;
> >>       char *nm;
> >>       struct ebitmap *e;
> >>       struct ebitmap_node *node;
> >> +    struct ebitmap_range range;
> >> +    unsigned int rlen;
> >>       if (!p->mls_enabled)
> >>           return 0;
> >> @@ -49,24 +51,14 @@ int mls_compute_context_len(struct policydb *p,
> >> struct context *context)
> >>           len += strlen(sym_name(p, SYM_LEVELS, index_sens - 1));
> >>           /* categories */
> >> -        head = -2;
> >> -        prev = -2;
> >>           e = &context->range.level[l].cat;
> >> -        ebitmap_for_each_positive_bit(e, node, i) {
> >> -            if (i - prev > 1) {
> >> -                /* one or more negative bits are skipped */
> >> -                if (head != prev) {
> >> -                    nm = sym_name(p, SYM_CATS, prev);
> >> -                    len += strlen(nm) + 1;
> >> -                }
> >> -                nm = sym_name(p, SYM_CATS, i);
> >> +        ebitmap_for_each_positive_range(e, node, range) {
> >> +            rlen = range.end - range.start;
> >> +            if (rlen > 1) {
> >> +                nm = sym_name(p, SYM_CATS, range.start);
> >>                   len += strlen(nm) + 1;
> >> -                head = i;
> >>               }
> >> -            prev = i;
> >> -        }
> >> -        if (prev != head) {
> >> -            nm = sym_name(p, SYM_CATS, prev);
> >> +            nm = sym_name(p, SYM_CATS, range.end - 1);
> >>               len += strlen(nm) + 1;
> >>           }
> >>           if (l == 0) {
> >> @@ -91,9 +83,11 @@ void mls_sid_to_context(struct policydb *p,
> >>               char **scontext)
> >>   {
> >>       char *scontextp, *nm;
> >> -    int i, l, head, prev;
> >> +    int l, first;
> >>       struct ebitmap *e;
> >>       struct ebitmap_node *node;
> >> +    struct ebitmap_range range;
> >> +    unsigned int rlen;
> >>       if (!p->mls_enabled)
> >>           return;
> >> @@ -109,43 +103,27 @@ void mls_sid_to_context(struct policydb *p,
> >>           scontextp += strlen(scontextp);
> >>           /* categories */
> >> -        head = -2;
> >> -        prev = -2;
> >> +        first = 1;
> >>           e = &context->range.level[l].cat;
> >> -        ebitmap_for_each_positive_bit(e, node, i) {
> >> -            if (i - prev > 1) {
> >> -                /* one or more negative bits are skipped */
> >> -                if (prev != head) {
> >> -                    if (prev - head > 1)
> >> -                        *scontextp++ = '.';
> >> -                    else
> >> -                        *scontextp++ = ',';
> >> -                    nm = sym_name(p, SYM_CATS, prev);
> >> -                    strcpy(scontextp, nm);
> >> -                    scontextp += strlen(nm);
> >> -                }
> >> -                if (prev < 0)
> >> -                    *scontextp++ = ':';
> >> -                else
> >> -                    *scontextp++ = ',';
> >> -                nm = sym_name(p, SYM_CATS, i);
> >> -                strcpy(scontextp, nm);
> >> -                scontextp += strlen(nm);
> >> -                head = i;
> >> -            }
> >> -            prev = i;
> >> -        }
> >> -
> >> -        if (prev != head) {
> >> -            if (prev - head > 1)
> >> -                *scontextp++ = '.';
> >> -            else
> >> +        ebitmap_for_each_positive_range(e, node, range) {
> >> +            if (first) {
> >> +                first = 0;
> >> +                *scontextp++ = ':';
> >> +            } else {
> >>                   *scontextp++ = ',';
> >> -            nm = sym_name(p, SYM_CATS, prev);
> >> +            }
> >> +            nm = sym_name(p, SYM_CATS, range.start);
> >>               strcpy(scontextp, nm);
> >>               scontextp += strlen(nm);
> >> -        }
> >> +            rlen = range.end - range.start;
> >> +            if (rlen > 1) {
> >> +                *scontextp++ = rlen > 2 ? '.' : ',';
> >> +                nm = sym_name(p, SYM_CATS, range.end - 1);
> >> +                strcpy(scontextp, nm);
> >> +                scontextp += strlen(nm);
> >> +            }
> >> +        }
> >>           if (l == 0) {
> >>               if (mls_level_eq(&context->range.level[0],
> >>                        &context->range.level[1]))
> >>
> >
>


--
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.