On Mon, Mar 5, 2018 at 8:12 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote: > On Mon, Mar 05, 2018 at 04:17:45AM -0800, Linus Torvalds wrote: >> Restoring the segments can cause exceptions that need to be >> handled. With PTI enabled, we still need to be on kernel cr3 >> when the exception happens. For the cr3-switch we need >> at least one integer scratch register, so we can't switch >> with the user integer registers already loaded. >> >> >> This fundamentally seems wrong. > > Okay, right, with v3 it is wrong, in v2 I still thought I could get away > without remembering the entry-cr3, but didn't think about the #DB case > then. > > In v3 I added code which remembers the entry-cr3 and handles the > entry-from-kernel-mode-with-user-cr3 case for all exceptions including > #DB. > >> The things is, we *know* that we will restore two segment registers with the >> user cr3 already loaded: CS and SS get restored with the final iret. > > Yeah, I know, but the iret-exception path is fine because it will > deliver a SIGILL and doesn't return to the faulting iret. > > Anyway, I will remove these restore-reorderings, they are not needed > anymore. > >> So has this been tested with >> >> - single-stepping through sysenter >> >> This takes a DB fault in the first kernel instruction. We're in kernel mode, >> but with user cr3. >> >> - ptracing and setting CS/SS to something bad >> >> That should test the "exception on iret" case - again in kernel mode, but >> with user cr3 restored for the return. > > The iret-exception case is tested by the ldt_gdt selftest (the > do_multicpu_tests subtest). But I didn't actually tested single-stepping > through sysenter yet. I just re-ran the same tests I did with v2 on this > patch-set. > > Regards, > > Joerg > For the IRET fault case you will still need to catch it in the exception code. See the 64-bit code (.Lerror_bad_iret) for example. For 32-bit, you could just expand that check to cover the whole exit prologue after the CR3 switch, including the data segment loads. I do wonder though, how expensive is a CR3 read? The SDM implies that only writes are serializing. It may be simpler to just unconditionally check it. -- Brian Gerst -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
