Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Aug 11, 2015 at 3:38 PM, Frederic Weisbecker <fweisbec@gmail.com> 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 <luto@kernel.org>
>> AuthorDate: Fri, 3 Jul 2015 12:44:31 -0700
>> Committer:  Ingo Molnar <mingo@kernel.org>
>> 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 <luto@kernel.org>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Borislav Petkov <bp@alien8.de>
>> Cc: Brian Gerst <brgerst@gmail.com>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Denys Vlasenko <vda.linux@googlemail.com>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: paulmck@linux.vnet.ibm.com
>> Link: http://lkml.kernel.org/r/60e90901eee611e59e958bfdbbe39969b4f88fe5.1435952415.git.luto@kernel.org
>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>> ---
>>  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