arm64: sdei: move uaccess logic to arch/arm64/

The SDEI support code is split across arch/arm64/ and drivers/firmware/,
largley this is split so that the arch-specific portions are under
arch/arm64, and the management logic is under drivers/firmware/.
However, exception entry fixups are currently under drivers/firmware.

Let's move the exception entry fixups under arch/arm64/. This
de-clutters the management logic, and puts all the arch-specific
portions in one place. Doing this also allows the fixups to be applied
earlier, so things like PAN and UAO will be in a known good state before
we run other logic. This will also make subsequent refactoring easier.

Signed-off-by: Mark Rutland <[e-mail]>
Reviewed-by: James Morse <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/kernel/sdei.c

···
 		sdei_api_event_context(i, &regs->regs[i]);
 	}
 
-	/*
-	 * We didn't take an exception to get here, set PAN. UAO will be cleared
-	 * by sdei_event_handler()s force_uaccess_begin() call.
-	 */
-	__uaccess_enable_hw_pan();
-
 	err = sdei_event_handler(regs, arg);
 	if (err)
 		return SDEI_EV_FAILED;
···
 __sdei_handler(struct pt_regs *regs, struct sdei_registered_event *arg)
 {
 	unsigned long ret;
+	mm_segment_t orig_addr_limit;
+
+	/*
+	 * We didn't take an exception to get here, so the HW hasn't set PAN or
+	 * cleared UAO, and the exception entry code hasn't reset addr_limit.
+	 * Set PAN, then use force_uaccess_begin() to clear UAO and reset
+	 * addr_limit.
+	 */
+	__uaccess_enable_hw_pan();
+	orig_addr_limit = force_uaccess_begin();
 
 	nmi_enter();
 
 	ret = _sdei_handler(regs, arg);
 
 	nmi_exit();
+
+	force_uaccess_end(orig_addr_limit);
 
 	return ret;
 }

drivers/firmware/arm_sdei.c

···
 #include <linux/slab.h>
 #include <linux/smp.h>
 #include <linux/spinlock.h>
-#include <linux/uaccess.h>
 
 /*
  * The call to use to reach the firmware.
···
 		       struct sdei_registered_event *arg)
 {
 	int err;
-	mm_segment_t orig_addr_limit;
 	u32 event_num = arg->event_num;
-
-	/*
-	 * Save restore 'fs'.
-	 * The architecture's entry code save/restores 'fs' when taking an
-	 * exception from the kernel. This ensures addr_limit isn't inherited
-	 * if you interrupted something that allowed the uaccess routines to
-	 * access kernel memory.
-	 * Do the same here because this doesn't come via the same entry code.
-	*/
-	orig_addr_limit = force_uaccess_begin();
 
 	err = arg->callback(event_num, regs, arg->callback_arg);
 	if (err)
 		pr_err_ratelimited("event %u on CPU %u failed with error: %d\n",
 				   event_num, smp_processor_id(), err);
-
-	force_uaccess_end(orig_addr_limit);
 
 	return err;
 }