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Linux Internals: How /proc/self/mem writes to unwritable memory (2021)

122 points by medbar 2 days ago | 26 comments

hansendc 23 hours ago

"On x86-64, there are two CPU settings which control the kernel’s ability to access memory."

There are a couple more than two, even in 2021.

Memory Protection Keys come to mind, as do the NPT/EPT tables when virtualization is in play. SEV and SGX also have their own ways of preventing the kernel from writing to memory. The CPU also has range registers that protect certain special physical address ranges, like the TDX module's range. You can't write there either.

That's all that comes to mind at the moment. It's definitely a fun question!

karlgkk 18 hours ago

a thought: do MPK actually control the kernel's ability to access memory? on intel, i think if you try to read that memory, a page fault wont be thrown. although with PKS, kernel reads will cause a page fault.

so can the kernel (ring0) freely read/write to memory encrypted with MPK? I think so, yes. good luck with whatever happens next tho lol

als0 11 hours ago

There are two versions of MPK. One is only applicable to userspace pages. The other is newer and can be applied to kernel space pages; last time I checked, this was only available on newer Xeon processors.

By the way, MPK memory is not encrypted. The key is just an identifier for the requestor. If the requestor key doesn’t match the same identifier for the memory page, then an exception is raised.

Funnily enough, MPK isn’t new at all. It’s almost a reintroduction of a feature from Itanium.

aliceryhl 16 hours ago

Interesting. Though looking at the code, it does still check VM_MAYWRITE, so the mapping needs to be something you could remap as writable.

KenoFischer 20 hours ago

I'm still surprised I was the first one to notice when Linus tried to change this - I always thought it was a pretty well known behavior.

anthk 14 hours ago

/proc it's a bad imitation of plan9's /proc.

bluepeter 23 hours ago

The kernel owns the page tables. It can always find another way in.

vlovich123 19 hours ago

But the point here is that userspace can use this to bypass kernel protections that would otherwise prevent it from mutating R^X pages for example, not that the kernel can bypass its own.

im3w1l 11 hours ago

Those protections are mainly about preventing well intentioned people from accidentally shooting themselves in the foot though, right? So it's not really a big deal that there is a way around it.

jcalvinowens 8 hours ago

No, page table write access allows arbitrary memory access because I can map any PFN I want. It's certainly a vector to execute arbitrary code in ring 0.

vlovich123 8 hours ago

It’s a huge deal. It’s a trivial gadget for building a larger exploit chain

pjmlp 16 hours ago

Not really, of the security measures on Windows, is exactly to control how kernel can access secure process memory, as possible mitigation to attacks by rogue drivers.

Naturally it is the kind of stuff that requires Windows 11 vlatest with the nice Pluton security CPU, as part of CoPilot+ PCs design.

mschuster91 23 hours ago

> The kernel owns the page tables.

not entirely, IOMMU is a thing, that is IIRC how Amazon and other hyperscalers can promise you virtual machines whose memory cannot be touched even in the case the host is compromised (and, by extension, also if the feds arrive to v& your server).

gruez 20 hours ago

>how Amazon and other hyperscalers can promise you virtual machines whose memory cannot be touched even in the case the host is compromised (and, by extension, also if the feds arrive to v& your server).

Even if we take those promises at face value, it practically doesn't mean much because every server still needs to handle reboots, which is when they can inject their evil code.

Borealid 20 hours ago

MK-TME allows having memory encrypted at run time, and the platform TPM signs an attestation saying the memory was not altered.

Malicious code can't be injected at boot without breaking that TPM.

fc417fc802 19 hours ago

Subject to the huge caveat that the attacker does not have physical access. https://tee.fail/

Borealid 17 hours ago

An interesting implementation flaw, but not a conceptual problem with the design.

fc417fc802 16 hours ago

Well, it kind of is actually. The previous iteration of the design didn't have that vulnerability but it was slower because managing IVs within the given constraints adds an additional layer of complexity. This is the pragmatic compromise so to speak.

Does it count as a conceptual problem when technical challenges without an acceptable solution block your goal?

ronsor 22 hours ago

If your threat model is being v& by feds, maybe you should keep your server at home behind Tor.

iberator 13 hours ago

Hosting tor outbound server at home is stupid idea.

Your home is gonna be raided by Police and you will wait months or year to get your shit back and then if nothing, gonna be charged for having pirated windows and Photoshop lol

real story

r_lee 10 hours ago

lmao please tell more

mschuster91 8 hours ago

Not even two years ago, see https://www.golem.de/news/nach-hausdurchsuchung-deutscher-to...

And it's not just a one off occurrence either. Tor exit node operators getting v& has been a thing for decades: https://www.heise.de/news/Anonymisierungsserver-bei-Razzia-b...

mschuster91 10 hours ago

These days, every American's threat model should include being v& by the feds, and here in Germany, the situation isn't much better, you can get v& for saying the Minister of Interior is a dick [1].

Yes, this was later on ruled unconstitutional, but it doesn't change the facts, and, worse, Germany doesn't have a "fruit of the forbidden tree" rule.

[1] https://www.spiegel.de/panorama/justiz/hamburg-wohnungsdurch...

haberman 21 hours ago

TL;DR: when a user writes to /proc/self/mem, the kernel bypasses the MMU and hardware address translation, opting to emulate it in software (including emulated page faults!), which allows it to disregard any memory protection that is currently setup in the page tables.

IAmLiterallyAB 19 hours ago

It doesn't bypass it exactly, it's still accessing it via virtual memory and the page tables. It's just that the kernel maintains one big linear memory map of RAM that's writable.

rramadass 20 hours ago

Thank You.