On Tue, Aug 11, 2015 at 3:38 PM, Frederic Weisbecker <[email protected]> wrote: > On Tue, Jul 07, 2015 at 03:53:29AM -0700, tip-bot for Andy Lutomirski wrote: >> Commit-ID: 02bc7768fe447ae305e924b931fa629073a4a1b9 >> Gitweb: >> http://git.kernel.org/tip/02bc7768fe447ae305e924b931fa629073a4a1b9 >> Author: Andy Lutomirski <[email protected]> >> AuthorDate: Fri, 3 Jul 2015 12:44:31 -0700 >> Committer: Ingo Molnar <[email protected]> >> CommitDate: Tue, 7 Jul 2015 10:59:08 +0200 >> >> x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old >> assembly code >> >> Signed-off-by: Andy Lutomirski <[email protected]> >> Cc: Andy Lutomirski <[email protected]> >> Cc: Borislav Petkov <[email protected]> >> Cc: Brian Gerst <[email protected]> >> Cc: Denys Vlasenko <[email protected]> >> Cc: Denys Vlasenko <[email protected]> >> Cc: Frederic Weisbecker <[email protected]> >> Cc: H. Peter Anvin <[email protected]> >> Cc: Kees Cook <[email protected]> >> Cc: Linus Torvalds <[email protected]> >> Cc: Oleg Nesterov <[email protected]> >> Cc: Peter Zijlstra <[email protected]> >> Cc: Rik van Riel <[email protected]> >> Cc: Thomas Gleixner <[email protected]> >> Cc: [email protected] >> Link: >> http://lkml.kernel.org/r/60e90901eee611e59e958bfdbbe39969b4f88fe5.1435952415.git.l...@kernel.org >> Signed-off-by: Ingo Molnar <[email protected]> >> --- >> arch/x86/entry/entry_64.S | 64 >> +++++++++++----------------------------- >> arch/x86/entry/entry_64_compat.S | 5 ++++ >> 2 files changed, 23 insertions(+), 46 deletions(-) >> >> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S >> index 83eb63d..168ee26 100644 >> --- a/arch/x86/entry/entry_64.S >> +++ b/arch/x86/entry/entry_64.S >> @@ -1088,7 +1055,12 @@ ENTRY(error_entry) >> SWAPGS >> >> .Lerror_entry_from_usermode_after_swapgs: >> +#ifdef CONFIG_CONTEXT_TRACKING >> + call enter_from_user_mode >> +#endif > > This makes me very nervous as well! > > It means that instead of using the context tracking save/restore model that > we had > with exception_enter/exception_exit(), now we rely on the CS register. > > I don't think we can do that because our "context tracking" is a soft > tracking whereas > CS is hard tracking and both are not atomically synchronized together. > > Imagine this situation: we are running in userspace. Context tracking knows > it, everything > is fine. Now we do a syscall, we enter in kernel entry code but we trigger an > exception > (DEBUG for example) before we got a chance to call user_exit(), which means > that the context > tracking code still thinks we are in userspace, so we look at CS from the > exception entry code > and it says the exception happened in the kernel. Hence we don't call > user_exit() before calling > the exception handler. There is the bug because the exception handler may use > RCU which still > thinks we run in userspace.
#DB doesn't go through this patch -- it uses the paranoid entry path and ist_enter. But I see your point. I think that, if we have a problem like this in practice, then we should fix it. But the old code had the same issue. If we got an exception (the most likely one is probably a vmalloc fault) during user_exit and we then hit exception_enter, the result would probably be bad. > > In early context tracking days we have relied on CS. But I changed that > because of such > issue. The only reliable source for soft context tracking is the soft context > tracking itself. I don't see why the soft state is more reliable. The only bad case is where the entry itself (HW entry up to user_exit) is not atomic enough, but that path should be at least as atomic as user_exit itself is. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
