MIPS: Flush RPS on kernel entry with EVA

When EVA is enabled, flush the Return Prediction Stack (RPS) present on
some MIPS cores on entry to the kernel from user mode.

This is important specifically for interAptiv with EVA enabled,
otherwise kernel mode RPS mispredicts may trigger speculative fetches of
user return addresses, which may be sensitive in the kernel address
space due to EVA's overlapping user/kernel address spaces.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.15.x-
Patchwork: https://patchwork.linux-mips.org/patch/10812/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
index 28d6d9364bd1..a71da576883c 100644
--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -152,6 +152,31 @@
                 .set    noreorder
                 bltz    k0, 8f
                  move   k1, sp
+#ifdef CONFIG_EVA
+                /*
+                 * Flush interAptiv's Return Prediction Stack (RPS) by writing
+                 * EntryHi. Toggling Config7.RPS is slower and less portable.
+                 *
+                 * The RPS isn't automatically flushed when exceptions are
+                 * taken, which can result in kernel mode speculative accesses
+                 * to user addresses if the RPS mispredicts. That's harmless
+                 * when user and kernel share the same address space, but with
+                 * EVA the same user segments may be unmapped to kernel mode,
+                 * even containing sensitive MMIO regions or invalid memory.
+                 *
+                 * This can happen when the kernel sets the return address to
+                 * ret_from_* and jr's to the exception handler, which looks
+                 * more like a tail call than a function call. If nested calls
+                 * don't evict the last user address in the RPS, it will
+                 * mispredict the return and fetch from a user controlled
+                 * address into the icache.
+                 *
+                 * More recent EVA-capable cores with MAAR to restrict
+                 * speculative accesses aren't affected.
+                 */
+                MFC0    k0, CP0_ENTRYHI
+                MTC0    k0, CP0_ENTRYHI
+#endif
                 .set    reorder
                 /* Called from user mode, new stack. */
                 get_saved_sp