Messages in this thread | | | Date | Fri, 31 Jul 2020 15:29:05 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas |
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On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote: > On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:
> > +static int secretmem_mmap(struct file *file, struct vm_area_struct *vma) > > +{ > > + struct secretmem_ctx *ctx = file->private_data; > > + unsigned long mode = ctx->mode; > > + unsigned long len = vma->vm_end - vma->vm_start; > > + > > + if (!mode) > > + return -EINVAL; > > + > > + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0) > > + return -EINVAL; > > + > > + if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len)) > > + return -EAGAIN; > > + > > + switch (mode) { > > + case SECRETMEM_UNCACHED: > > + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); > > + fallthrough; > > + case SECRETMEM_EXCLUSIVE: > > + vma->vm_ops = &secretmem_vm_ops; > > + break; > > + default: > > + return -EINVAL; > > + } > > + > > + vma->vm_flags |= VM_LOCKED; > > + > > + return 0; > > +} > > I think the uncached mapping is not the right thing for arm/arm64. First > of all, pgprot_noncached() gives us Strongly Ordered (Device memory) > semantics together with not allowing unaligned accesses. I suspect the > semantics are different on x86.
> The second, more serious problem, is that I can't find any place where > the caches are flushed for the page mapped on fault. When a page is > allocated, assuming GFP_ZERO, only the caches are guaranteed to be > zeroed. Exposing this subsequently to user space as uncached would allow > the user to read stale data prior to zeroing. The arm64 > set_direct_map_default_noflush() doesn't do any cache maintenance.
It's also worth noting that in a virtual machine this is liable to be either broken (with a potential loss of coherency if the host has a cacheable alias as existing KVM hosts have), or pointless (if the host uses S2FWB to upgrade Stage-1 attribues to cacheable as existing KVM hosts also have).
I think that trying to avoid the data caches creates many more problems than it solves, and I don't think there's a strong justification for trying to support that on arm64 to begin with, so I'd rather entirely opt-out on supporting SECRETMEM_UNCACHED.
Thanks, Mark.
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