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

From: Andy Lutomirski
Date: Tue Aug 11 2015 - 18:51:51 EST

On Tue, Aug 11, 2015 at 3:38 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> On Tue, Jul 07, 2015 at 03:53:29AM -0700, tip-bot for Andy Lutomirski wrote:
>> Commit-ID: 02bc7768fe447ae305e924b931fa629073a4a1b9
>> Gitweb:
>> Author: Andy Lutomirski <luto@xxxxxxxxxx>
>> AuthorDate: Fri, 3 Jul 2015 12:44:31 -0700
>> Committer: Ingo Molnar <mingo@xxxxxxxxxx>
>> 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@xxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Brian Gerst <brgerst@xxxxxxxxx>
>> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> Cc: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: paulmck@xxxxxxxxxxxxxxxxxx
>> Link:
>> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
>> ---
>> 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)
>> .Lerror_entry_from_usermode_after_swapgs:
>> + 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

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