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x86/speculation/mds: Improve CPU buffer clear documentation

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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: 04dcbdb805 ("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>
This commit is contained in:
Andy Lutomirski 2019-05-14 13:24:40 -07:00 committed by Ingo Molnar
parent 88640e1dcd
commit 9d8d0294e7
1 changed files with 6 additions and 31 deletions
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37
Documentation/x86/mds.rst
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@ -142,38 +142,13 @@ Mitigation points
mds_user_clear. mds_user_clear.
The mitigation is invoked in prepare_exit_to_usermode() which covers The mitigation is invoked in prepare_exit_to_usermode() which covers
most of the kernel to user space transitions. There are a few exceptions all but one of the kernel to user space transitions. The exception
which are not invoking prepare_exit_to_usermode() on return to user is when we return from a Non Maskable Interrupt (NMI), which is
space. These exceptions use the paranoid exit code. handled directly in do_nmi().
- Non Maskable Interrupt (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
Access to sensible data like keys, credentials in the NMI context is enable IRQs with NMIs blocked.)
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.
2. C-State transition 2. C-State transition