[Xen-changelog] [xen stable-4.12] xen/arm: Place a speculation barrier sequence following an eret instruction

[patchbot@xxxxxxx]

commit a5fcafbfbee55261853fba07149c1c795f2baf58
Author:     Julien Grall <julien@xxxxxxx>
AuthorDate: Thu Dec 19 08:12:21 2019 +0000
Commit:     Julien Grall <julien@xxxxxxx>
CommitDate: Tue Jan 14 14:25:16 2020 +0000

    xen/arm: Place a speculation barrier sequence following an eret instruction
    
    Some CPUs can speculate past an ERET instruction and potentially perform
    speculative accesses to memory before processing the exception return.
    Since the register state is often controlled by lower privilege level
    at the point of an ERET, this could potentially be used as part of a
    side-channel attack.
    
    Newer CPUs may implement a new SB barrier instruction which acts
    as an architected speculation barrier. For current CPUs, the sequence
    DSB; ISB is known to prevent speculation.
    
    The latter sequence is heavier than SB but it would never be executed
    (this is speculation after all!).
    
    Introduce a new macro 'sb' that could be used when a speculation barrier
    is required. For now it is using dsb; isb but this could easily be
    updated to cater SB in the future.
    
    This is XSA-312.
    
    Signed-off-by: Julien Grall <julien@xxxxxxx>
---
 xen/arch/arm/arm32/entry.S   | 1 +
 xen/arch/arm/arm64/entry.S   | 3 +++
 xen/include/asm-arm/macros.h | 9 +++++++++
 3 files changed, 13 insertions(+)

diff --git a/xen/arch/arm/arm32/entry.S b/xen/arch/arm/arm32/entry.S
index 31ccfb2631..b228d44b19 100644
--- a/xen/arch/arm/arm32/entry.S
+++ b/xen/arch/arm/arm32/entry.S
@@ -426,6 +426,7 @@ return_to_hypervisor:
         add sp, #(UREGS_SP_usr - UREGS_sp); /* SP, LR, SPSR, PC */
         clrex
         eret
+        sb
 
 /*
  * struct vcpu *__context_switch(struct vcpu *prev, struct vcpu *next)
diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
index 5b0fe83e4c..97bd06217b 100644
--- a/xen/arch/arm/arm64/entry.S
+++ b/xen/arch/arm/arm64/entry.S
@@ -359,6 +359,7 @@ guest_sync:
          */
         mov     x1, xzr
         eret
+        sb
 
 check_wa2:
         /* ARM_SMCCC_ARCH_WORKAROUND_2 handling */
@@ -398,6 +399,7 @@ wa2_end:
 #endif /* !CONFIG_ARM_SSBD */
         mov     x0, xzr
         eret
+        sb
 guest_sync_slowpath:
         /*
          * x0/x1 may have been scratch by the fast path above, so avoid
@@ -462,6 +464,7 @@ return_from_trap:
         ldr     lr, [sp], #(UREGS_SPSR_el1 - UREGS_LR) /* CPSR, PC, SP, LR */
 
         eret
+        sb
 
 /*
  * Consume pending SError generated by the guest if any.
diff --git a/xen/include/asm-arm/macros.h b/xen/include/asm-arm/macros.h
index 91ea3505e4..4833671f4c 100644
--- a/xen/include/asm-arm/macros.h
+++ b/xen/include/asm-arm/macros.h
@@ -20,4 +20,13 @@
     .endr
     .endm
 
+    /*
+     * Speculative barrier
+     * XXX: Add support for the 'sb' instruction
+     */
+    .macro sb
+    dsb nsh
+    isb
+    .endm
+
 #endif /* __ASM_ARM_MACROS_H */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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