Last commit made on 2015-07-29
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f3caf1a... by Andy Lutomirski <email address hidden> on 2015-07-11

x86/nmi/64: Use DF to avoid userspace RSP confusing nested NMI detection

commit 810bc075f78ff2c221536eb3008eac6a492dba2d upstream.

We have a tricky bug in the nested NMI code: if we see RSP pointing
to the NMI stack on NMI entry from kernel mode, we assume that we
are executing a nested NMI.

This isn't quite true. A malicious userspace program can point RSP
at the NMI stack, issue SYSCALL, and arrange for an NMI to happen
while RSP is still pointing at the NMI stack.

Fix it with a sneaky trick. Set DF in the region of code that the RSP
check is intended to detect. IRET will clear DF atomically.

(Note: other than paravirt, there's little need for all this complexity.
 We could check RIP instead of RSP.)

Fixes CVE-2015-3291.

Cc: <email address hidden>
Reviewed-by: Steven Rostedt <email address hidden>
Signed-off-by: Andy Lutomirski <email address hidden>
[bwh: Backported to 4.0: adjust filename, context]
Signed-off-by: Ben Hutchings <email address hidden>
Acked-by: John Johansen <email address hidden>
Acked-by: Andy Whitcroft <email address hidden>
Signed-off-by: Luis Henriques <email address hidden>
Signed-off-by: Andy Whitcroft <email address hidden>

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