Messages in this thread | | | Date | Thu, 07 May 2015 20:48:14 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCH 00/12] [RFC] x86: Memory Protection Keys |
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On 05/07/2015 08:09 PM, Dave Hansen wrote: > On 05/07/2015 10:57 AM, Ingo Molnar wrote: >>>> There are two new instructions (RDPKRU/WRPKRU) for reading and >>>> writing to the new register. The feature is only available in >>>> 64-bit mode, even though there is theoretically space in the PAE >>>> PTEs. These permissions are enforced on data access only and have >>>> no effect on instruction fetches. >> So I'm wondering what the primary usecases are for this feature? >> Could you outline applications/workloads/libraries that would >> benefit from this? > > There are lots of things that folks would _like_ to mprotect(), but end > up not being feasible because of the overhead of going and mucking with > thousands of PTEs and shooting down remote TLBs every time you want to > change protections. > > Data structures like logs or journals that are only written to in very > limited code paths, but that you want to protect from "stray" writes. > > Maybe even a database where a query operation will never need to write > to memory, but an insert would. You could keep the data R/O during the > entire operation except when an insert is actually in progress. It > narrows the window where data might be corrupted. This becomes even > more valuable if a stray write to memory is guaranteed to hit storage... > like with persistent memory. > > Someone mentioned to me that valgrind does lots of mprotect()s and might > benefit from this. > > We could keep heap metadata as R/O and only make it R/W inside of > malloc() itself to catch corruption more quickly.
But that metadata is typically within the same page as the data itself (for small objects at least), no?
> More crazy ideas welcome. :)
Since you asked :) I wonder if the usefulness could be extended by making it possible for a thread to revoke its access to WRPKRU (it's not privileged, right?). Then I could imagine some extra security for sandbox/bytecode/JIT code so it doesn't interfere with the runtime. But since it doesn't block instruction fetches, then maybe it wouldn't make much difference...
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