Στις 2021-05-06 20:05, James Bottomley έγραψε:

On Thu, 2021-05-06 at 18:45 +0200, David Hildenbrand wrote:

quoted

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 ...

quoted

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).

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