Re: [PATCH v6 6/9] kernel: entry: Support Syscall User Dispatch for common syscall entry

[Andy Lutomirski]

> On Sep 7, 2020, at 3:15 AM, Christian Brauner <christian.brauner@xxxxxxxxxx> wrote:
> 
> On Fri, Sep 04, 2020 at 04:31:44PM -0400, Gabriel Krisman Bertazi wrote:
>> Syscall User Dispatch (SUD) must take precedence over seccomp, since the
>> use case is emulation (it can be invoked with a different ABI) such that
>> seccomp filtering by syscall number doesn't make sense in the first
>> place.  In addition, either the syscall is dispatched back to userspace,
>> in which case there is no resource for seccomp to protect, or the
> 
> Tbh, I'm torn here. I'm not a super clever attacker but it feels to me
> that this is still at least a clever way to circumvent a seccomp
> sandbox.
> If I'd be confined by a seccomp profile that would cause me to be
> SIGKILLed when I try do open() I could prctl() myself to do user
> dispatch to prevent that from happening, no?
> 

Not really, I think. The idea is that you didn’t actually do open(). You did a SYSCALL instruction which meant something else, and the syscall dispatch correctly prevented the kernel from misinterpreting it as open().