drm/i915: Protect context while grabbing its name for the request

Inside print_request(), we query the context/timeline name. Nothing
immediately protects the context from being freed if the request is
complete -- we rely on serialisation by the caller to keep the name
valid until they finish using it. Inside intel_engine_dump(), we
generally only print the requests in the execution queue protected by the
engine->active.lock, but we also show the pending execlists ports which
are not protected and so require a rcu_read_lock to keep the pointer
valid.

[ 1695.700883] BUG: KASAN: use-after-free in i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.700981] Read of size 8 at addr ffff8887344f4d50 by task gem_ctx_persist/2968
[ 1695.701068]
[ 1695.701156] CPU: 1 PID: 2968 Comm: gem_ctx_persist Tainted: G     U            5.4.0-rc6+ #331
[ 1695.701246] Hardware name: Intel Corporation NUC7i5BNK/NUC7i5BNB, BIOS BNKBL357.86A.0052.2017.0918.1346 09/18/2017
[ 1695.701334] Call Trace:
[ 1695.701424]  dump_stack+0x5b/0x90
[ 1695.701870]  ? i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.701964]  print_address_description.constprop.7+0x36/0x50
[ 1695.702408]  ? i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.702856]  ? i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.702947]  __kasan_report.cold.10+0x1a/0x3a
[ 1695.703390]  ? i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.703836]  i915_fence_get_timeline_name+0x53/0x90 [i915]
[ 1695.704241]  print_request+0x82/0x2e0 [i915]
[ 1695.704638]  ? fwtable_read32+0x133/0x360 [i915]
[ 1695.705042]  ? write_timestamp+0x110/0x110 [i915]
[ 1695.705133]  ? _raw_spin_lock_irqsave+0x79/0xc0
[ 1695.705221]  ? refcount_inc_not_zero_checked+0x91/0x110
[ 1695.705306]  ? refcount_dec_and_mutex_lock+0x50/0x50
[ 1695.705709]  ? intel_engine_find_active_request+0x202/0x230 [i915]
[ 1695.706115]  intel_engine_dump+0x2c9/0x900 [i915]

Fixes: c36eebd9 ("drm/i915/gt: execlists->active is serialised by the tasklet")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111114323.5833-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -1373,6 +1373,7 @@ static void intel_engine_print_registers(struct intel_engine_cs *engine,
 		}
 
 		execlists_active_lock_bh(execlists);
+		rcu_read_lock();
 		for (port = execlists->active; (rq = *port); port++) {
 			char hdr[80];
 			int len;
@@ -1410,6 +1411,7 @@ static void intel_engine_print_registers(struct intel_engine_cs *engine,
 			if (tl)
 				intel_timeline_put(tl);
 		}
+		rcu_read_unlock();
 		execlists_active_unlock_bh(execlists);
 	} else if (INTEL_GEN(dev_priv) > 6) {
 		drm_printf(m, "\tPP_DIR_BASE: 0x%08x\n",