arm/xen: Enable user access to the kernel before issuing a privcmd call

When Xen is copying data to/from the guest it will check if the kernel
has the right to do the access. If not, the hypercall will return an
error.

After the commit a5e090acbf545c0a3b04080f8a488b17ec41fe02 "ARM:
software-based privileged-no-access support", the kernel can't access
any longer the user space by default. This will result to fail on every
hypercall made by the userspace (i.e via privcmd).

We have to enable the userspace access and then restore the correct
permission every time the privcmd is used to made an hypercall.

I didn't find generic helpers to do a these operations, so the change
is only arm32 specific.

Reported-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 15 insertions, 0 deletions

diff --git a/arch/arm/xen/hypercall.S b/arch/arm/xen/hypercall.S
index f00e08075938..10fd99c568c6 100644
--- a/arch/arm/xen/hypercall.S
+++ b/arch/arm/xen/hypercall.S
@@ -98,8 +98,23 @@ ENTRY(privcmd_call)
         mov r1, r2
         mov r2, r3
         ldr r3, [sp, #8]
+        /*
+         * Privcmd calls are issued by the userspace. We need to allow the
+         * kernel to access the userspace memory before issuing the hypercall.
+         */
+        uaccess_enable r4
+
+        /* r4 is loaded now as we use it as scratch register before */
         ldr r4, [sp, #4]
         __HVC(XEN_IMM)
+
+        /*
+         * Disable userspace access from kernel. This is fine to do it
+         * unconditionally as no set_fs(KERNEL_DS)/set_fs(get_ds()) is
+         * called before.
+         */
+        uaccess_disable r4
+
         ldm sp!, {r4}
         ret lr
 ENDPROC(privcmd_call);