Re: [PATCH v18 0/9] mm: introduce memfd_secret system call to create "secret" memory areas

Nick Kossifidis
2021-05-06T23:16:55Z

Στις 2021-05-06 20:05, James Bottomley έγραψε:
> On Thu, 2021-05-06 at 18:45 +0200, David Hildenbrand wrote:
>>
>> Also, there is a way to still read that memory when root by
>>
>> 1. Having kdump active (which would often be the case, but maybe not
>> to dump user pages )
>> 2. Triggering a kernel crash (easy via proc as root)
>> 3. Waiting for the reboot after kump() created the dump and then
>> reading the content from disk.
>
> Anything that can leave physical memory intact but boot to a kernel
> where the missing direct map entry is restored could theoretically
> extract the secret. However, it's not exactly going to be a stealthy
> extraction ...
>
>> Or, as an attacker, load a custom kexec() kernel and read memory
>> from the new environment. Of course, the latter two are advanced
>> mechanisms, but they are possible when root. We might be able to
>> mitigate, for example, by zeroing out secretmem pages before booting
>> into the kexec kernel, if we care :)
>
> I think we could handle it by marking the region, yes, and a zero on
> shutdown might be useful ... it would prevent all warm reboot type
> attacks.
>

I had similar concerns about recovering secrets with kdump, and
considered cleaning up keyrings before jumping to the new kernel. The
problem is we can't provide guarantees in that case, once the kernel has
crashed and we are on our way to run crashkernel, we can't be sure we
can reliably zero-out anything, the more code we add to that path the
more risky it gets. However during reboot/normal kexec() we should do
some cleanup, it makes sense and secretmem can indeed be useful in that
case. Regarding loading custom kexec() kernels, we mitigate this with
the kexec file-based API where we can verify the signature of the loaded
kimage (assuming the system runs a kernel provided by a trusted 3rd
party and we 've maintained a chain of trust since booting).