[PATCH 1/2] xen/arm: entry: Place a speculation barrier following an ret instruction

[Julien Grall <julien@xen.org>]

From: Julien Grall <jgrall@amazon.com>

Some CPUs can speculate past a RET instruction and potentially perform
speculative accesses to memory before processing the return.

There is no known gadget available after the RET instruction today.
However some of the registers (such as in check_pending_guest_serror())
may contain a value provided the guest.

In order to harden the code, it would be better to add a speculation
barrier after each RET instruction. The performance is meant to be
negligeable as the speculation barrier is not meant to be archicturally
executed.

Note that on arm32, the ldmia instruction will act as a return from the
function __context_switch(). While the whitepaper doesn't suggest it is
possible to speculate after the instruction, add preventively a
speculation barrier after it as well.

This is part of the work to mitigate straight-line speculation.

Signed-off-by: Julien Grall <jgrall@amazon.com>

---

I am still unsure whether we preventively should add a speculation barrier
preventively after all the RET instructions in arm*/lib/. The smc call be
taken care in a follow-up patch.
---
 xen/arch/arm/arm32/entry.S | 1 +
 xen/arch/arm/arm64/entry.S | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S
index b228d44b190c..bd54fc43558b 100644
--- a/xen/arch/arm/arm32/entry.S
+++ b/xen/arch/arm/arm32/entry.S
@@ -442,6 +442,7 @@ ENTRY(__context_switch)
 
         add     r4, r1, #VCPU_arch_saved_context
         ldmia   r4, {r4 - sl, fp, sp, pc}       /* Load registers and return */
+        sb
 
 /*
  * Local variables:
diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index 175ea2981e72..b37d8fe09dc2 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -522,6 +522,7 @@ abort_guest_exit_end:
         cset    x19, ne
 
         ret
+        sb
 ENDPROC(check_pending_guest_serror)
 
 /*
@@ -583,6 +584,7 @@ ENTRY(__context_switch)
         ldr     lr, [x8]
         mov     sp, x9
         ret
+        sb
 
 /*
  * Local variables:
-- 
2.17.1