On Thu, 2021-05-06 at 18:45 +0200, David Hildenbrand wrote:
On 06.05.21 17:26, James Bottomley wrote:
On Wed, 2021-05-05 at 12:08 -0700, Andrew Morton wrote:
On Wed, 3 Mar 2021 18:22:00 +0200 Mike Rapoport <rppt@kernel.org
wrote:
This is an implementation of "secret" mappings backed by a file descriptor.
The file descriptor backing secret memory mappings is created using a dedicated memfd_secret system call The desired protection mode for the memory is configured using flags parameter of the system call. The mmap() of the file descriptor created with memfd_secret() will create a "secret" memory mapping. The pages in that mapping will be marked as not present in the direct map and will be present only in the page table of the owning mm.
Although normally Linux userspace mappings are protected from other users, such secret mappings are useful for environments where a hostile tenant is trying to trick the kernel into giving them access to other tenants mappings.
I continue to struggle with this and I don't recall seeing much enthusiasm from others. Perhaps we're all missing the value point and some additional selling is needed.
Am I correct in understanding that the overall direction here is to protect keys (and perhaps other things) from kernel bugs? That if the kernel was bug-free then there would be no need for this feature? If so, that's a bit sad. But realistic I guess.
Secret memory really serves several purposes. The "increase the level of difficulty of secret exfiltration" you describe. And, as you say, if the kernel were bug free this wouldn't be necessary.
But also:
1. Memory safety for use space code. Once the secret memory is allocated, the user can't accidentally pass it into thekernel to be transmitted somewhere.
That's an interesting point I didn't realize so far.
2. It also serves as a basis for context protection of virtual machines, but other groups are working on this aspect, andit is broadly similar to the secret exfiltration from the kernel problem.
I was wondering if this also helps against CPU microcode issues like spectre and friends.
It can for VMs, but not really for the user space secret memory use cases ... the in-kernel mitigations already present are much more effective.
Is this intended to protect keys/etc after the attacker has gained the ability to run arbitrary kernel-mode code? If so, that seems optimistic, doesn't it?
Not exactly: there are many types of kernel attack, but mostly the attacker either manages to effect a privilege escalation to root or gets the ability to run a ROP gadget. The object of this code is to be completely secure against root trying to extract the secret (some what similar to the lockdown idea), thus defeating privilege escalation and to provide "sufficient" protection against ROP gadget.
What stops "root" from mapping /dev/mem and reading that memory?
/dev/mem uses the direct map for the copy at least for read/write, so it gets a fault in the same way root trying to use ptrace does. I think we've protected mmap, but Mike would know that better than I.
IOW, would we want to enforce "CONFIG_STRICT_DEVMEM" with CONFIG_SECRETMEM?
Unless there's a corner case I haven't thought of, I don't think it adds much. However, doing a full lockdown on a public system where users want to use secret memory is best practice I think (except I think you want it to be the full secure boot lockdown to close all the root holes).
Also, there is a way to still read that memory when root by
- 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.
James