alt.hn

3/8/2026 at 10:50:55 PM

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

 https://offlinemark.com/an-obscure-quirk-of-proc/

by medbar

3/9/2026 at 12:26:14 AM

"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!

by hansendc

3/9/2026 at 5:06:20 AM

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

by karlgkk

3/9/2026 at 12:28:50 PM

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.

by als0

3/9/2026 at 7:14:05 AM

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

by aliceryhl

3/9/2026 at 3:03:33 AM

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.

by KenoFischer

3/9/2026 at 9:01:45 AM

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

by anthk

3/9/2026 at 12:07:06 AM

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

by bluepeter

3/9/2026 at 4:58:31 AM

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.

by vlovich123

3/9/2026 at 12:07:00 PM

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.

by im3w1l

3/9/2026 at 3:21:24 PM

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.

by jcalvinowens

3/9/2026 at 3:28:15 PM

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

by vlovich123

3/9/2026 at 7:41:24 AM

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.

by pjmlp

3/9/2026 at 12:23:00 AM

> 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).

by mschuster91

3/9/2026 at 3:35:01 AM

>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.

by gruez

3/9/2026 at 3:55:52 AM

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.

by Borealid

3/9/2026 at 4:18:51 AM

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

by fc417fc802

3/9/2026 at 6:53:34 AM

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

by Borealid

3/9/2026 at 7:46:06 AM

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?

by fc417fc802

3/9/2026 at 2:00:38 AM

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

by ronsor

3/9/2026 at 10:34:34 AM

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

by iberator

3/9/2026 at 1:01:57 PM

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...

by mschuster91

3/9/2026 at 2:13:19 AM

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.

by haberman

3/9/2026 at 4:59:12 AM

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.

by IAmLiterallyAB

3/9/2026 at 3:34:29 AM

Thank You.

by rramadass