[PATCH v2] selinux: log invalid contexts in AVCs
Stephen Smalley
sds at tycho.nsa.gov
Tue Jan 22 20:00:25 UTC 2019
On 1/22/19 2:42 PM, Paul Moore wrote:
> On Mon, Jan 21, 2019 at 10:36 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>> In case a file has an invalid context set, in an AVC record generated
>> upon access to such file, the target context is always reported as
>> unlabeled. This patch adds new optional fields to the AVC record
>> (srawcon and trawcon) that report the actual context string if it
>> differs from the one reported in scontext/tcontext. This is useful for
>> diagnosing SELinux denials involving invalid contexts.
>>
>> To trigger an AVC that illustrates this situation:
>>
>> # setenforce 0
>> # touch /tmp/testfile
>> # setfattr -n security.selinux -v system_u:object_r:banana_t:s0 /tmp/testfile
>> # runcon system_u:system_r:sshd_t:s0 cat /tmp/testfile
>>
>> AVC before:
>>
>> type=AVC msg=audit(1547801083.248:11): avc: denied { open } for pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 tclass=file permissive=1
>>
>> AVC after:
>>
>> type=AVC msg=audit(1547801083.248:11): avc: denied { open } for pid=1149 comm="cat" path="/tmp/testfile" dev="tmpfs" ino=6608 scontext=system_u:system_r:sshd_t:s0 tcontext=system_u:object_r:unlabeled_t:s15:c0.c1023 trawcon=system_u:object_r:banana_t:s0 tclass=file permissive=1
>
> I would like us to add new fields at the end of existing records; the
> recent audit config changes are a bit of a special case as discussed
> previously.
>
> Also, under what cases would we ever see a srawcon field? This is
> only going to happen if we have a running process whose domain is
> removed during a policy reload, correct? I'm find with including this
> for the sake of completeness, but I would mention this in the patch
> description for the next revision.
Technically could occur on other permission checks where the source
context isn't a process context, e.g. filesystem associate check
(scontext/srawcon is a file context), socket checks (scontext/srawcon is
a socket context, which may not correspond to any running process if
passed to another or using /proc/self/attr/sockcreate), msgq enqueue
(scontext/srawcon is context of message). Common property is that the
context had to have been valid at the point it was converted to a SID
and then invalidated by a policy reload. In contrast, trawcon can occur
for file contexts read from the filesystem that were never valid under
any policy loaded into the kernel since boot.
>
>> Cc: Daniel Walsh <dwalsh at redhat.com>
>> Link: https://bugzilla.redhat.com/show_bug.cgi?id=1135683
>> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
>> ---
>>
>> v2: Rename fields to "(s|t)rawcon".
>>
>> security/selinux/avc.c | 49 +++++++++++++++++++++++++-----------------
>> 1 file changed, 29 insertions(+), 20 deletions(-)
>>
>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>> index 9b63d8ee1687..df5490db575b 100644
>> --- a/security/selinux/avc.c
>> +++ b/security/selinux/avc.c
>> @@ -165,6 +165,32 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
>> audit_log_format(ab, " }");
>> }
>>
>> +static void avc_dump_sid(struct audit_buffer *ab, struct selinux_state *state,
>> + u32 sid, char type)
>> +{
>> + int rc;
>> + char *context, *rcontext;
>> + u32 context_len, rcontext_len;
>> +
>> + rc = security_sid_to_context(state, sid, &context, &context_len);
>> + if (rc) {
>> + audit_log_format(ab, "%csid=%d ", type, sid);
>> + return;
>> + }
>> +
>> + audit_log_format(ab, "%ccontext=%s ", type, context);
>> +
>> + /* in case of invalid context report also the actual context string */
>> + rc = security_sid_to_context_force(state, sid, &rcontext,
>> + &rcontext_len);
>> + if (!rc) {
>> + if (strcmp(context, rcontext))
>> + audit_log_format(ab, "%crawcon=%s ", type, rcontext);
>> + kfree(rcontext);
>> + }
>> + kfree(context);
>> +}
>> +
>> /**
>> * avc_dump_query - Display a SID pair and a class in human-readable form.
>> * @ssid: source security identifier
>> @@ -174,28 +200,11 @@ static void avc_dump_av(struct audit_buffer *ab, u16 tclass, u32 av)
>> static void avc_dump_query(struct audit_buffer *ab, struct selinux_state *state,
>> u32 ssid, u32 tsid, u16 tclass)
>> {
>> - int rc;
>> - char *scontext;
>> - u32 scontext_len;
>> -
>> - rc = security_sid_to_context(state, ssid, &scontext, &scontext_len);
>> - if (rc)
>> - audit_log_format(ab, "ssid=%d", ssid);
>> - else {
>> - audit_log_format(ab, "scontext=%s", scontext);
>> - kfree(scontext);
>> - }
>> -
>> - rc = security_sid_to_context(state, tsid, &scontext, &scontext_len);
>> - if (rc)
>> - audit_log_format(ab, " tsid=%d", tsid);
>> - else {
>> - audit_log_format(ab, " tcontext=%s", scontext);
>> - kfree(scontext);
>> - }
>> + avc_dump_sid(ab, state, ssid, 's');
>> + avc_dump_sid(ab, state, tsid, 't');
>>
>> BUG_ON(!tclass || tclass >= ARRAY_SIZE(secclass_map));
>> - audit_log_format(ab, " tclass=%s", secclass_map[tclass-1].name);
>> + audit_log_format(ab, "tclass=%s", secclass_map[tclass-1].name);
>> }
>>
>> /**
>> --
>> 2.20.1
>
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