Re: [PATCH v5 3/4] LSM: Define SELinux function to measure state and policy

[Stephen Smalley]

On 8/3/20 6:08 PM, Lakshmi Ramasubramanian wrote:

> On 8/3/20 2:07 PM, Stephen Smalley wrote:
>
> > > > > [   68.870715] irq event stamp: 23486085
> > > > > [   68.870715] hardirqs last  enabled at (23486085):
> > > > > [<ffffffffaa419406>] _raw_spin_unlock_irqrestore+0x46/0x60
> > > > > [   68.870715] hardirqs last disabled at (23486084):
> > > > > [<ffffffffaa419443>] _raw_spin_lock_irqsave+0x23/0x90
> > > > > [   68.870715] softirqs last  enabled at (23486074):
> > > > > [<ffffffffaa8004f3>] __do_softirq+0x4f3/0x662
> > > > > [   68.870715] softirqs last disabled at (23486067):
> > > > > [<ffffffffaa601072>] asm_call_on_stack+0x12/0x20
> > > > > [   68.870715] ---[ end trace fb02740ff6f4d0cd ]---
> > > >
> > > > I think one issue here is that systemd loads SELinux policy first, 
> > > > then IMA policy, so it doesn't know whether it needs to measure 
> > > > SELinux policy on first policy load, and another issue is that the 
> > > > policy is too large to just queue the policy data itself this way 
> > > > (or you need to use an allocator that can handle larger sizes).
> > >
> > > The problem seems to be that a lock is held when the IMA hook to 
> > > measure the LSM state is called. So memory allocation is not 
> > > allowed, but the hook is doing an allocation. I'll address this - 
> > > thanks for catching it.
> > >
> > > I have the following CONFIGs enabled, but I still don't see the 
> > > above issue on my machine.
> > The warning has to do with the memory allocation order being above 
> > the max order supported for kmalloc.  I think the problem is that 
> > ima_alloc_data_entry() is using kmemdup() to duplicate a payload of 
> > arbitrary size.  Policies on e.g. Fedora can be quite large, so you 
> > can't assume they can be allocated with kmalloc and friends.
>
> Thanks for clarifying. Yes ima_alloc_entry() does use kmemdup to save 
> the given buffer (to be measured) until IMA loads custom policy.
>
> On my machine the SELinux policy size is about 2MB.
>
> Perhaps vmalloc would be better than using kmalloc? If there are 
> better options for such large buffer allocation, please let me know.

kvmalloc() can be used to select whichever one is most appropriate.