On 15/04/2026 14:48, David Hildenbrand (Arm) wrote: > On 2/27/26 18:54, Kevin Brodsky wrote: >> NEW in v6: support for large block mappings through a dedicated page table >> allocator (patch 14-17) > Heh, I had to read till the very end to realize that this is an RFC, and > then saw your other mail. > > I can recommend using b4 for patch management, where you can configure a > sticky prefix through > > b4 prep --set-prefixes RFC > > And using "b4 send" to automate all the rest.
I certainly should... sorry for the confusion! >> Threat model >> ============ >> >> The proposed scheme aims at mitigating data-only attacks (e.g. >> use-after-free/cross-cache attacks). In other words, it is assumed that >> control flow is not corrupted, and that the attacker does not achieve >> arbitrary code execution. Nothing prevents the pkey register from being >> set to its most permissive state - the assumption is that the register >> is only modified on legitimate code paths. >> >> A few related notes: >> >> - Functions that set the pkey register are all implemented inline. >> Besides performance considerations, this is meant to avoid creating >> a function that can be used as a straightforward gadget to set the >> pkey register to an arbitrary value. >> >> - kpkeys_set_level() only accepts a compile-time constant as argument, >> as a variable could be manipulated by an attacker. This could be >> relaxed but it seems unlikely that a variable kpkeys level would be >> needed in practice. >> > I see a lot of value for that also as a debugging mechanism. I hear that > other people had private patches that would attempt to only map leaf > pages in the direct map in pte_offset_map_lock() and friends. I assume > there are some tricky bits to that (concurrent access to page tables). Indeed, this should be a much better solution, not only because it means a lot fewer TLBIs, but also because it is truly per-thread (so concurrency is not a concern). > What's the general take regarding the thread model you describe vs. MTE? I'd say quite similar, although corrupting pointers (specifically the tag bits) remains possible in a data-only attack, while corrupting the POR_EL1 register would require some control flow hijack (only constant values are written to POR_EL1). > Regarding use-after-free, I'd assume KASAN would achieve something > similar. And with MTE "reasonably" fast. Or what is the biggest > difference you see, there? For use-after-free specifically, yes that sounds about right. > I'd assume that one difference would be, that not even match-all > pointers could accidentally modify page tables. Yep that's pretty much what I tried to say above - with pkeys you have to corrupt a system register to bypass the protection. > In the future, would you think that both mechanisms (pkey PT table > protection + KASAN) would be active at the same time, or wouldn't there > really be a lot of value in having both enabled? I think these are fairly orthogonal, KASAN gives you probabilistic spatial+temporal safety for most allocations, while kpkeys restricts access to key data to a small set of functions. I don't think one reduces the usefulness of the other. Of course KASAN makes it harder to use an arbitrary pointer to write to page tables, but kpkeys gives a clear guarantee (assuming CFI is preserved). > [...] > >> >> Open questions >> ============== >> >> A few aspects in this RFC that are debatable and/or worth discussing: >> >> - Can the pkeys block allocator be abstracted into something more >> generic? This seems desirable considering other use-cases for changing >> attributes of regions of the linear map, but the handling of page >> tables while splitting may be difficult to integrate in a generic >> allocator. >> >> - There is currently no restriction on how kpkeys levels map to pkeys >> permissions. A typical approach is to allocate one pkey per level and >> make it writable at that level only. As the number of levels >> increases, we may however run out of pkeys, especially on arm64 (just >> 8 pkeys with POE). Depending on the use-cases, it may be acceptable to >> use the same pkey for the data associated to multiple levels. >> >> >> Any comment or feedback is highly appreciated, be it on the high-level >> approach or implementation choices! > How crucial would the dedicated page table allocator be for a first up > streamed version? > > Assuming we introduce this as a debugging feature first, it would be > perfectly reasonable to just disallow large block mappings in the direct > map when enabled. > > That means, we could merge basic support first and think about how to > deal with page tables in a different way with most of the pkey details > out of the picture. I think that makes perfect sense, at least on arm64 where it's just a matter of configuring force_pte_mapping() appropriately. I'm not sure whether there is such an option on x86, though. - Kevin
