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Clamav/SeLinux, issue with system call recvmsg, and auxilary data.




Hello All

I triggered this issue with clamav/clamav-milter 0.95.2 from rpmforge running on a test box with Centos 5.3

Clamd opens a socket /var/run/clamav/clamd.sock to accept requests to scan things.

ls -Z /var/run/clamav/clamd.sock
srwxrwxrwx  clamav clamav root:object_r:clamd_var_run_t    /var/run/clamav/clamd.sock

Requests are read using the system call recvmsg, this allows for the passing auxiliary control data.

Clamav-milter 0.95.2 uses this to pass a handle to the temp file containing the data to be scanned

With SeLinux set to  targeted enforcing, this call reads and returns the normal data fine, but returns with the
flag MSG_CTRUNC set.

according to the man page this is
"indicates that some control data were discarded due to lack of space in the buffer for ancillary data."

clamd responded by closing the socket, clamav-milter responded to the closed socket by looping a 100% CPU. :-(


Running the audit log through audit2allow suggests 

grep clam /var/log/audit/audit.log | audit2allow -m local > local.te
[root fallback0 selinux]# cat local.te

module local 1.0;

require {
        type initrc_tmp_t;
        type proc_t;
        type sysctl_kernel_t;
        type clamd_t;
        class dir search;
        class file { read write getattr };
}

#============= clamd_t ==============
allow clamd_t initrc_tmp_t:file { read write getattr };
allow clamd_t proc_t:file { read getattr };
allow clamd_t sysctl_kernel_t:dir search;
allow clamd_t sysctl_kernel_t:file read;


The allow clamd_t proc_t:file { read getattr }; looks to relate to reading /proc/meminfo

allow clamd_t sysctl_kernel_t:dir search;
allow clamd_t sysctl_kernel_t:file read;
Look to relate to these log entries 
type=AVC msg=audit(1254139856.343:48724): avc:  denied  { search } for  pid=14771 comm="clamd" name="kernel" dev=proc ino=-268435416 scontext=root:system_r:clamd_t:s0 tcontext=system_u:object_r:sysctl_kernel_t:s0 tclass=dir
type=AVC msg=audit(1254139856.343:48724): avc:  denied  { read } for  pid=14771 comm="clamd" name="ngroups_max" dev=proc ino=-268435368 scontext=root:system_r:clamd_t:s0 tcontext=system_u:object_r:sysctl_kernel_t:s0 tclass=file
type=AVC msg=audit(1254149740.665:48885): avc:  denied  { search } for  pid=1261 comm="clamd" name="kernel" dev=proc ino=-268435416 scontext=root:system_r:clamd_t:s0 tcontext=system_u:object_r:sysctl_kernel_t:s0 tclass=dir

This if I have figured it out right relate to something that clamd is calling in turn trying to read /proc/sys/kernel/ngroups_max


So by elimination 
allow clamd_t initrc_tmp_t:file { read write getattr };

Must relate to the the use of auxiliary data with the socket, and the following log entries but I do not see why.
Can anyone explain?

type=AVC msg=audit(1254150147.188:48924): avc:  denied  { read write } for  pid=1288 comm="clamd" path=2F746D702F636C616D61762D3063666237656532666331656139656636323364373463316236626532623735202864656C6574656429 dev=dm-0 ino=34668546 scontext=root:system_r:clamd_t:s0 tcontext=root:object_r:initrc_tmp_t:s0 tclass=file
type=AVC msg=audit(1254150153.681:48925): avc:  denied  { read write } for  pid=1288 comm="clamd" path=2F746D702F636C616D61762D3336316332323033323138613239633865363633633937303962663133363664202864656C6574656429 dev=dm-0 ino=34668546 scontext=root:system_r:clamd_t:s0 tcontext=root:object_r:initrc_tmp_t:s0 tclass=file
type=AVC msg=audit(1254150177.903:48926): avc:  denied  { read write } for  pid=1288 comm="clamd" path=2F746D702F636C616D61762D3366636162623138633237636231383466643064656630643838353063363933202864656C6574656429 dev=dm-0 ino=34668546 scontext=root:system_r:clamd_t:s0 tcontext=root:object_r:initrc_tmp_t:s0 tclass=file
type=AVC msg=audit(1254150188.366:48927): avc:  denied  { read write } for  pid=1288 comm="clamd" path=2F746D702F636C616D61762D6366393131623632353130333564353832656435396466663136373362626131202864656C6574656429 dev=dm-0 ino=34668546 scontext=root:system_r:clamd_t:s0 tcontext=root:object_r:initrc_tmp_t:s0 tclass=file
type=AVC msg=audit(1254150220.428:48928): avc:  denied  { read write } for  pid=1288 comm="clamd" path=2F746D702F636C616D61762D3931633534623761393630653531386630363539653033363537303937323135202864656C6574656429 dev=dm-0 ino=34668546 scontext=root:system_r:clamd_t:s0 tcontext=root:object_r:initrc_tmp_t:s0 tclass=file


Yours

J. David Rye 











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