[Xen-devel] [PATCH] xen/arm: traps: Avoid using BUG_ON() to check guest state in advance_pc()

[Julien Grall <julien.grall@xxxxxxx>]

The condition of the BUG_ON() in advance_pc() is pretty wrong because
the bits [26:25] and [15:10] have a different meaning between AArch32
and AArch64 state.

On AArch32, they are used to store PSTATE.IT. On AArch64, they are RES0
or used for new feature (e.g ARMv8.0-SSBS, ARMv8.5-BTI).

This means a 64-bit guest will hit the BUG_ON() if it is trying to use
any of these features.

More generally, RES0 means that the bits is reserved for future use. So
crashing the host is definitely not the right solution.

In this particular case, we only need to know the guest was using 32-bit
Mode and the Thumb instructions. So replace the BUG_ON() by a proper
check.

Reported-by: Lukas Jünger <lukas.juenger@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    This patch needs to be backported as far as possible. Otherwise Xen
    would not be able to run on processor implementing ARMv8.0-SSBS,
    ARMv8.5-BTI or ARMv8.5-MemTag. The former is actually the most
    critical as this is used for controlling mitagion for SSBD (aka
    Spectre v4) in hardware.
---
 xen/arch/arm/traps.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index d8b9a8a0f0..798a3a45a4 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1650,12 +1650,9 @@ int check_conditional_instr(struct cpu_user_regs *regs, 
const union hsr hsr)
 void advance_pc(struct cpu_user_regs *regs, const union hsr hsr)
 {
     unsigned long itbits, cond, cpsr = regs->cpsr;
+    bool is_thumb = psr_mode_is_32bit(cpsr) && (cpsr & PSR_THUMB);
 
-    /* PSR_IT_MASK bits can only be set for 32-bit processors in Thumb mode. */
-    BUG_ON( (!psr_mode_is_32bit(cpsr)||!(cpsr&PSR_THUMB))
-            && (cpsr&PSR_IT_MASK) );
-
-    if ( cpsr&PSR_IT_MASK )
+    if ( is_thumb && (cpsr & PSR_IT_MASK) )
     {
         /* The ITSTATE[7:0] block is contained in CPSR[15:10],CPSR[26:25]
          *
-- 
2.11.0


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