Re: Defense in depth: LSM *modules*, not a static interface

Tetsuo Handa
2007-11-07T04:14:52Z

Hello.

Casey Schaufler wrote:
> Fine grained capabilities are a bonus, and there are lots of
> people who think that it would be really nifty if there were a
> separate capability for each "if" in the kernel. I personally
> don't see need for more than about 20. That is a matter of taste.
> DG/UX ended up with 330 and I say that's too many.

TOMOYO Linux has own (non-POSIX) capability that can support 65536 capabilities
if there *were* a separate capability for each "if" in the kernel.
http://svn.sourceforge.jp/cgi-bin/viewcvs.cgi/trunk/2.1.x/tomoyo-lsm/patches/tomoyo-capability.diff?root=tomoyo&view=markup

The reason I don't use POSIX capability is that the maximum types are limited to
bitwidth of a variable (i.e. currently 32, or are we going to extend it to 64).
This leads to abuse of CAP_SYS_ADMIN capability.
In other words, it makes fine-grained privilege division impossible.

Since security_capable() cannot receive fine-grained values,
TOMOYO can't do fine-grained privilege division.

I wish if capability machanism has mapping layer like:

#define CAP_DIVIDED_FOO1 0
#define CAP_DIVIDED_FOO2 1
#define CAP_DIVIDED_FOO3 2
...
#define CAP_DIVIDED_BAR1 100
#define CAP_DIVIDED_BAR2 101
#define CAP_DIVIDED_BAR3 102

const int cap_divided_to_grouped(int cap_divided)
{
static const int cap_mapping_array[] = {
/* [divided index value] = POSIX compatible index value (i.e. 0-31) */
[CAP_DIVIDED_FOO1] = 0,
[CAP_DIVIDED_FOO2] = 0,
[CAP_DIVIDED_FOO3] = 0,
[CAP_DIVIDED_BAR1] = 1,
[CAP_DIVIDED_BAR2] = 1,
[CAP_DIVIDED_BAR3] = 1,
};
return cap_mapping_array[cap_divided];
}

int capable(int cap_divided)
{
return security_capable(cap_divided);
}

int security_capable(int cap_divided)
{
/* Allow LSM to decide based on fine-grained capability index. */
return LSM_implementation_specific_capability_check(cap_divided_to_grouped(cap_divided));
}

int function_foo(void)
{
if (!capable(CAP_DIVIDED_FOO1))
return -EPERM;
return 0;
}

int function_bar(void)
{
if (!capable(CAP_DIVIDED_BAR2))
return -EPERM;
return 0;
}

Thanks.