On 2/16/2024 6:21 PM, Jonathan Corbet wrote:
Petr Tesařík <petr@xxxxxxxxxxx> writes:
On Fri, 16 Feb 2024 07:38:30 -0800
Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
I'm confused by this. The kernel doesn't (appear to) have a PGP parser
today. So are you saying that it *should* have one and it's only
feasible if its confined in a sandbox?
I'm sorry if this is confusing. Yes, your understanding is correct.
This patch series demonstrates that SBM (even in the initial version
that was submitted) allows to write a PGP parser which can survive
memory safety bugs withoug compromising the rest of the kernel.
So I have a different question: some years ago we added the "usermode
blob" feature for just this kind of use case - parsing firewall rules at
the time. It has never been used for that, but it's still there in
kernel/usermode_driver.c. Is there a reason why this existing
functionality can't be used for tasks like PGP parsing as well?
Yes, it was an option I explored last year (briefly talked about it as a
BoF at LSS NA 2023).
You are right, there is such feature that seemed to fit well.
User space blob embedded in a kernel module, so signed. User space
process connected only to the kernel through a pipe.
I even went ahead, and created a framework:
https://lore.kernel.org/linux-kernel/20230317145240.363908-1-roberto.sassu@xxxxxxxxxxxxxxx/
so that anyone can implement similar use cases.
The further step is: how can I ensure that the process launched by the
kernel is not attacked by root (which I assumed to be less powerful than
the kernel in a locked-down system).
I handled this in both directions:
- The process launched by the kernel is under a seccomp strict profile,
and can only read/write through the pipe created by the kernel (and
call few other system calls, such as exit()). Otherwise it is killed.
Cannot create any new communication channel.
- I created an LSM that denies any attempt to ptrace/signal to the
process launched by the kernel. Jann Horn also suggested to make the
process non-swappable.
However, despite these attempts, security people don't feel confident on
offloading a kernel workload outside the kernel.
This is why this work started.
Roberto
