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Linux 2.6 Kernel Capability LSM Module Local Privilege Elevation
------------------------------------------------------------------------
SUMMARY
POSIX.1e Capability is a very important component of Linux kernel. In the
original Linux kernel, system security relied on it and DAC. In new the
kernel version, Linux Security Modules (LSM) framework was introduced to
provide a lightweight, general-purpose framework for access control. Some
Linux security projects were ported to LSM and accepted by kernel source,
such as POSIX.1e Capability and SE-Linux. Users can compile Capability as
a Linux Loadable Kernel Module, and insert it into kernel at any time he
or she wants to. Under this situation, after inserting Capability module,
due to an error in handling of credentials existing processes (normal user
processes) will posses root privileges.
DETAILS
When privileged operations are controlled by the Capability LSM module, it
mediates privileged operations based on the credentials of processes. The
credentials consists of three fields of task_struct, namely,
cap_permitted, cap_inheritable and cap_effective.
Before a user processes carries out a privileged operation (such as
sethostname, override DAC, raw IO etc.), the operating system will check
cap_effective field (in cap_capable hooks function of security/commoncap.c
(2.6.*) or security/capability.c (2.5.72-lsm1)):
if (cap_raised (tsk->cap_effective, cap)))
return 0;
else
return -EPERM;
In general, only root processes can possess Capability privileges.
When the Capability is enabled in the kernel and no other LSM security
modules are loaded, the kernel uses default security function operations
(security/dummy.c) to mediate privilege operation.
The logic of dummy operations is very simple: if a process wants to
perform a privileged operation, the euid property of it must be zero
(root), or fsuid property must be zero when this privileged operation
involved with file system.
However, dummy operations do nothing about credentials of processes, the
credentials of any process is a clone of its parent process. As results,
the credentials of all process, even normal user process, are the same as
that of the Init process. As the Init process is a privileged process, it
is assigned total Capability privileges when the system starts.
Unfortunately, after Capability LSM module is loaded, it does not
recomputed the credentials of existing processes (before the insertion of
Capability module).
Before the insertion, only root processes could have performed privileged
operations controlled, as it is defined by the dummy operations. After
insertion, the control of privileged operations is switched from the dummy
operations to the Capability module based on the process's credentials.
As a result, all existing processes gain the same privileges as that set
to the Init process, even if they are normal user processes.
Example:
Before loading Capability module, run a vim editor as a normal user. In
vim, enter command ":r /etc/shadow" vim response "can't open file
/etc/shadow" the request to access a root file is denied.
Don't end vim, switch to another console and login as root, insert
Capability module into kernel.
#modprobe capability
After inserting, go back to vim and try to open file /etc/shadow again,
you will find you can read, edit and save(w!) this file as a normal user!
The reason for this wrong access control is error credentials of vim so as
to it has Capability privilege CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH.
Let's view the credentials with a shell command.
$cat /proc/2454/status (2454 is the pid of vim)
Name: vim
State: S (sleeping)
SleepAVG: 91%
Tgid: 2454
Pid: 2454
PPid: 1552
TracerPid: 0
Uid: 500 500 500 500
Gid: 500 500 500 500
FDSize: 256
Groups: 500
VmSize: 9356 kB
VmLck: 0 kB
VmRSS: 2728 kB
VmData: 856 kB
VmStk: 16 kB
VmExe: 1676 kB
VmLib: 3256 kB
Threads: 1
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 8000000000003000
SigCgt: 00000000ef824eff
CapInh: 0000000000000000
CapPrm: 00000000ffffffff
CapEff: 00000000fffffeff
The last three lines are the credentials of vim, it has all Capability
privileges besides CAP_SETPCAP.
Above test was performed under 2.6.* and 2.5.72-lsm1.
Workaround:
In order to fix this bug, we may have two methods. One is moving the
computation of credentials from Capability module to kernel. The other is
adding some code in Capability module to re-compute credentials of existed
process. flashsky choose the second method, add following code in
security/capability.c:
static void recompute_capability_creds(struct task_struct *task)
{
if(task->pid <= 1)
return;
task_lock(task);
task->keep_capabilities = 0;
if ((task->uid && task->euid && task->suid) &&
!task->keep_capabilities)
cap_clear (task->cap_permitted);
else
task->cap_permitted = CAP_INIT_EFF_SET;
if (task->euid != 0){
cap_clear (task->cap_effective);
}
else{
task->cap_effective = CAP_INIT_EFF_SET;
}
if(task->fsuid)
task->cap_effective &= ~CAP_FS_MASK;
else
task->cap_effective |= CAP_FS_MASK;
task_unlock(task);
return;
}
And add following code in Capability init function capability_init before
it return:
struct task_struct *task;
read_lock(&tasklist_lock);
for_each_process(task){
recompute_capability_creds(task);
}
read_unlock(&tasklist_lock);
return 0;
After modifying capability.c, we need "make" and "make modules_install"
again. Unload Capability module (rmmod capability; rmmod commoncap) and
retry the above example, all the accesses to a root file by normal user
existed process are always denied before and after inserting Capability
module.
Test and view the credentials again.
$cat /proc/(pid of vim)/status
Name: vim
State: S (sleeping)
SleepAVG: 91%
Tgid: 2864
Pid: 2864
PPid: 1552
TracerPid: 0
Uid: 500 500 500 500
Gid: 500 500 500 500
FDSize: 256
Groups: 500
VmSize: 9360 kB
VmLck: 0 kB
VmRSS: 2816 kB
VmData: 860 kB
VmStk: 16 kB
VmExe: 1676 kB
VmLib: 3256 kB
Threads: 1
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 8000000000003000
SigCgt: 00000000ef824eff
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
ADDITIONAL INFORMATION
The information has been provided by <mailto:liangbin@venustech.com.cn>
LiangBin.
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