On Wed, Jan 3, 2018 at 8:07 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > On Wed, Jan 3, 2018 at 5:24 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote: >> On Wed, 2018-01-03 at 21:11 +0100, Greg Kroah-Hartman wrote: >>> This is the start of the stable review cycle for the 4.9.75 release. >>> There are 39 patches in this series, all will be posted as a response >>> to this one. If anyone has any issues with these being applied, please >>> let me know. >> [...] >> >> This seems to be missing a backport of: >> >> commit 85900ea51577e31b186e523c8f4e068c79ecc7d3 >> Author: Andy Lutomirski <luto@xxxxxxxxxx> >> Date: Tue Dec 12 07:56:42 2017 -0800 >> >> x86/pti: Map the vsyscall page if needed >> >> without which, KPTI will break some older user-space. > > You're (kind of) right: I did warn Greg of that yesterday, and we were > both vaguely hoping that someone else could fill in that blank more > quickly than I'd get there. > > I doubt it's an actual backport of Andy's 85900ea51577 that's needed - > I'd expect his to depend on at least some of the improved > infrastructure in 4.15-rc, rather than the old-style kaiser-mapping > employed in these backports. > > Andrea has a RHEL7 patch for this, and I have a 3.2 patch derived from > Andrea's, but neither would be right for 4.9 or 4.4 as is: I think I'm > going to have to drop today's priority, and attend to this lacuna > tomorrow. > I think what you need to backport is: "x86/vsyscall/64: Explicitly set _PAGE_USER in the pagetable hierarchy" to get the permission bits right and maybe the big simplification of kaiser_whatever_its_called_set_pgd(). The latter is going to be a mess if you're still using the old code. Sorry. But even with the crappy old code, kaiser_add_mapping() on the vsyscall page plus the latter _PAGE_USER bit plus the call to the new _PAGE_USER code in "x86/pti: Map the vsyscall page if needed" should do it. If you skip all this and only get vsyscall emulation working such that vsyscall page *reads* don't work from user mode, you're going to encounter weird problems with programs like "pin". Apparently people use these things.