x86/entry: Add enter_from_user_mode() and use it in syscalls
x86/entry: Add enter_from_user_mode() and use it in syscalls

Changing the x86 context tracking hooks is dangerous because
there are no good checks that we track our context correctly.
Add a helper to check that we're actually in CONTEXT_USER when
we enter from user mode and wire it up for syscall entries.

Subsequent patches will wire this up for all non-NMI entries as
well.  NMIs are their own special beast and cannot currently
switch overall context tracking state.  Instead, they have their
own special RCU hooks.

This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a
layer of indirection).  Eventually, we should fix up the core
context tracking code to supply a function that does what we
want (and can be much simpler than user_exit), which will enable
us to get rid of the extra call.

Signed-off-by: Andy Lutomirski <2cc0d0d76de2e6f37b92deeb83c85cb306fc5dc1@kernel.org>
Cc: Andy Lutomirski <2cc0d0d76de2e6f37b92deeb83c85cb306fc5dc1@amacapital.net>
Cc: Borislav Petkov <0691a664b24b6f15b20cd5aee64b72271db08be1@alien8.de>
Cc: Brian Gerst <e60badcb99f652a0bcc15460d39f7ea12d6655ee@gmail.com>
Cc: Denys Vlasenko <50fbbd36fe0f9c8261d4a3c762c8c29ffb35e48f@redhat.com>
Cc: Denys Vlasenko <42afb3a19136e88700e8b77002ea6a5a94c16782@googlemail.com>
Cc: Frederic Weisbecker <e8a1bf9163cb25e93cfd6540f223b3872ea7ee55@gmail.com>
Cc: H. Peter Anvin <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>
Cc: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Cc: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@linux-foundation.org>
Cc: Oleg Nesterov <20b70f0af00562e63758b9ee42012ecc96c58590@redhat.com>
Cc: Peter Zijlstra <3fddac958924aef220f202ca567388ddab3f14a8@infradead.org>
Cc: Rik van Riel <a21938f5d463ddf41aa718934c205ca2cce8ebbc@redhat.com>
Cc: Thomas Gleixner <00e4cf8f46a57000a44449bf9dd8cbbcc209fd2a@linutronix.de>
Cc: 1e0ce936bb9b355d257bf5790d2513c3f28be22b@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/853b42420066ec3fb856779cdc223a6dcb5d355b.1435952415.git.2cc0d0d76de2e6f37b92deeb83c85cb306fc5dc1@kernel.org
Signed-off-by: Ingo Molnar <9dbbbf0688fedc85ad4da37637f1a64b8c718ee2@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs