On Wed, 2020-09-30 at 13:35 +0300, Mike Rapoport wrote:
On Tue, Sep 29, 2020 at 08:06:03PM +0000, Edgecombe, Rick P wrote:
On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote:
On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote:
On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote:
Introduce "memfd_secret" system call with the ability to create memory areas visible only in the context of the owning process and not mapped not only to other processes but in the kernel page tables as well.
The user will create a file descriptor using the memfd_secret() system call where flags supplied as a parameter to this system call will define the desired protection mode for the memory associated with that file descriptor.
Currently there are two protection modes:
- exclusive - the memory area is unmapped from the kernel
direct map and it is present only in the page tables of the owning mm.
Seems like there were some concerns raised around direct map efficiency, but in case you are going to rework this...how does this memory work for the existing kernel functionality that does things like this?
get_user_pages(, &page); ptr = kmap(page); foo = *ptr;
Not sure if I'm missing something, but I think apps could cause the kernel to access a not-present page and oops.
The idea is that this memory should not be accessible by the kernel, so the sequence you describe should indeed fail.
Probably oops would be to noisy and in this case the report needs to be less verbose.
I was more concerned that it could cause kernel instabilities.
I think kernel recovers nicely from such sort of page fault, at least on x86.
We are talking about the kernel taking a direct map NP fault and oopsing? Hmm, I thought it should often recover, but stability should be considered reduced. How could the kernel know whether to release locks or clean up other state? Pretty sure I've seen deadlocks in this case.
I see, so it should not be accessed even at the userspace address? I wonder if it should be prevented somehow then. At least get_user_pages() should be prevented I think. Blocking copy_*_user() access might not be simple.
I'm also not so sure that a user would never have any possible reason to copy data from this memory into the kernel, even if it's just convenience. In which case a user setup could break if a specific kernel implementation switched to get_user_pages()/kmap() from using copy_*_user(). So seems maybe a bit thorny without fully blocking access from the kernel, or deprecating that pattern.
You should probably call out these "no passing data to/from the kernel" expectations, unless I missed them somewhere.
You are right, I should have been more explicit in the description of the expected behavoir.
Our thinking was that copy_*user() would work in the context of the process that "owns" the secretmem and gup() would not allow access in general, unless requested with certail (yet another) FOLL_ flag.
Hmm, yes. I think one easier thing about this design over the series Kirill sent out is that the actual page will never transition to and from unmapped while it's mapped in userspace. If it could transition, you'd have to worry about a race window between get_user_pages(FOLL_foo) and the kmap() where the page might get unmapped.
Without the ability to transition pages though, using this for KVM guests memory remains a not completely worked through problem since it has the get_user_pages()/kmap() pattern quite a bit. Did you have an idea for that? (I thought I saw that use case mentioned somewhere).