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

[Stephen Smalley <sds@tycho.nsa.gov>]

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.

> 
>> +
>> +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]))
>>
>