On Mon, Mar 10, 2014 at 10:38 AM, H. Peter Anvin <h...@linux.intel.com> wrote: > On 03/10/2014 10:31 AM, Andy Lutomirski wrote: >>> >>>> For 64-bit, this is an entirely different story. The vsyscall page is >>>> stuck in the fixmap forever, although I want to add a way for >>>> userspace to opt out. The vvar page, hpet, etc could move into vmas, >>>> though. I kind of want to do that anyway to allow processes to turn >>>> off the ability to read the clock. >>> >>> Wait... you want to do what?! >> >> This isn't even my idea: >> >> commit 8fb402bccf203ecca8f9e0202b8fd3c937dece6f >> Author: Erik Bosman <ebn...@few.vu.nl> >> Date: Fri Apr 11 18:54:17 2008 +0200 >> >> generic, x86: add prctl commands PR_GET_TSC and PR_SET_TSC >> >> This patch adds prctl commands that make it possible >> to deny the execution of timestamp counters in userspace. >> If this is not implemented on a specific architecture, >> prctl will return -EINVAL. >> >> Currently anything that tries to use the vdso will just crash if you >> do that, and it fails to turn off direct HPET access. Fixing this >> might be nice, but the current vvar implementation makes it >> impossible. If you want to stick something in a seccomp sandbox and >> make it very difficult for it to exploit timing side channels, then >> this is important :) >> > > Yes, we'd have to switch the vdso to using syscall access. Doing that > from inside a system call is... "interesting".
It's a little less interesting if it just involves changing a vma. It's still tricky, though -- would each struct mm have its own struct file for the vvar page? Can this be done with some vm_operations_struct magic? There are possible races, too, though -- another thread could access the thing concurrently with a syscall. It might be nice in general for there to be a /dev/vdso and for the vdso to literally be a mapping of that device node. I bet that CRIU would appreciate this. (The mmap flags would be a little odd, since different pages have different protections.) Anyway, this is totally off topic for the current issue :) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
