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Subject[PATCH 4.4 002/241] x86/speculation/mds: Improve CPU buffer clear documentation
Date
From: Andy Lutomirski <luto@kernel.org>

commit 9d8d0294e78a164d407133dea05caf4b84247d6a upstream.

On x86_64, all returns to usermode go through
prepare_exit_to_usermode(), with the sole exception of do_nmi().
This even includes machine checks -- this was added several years
ago to support MCE recovery. Update the documentation.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jon Masters <jcm@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/999fa9e126ba6a48e9d214d2f18dbde5c62ac55c.1557865329.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
Documentation/x86/mds.rst | 39 +++++++--------------------------------
1 file changed, 7 insertions(+), 32 deletions(-)

--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -142,38 +142,13 @@ Mitigation points
mds_user_clear.

The mitigation is invoked in prepare_exit_to_usermode() which covers
- most of the kernel to user space transitions. There are a few exceptions
- which are not invoking prepare_exit_to_usermode() on return to user
- space. These exceptions use the paranoid exit code.
-
- - Non Maskable Interrupt (NMI):
-
- Access to sensible data like keys, credentials in the NMI context is
- mostly theoretical: The CPU can do prefetching or execute a
- misspeculated code path and thereby fetching data which might end up
- leaking through a buffer.
-
- But for mounting other attacks the kernel stack address of the task is
- already valuable information. So in full mitigation mode, the NMI is
- mitigated on the return from do_nmi() to provide almost complete
- coverage.
-
- - Machine Check Exception (#MC):
-
- Another corner case is a #MC which hits between the CPU buffer clear
- invocation and the actual return to user. As this still is in kernel
- space it takes the paranoid exit path which does not clear the CPU
- buffers. So the #MC handler repopulates the buffers to some
- extent. Machine checks are not reliably controllable and the window is
- extremly small so mitigation would just tick a checkbox that this
- theoretical corner case is covered. To keep the amount of special
- cases small, ignore #MC.
-
- - Debug Exception (#DB):
-
- This takes the paranoid exit path only when the INT1 breakpoint is in
- kernel space. #DB on a user space address takes the regular exit path,
- so no extra mitigation required.
+ all but one of the kernel to user space transitions. The exception
+ is when we return from a Non Maskable Interrupt (NMI), which is
+ handled directly in do_nmi().
+
+ (The reason that NMI is special is that prepare_exit_to_usermode() can
+ enable IRQs. In NMI context, NMIs are blocked, and we don't want to
+ enable IRQs with NMIs blocked.)


2. C-State transition

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