From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
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@xxxxxxxxx>
Fixes: 96601e8a4755 ("dma-buf: use new iterator in dma_resv_copy_fences")
Fixes: d3c80698c9f5 ("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@xxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
---
drivers/dma-buf/dma-resv.c | 18 ++++++++++--------
include/linux/dma-resv.h | 5 ++++-
2 files changed, 14 insertions(+), 9 deletions(-)
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index a480af9581bd..7b6d881c8904 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/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;
@@ -448,10 +452,8 @@ 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;
@@ -522,7 +524,7 @@ 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 */
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 8b6c20636a79..3e1bff147428 100644
--- a/include/linux/dma-resv.h
+++ b/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;
};
--
2.30.2
