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Re: [Xen-devel] [PATCH v3 4/4] xen/arm: Don't crash the domain on invalid HVC immediate

On 02/02/18 14:37, Andre Przywara wrote:


On 02/02/18 10:14, Julien Grall wrote:
domain_crash_synchronous() should only be used when something went wrong
in Xen. It is better to inject to the guest as it will be in a better
position to provide helpful information (stack trace...).

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


     We potentially want to return -1 instead. This would make Xen more
     future-proof if we decide to implement the other HVC immediate.

     Changes in v2:
         - Add Stefano's reviewed-by
  xen/arch/arm/traps.c | 13 ++++++++-----
  1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 1e85f99ec1..1cba7e584d 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1471,14 +1471,17 @@ static void do_debug_trap(struct cpu_user_regs *regs, 
unsigned int code)
static void do_trap_hypercall(struct cpu_user_regs *regs, register_t *nr,
-                              unsigned long iss)
+                              const union hsr hsr)
      arm_hypercall_fn_t call = NULL;
BUILD_BUG_ON(NR_hypercalls < ARRAY_SIZE(arm_hypercall_table) ); - if ( iss != XEN_HYPERCALL_TAG )
-        domain_crash_synchronous();
+    if ( hsr.iss != XEN_HYPERCALL_TAG )
+    {
+        gprintk(XENLOG_WARNING, "Invalid HVC imm 0x%x\n", hsr.iss);
+        return inject_undef_exception(regs, hsr);

Why is this UNDEF?
UNDEF is the exception used when either HVC is disabled or if it's
called from EL0.
I couldn't find anything that talks about how non-implemented immediates
should be handled, I guess they would just be ignored?
Do you have any sources that recommend UNDEF?

Per the SMCCC, all nonzero immediate for HVC are designated for use by the hypervisors. So I take as we can implement the way we want.

We have few solutions here:
1) Ignore. This means that a guest may wrongly call an HVC and will never notice it until implemented. This would lead to some trouble with introduce new ABI. 2) Return a value. We could return -1 (or -ENOSYS). But that would need to be defined.
        3) Undefined. The guest should not be allow to touch it, so it makes 

1# is not a solution because catching them wrong usage on old Xen would be a nightmare.

2# might be doable, thought I am not sure we want to commit on a return value now. But old Xen would see crash the domain.

So I choose #3 which still crash the domain but by injecting an undefined.


Julien Grall

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