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. 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. -- 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/
