dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion

Cache the count of shared fences in the iterator to avoid dereferencing
the dma_resv_object outside the RCU protection. Otherwise iterator and its
users can observe an incosistent state which makes it impossible to use
safely. Such as:

<6> [187.517041] [IGT] gem_sync: executing
<7> [187.536343] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl [i915]] HW context 1 created
<7> [187.536793] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl [i915]] HW context 1 created
<6> [187.551235] [IGT] gem_sync: starting subtest basic-many-each
<1> [188.935462] BUG: kernel NULL pointer dereference, address: 0000000000000010
<1> [188.935485] #PF: supervisor write access in kernel mode
<1> [188.935495] #PF: error_code(0x0002) - not-present page
<6> [188.935504] PGD 0 P4D 0
<4> [188.935512] Oops: 0002 [#1] PREEMPT SMP NOPTI
<4> [188.935521] CPU: 2 PID: 1467 Comm: gem_sync Not tainted 5.15.0-rc4-CI-Patchwork_21264+ #1
<4> [188.935535] Hardware name:  /NUC6CAYB, BIOS AYAPLCEL.86A.0049.2018.0508.1356 05/08/2018
<4> [188.935546] RIP: 0010:dma_resv_get_fences+0x116/0x2d0
<4> [188.935560] Code: 10 85 c0 7f c9 be 03 00 00 00 e8 15 8b df ff eb bd e8 8e c6 ff ff eb b6 41 8b 04 24 49 8b 55 00 48 89 e7 8d 48 01 41 89 0c 24 <4c> 89 34 c2 e8 41 f2 ff ff 49 89 c6 48 85 c0 75 8c 48 8b 44 24 10
<4> [188.935583] RSP: 0018:ffffc900011dbcc8 EFLAGS: 00010202
<4> [188.935593] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 0000000000000001
<4> [188.935603] RDX: 0000000000000010 RSI: ffffffff822e343c RDI: ffffc900011dbcc8
<4> [188.935613] RBP: ffffc900011dbd48 R08: ffff88812d255bb8 R09: 00000000fffffffe
<4> [188.935623] R10: 0000000000000001 R11: 0000000000000000 R12: ffffc900011dbd44
<4> [188.935633] R13: ffffc900011dbd50 R14: ffff888113d29cc0 R15: 0000000000000000
<4> [188.935643] FS:  00007f68d17e9700(0000) GS:ffff888277900000(0000) knlGS:0000000000000000
<4> [188.935655] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [188.935665] CR2: 0000000000000010 CR3: 000000012d0a4000 CR4: 00000000003506e0
<4> [188.935676] Call Trace:
<4> [188.935685]  i915_gem_object_wait+0x1ff/0x410 [i915]
<4> [188.935988]  i915_gem_wait_ioctl+0xf2/0x2a0 [i915]
<4> [188.936272]  ? i915_gem_object_wait+0x410/0x410 [i915]
<4> [188.936533]  drm_ioctl_kernel+0xae/0x140
<4> [188.936546]  drm_ioctl+0x201/0x3d0
<4> [188.936555]  ? i915_gem_object_wait+0x410/0x410 [i915]
<4> [188.936820]  ? __fget_files+0xc2/0x1c0
<4> [188.936830]  ? __fget_files+0xda/0x1c0
<4> [188.936839]  __x64_sys_ioctl+0x6d/0xa0
<4> [188.936848]  do_syscall_64+0x3a/0xb0
<4> [188.936859]  entry_SYSCALL_64_after_hwframe+0x44/0xae

If the shared object has changed during the RCU unlocked period
callers will correctly handle the restart on the next iteration.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 96601e8a ("dma-buf: use new iterator in dma_resv_copy_fences")
Fixes: d3c80698 ("dma-buf: use new iterator in dma_resv_get_fences v3")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4274
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20211008095007.972693-1-tvrtko.ursulin@linux.intel.comReviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/dma-buf/dma-resv.c

@@ -333,10 +333,14 @@ static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
 {
 	cursor->seq = read_seqcount_begin(&cursor->obj->seq);
 	cursor->index = -1;
-	if (cursor->all_fences)
+	cursor->shared_count = 0;
+	if (cursor->all_fences) {
 		cursor->fences = dma_resv_shared_list(cursor->obj);
-	else
+		if (cursor->fences)
+			cursor->shared_count = cursor->fences->shared_count;
+	} else {
 		cursor->fences = NULL;
+	}
 	cursor->is_restarted = true;
 }
 
@@ -363,7 +367,7 @@ static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
 				continue;
 
 		} else if (!cursor->fences ||
-			   cursor->index >= cursor->fences->shared_count) {
+			   cursor->index >= cursor->shared_count) {
 			cursor->fence = NULL;
 			break;
 
@@ -499,10 +503,8 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
 			dma_resv_list_free(list);
 			dma_fence_put(excl);
 
-			if (cursor.fences) {
-				unsigned int cnt = cursor.fences->shared_count;
-
-				list = dma_resv_list_alloc(cnt);
+			if (cursor.shared_count) {
+				list = dma_resv_list_alloc(cursor.shared_count);
 				if (!list) {
 					dma_resv_iter_end(&cursor);
 					return -ENOMEM;
@@ -573,7 +575,7 @@ int dma_resv_get_fences(struct dma_resv *obj, struct dma_fence **fence_excl,
 			if (fence_excl)
 				dma_fence_put(*fence_excl);
 
-			count = cursor.fences ? cursor.fences->shared_count : 0;
+			count = cursor.shared_count;
 			count += fence_excl ? 0 : 1;
 
 			/* Eventually re-allocate the array */

include/linux/dma-resv.h

@@ -170,9 +170,12 @@ struct dma_resv_iter {
 	/** @index: index into the shared fences */
 	unsigned int index;
 
-	/** @fences: the shared fences */
+	/** @fences: the shared fences; private, *MUST* not dereference  */
 	struct dma_resv_list *fences;
 
+	/** @shared_count: number of shared fences */
+	unsigned int shared_count;
+
 	/** @is_restarted: true if this is the first returned fence */
 	bool is_restarted;
 };