[PATCH v2] drm/gpuvm: skip setting kernel_alloc_node range on overflow

[Alice Ryhl <aliceryhl@google.com>]

When the reserved VA range overflows in drm_gpuvm_init(), insertion of
kernel_alloc_node is skipped, leaving its vm pointer NULL. However,
va.range is still set to a non-zero value, which causes drm_gpuvm_fini()
to dereference the NULL vm pointer when removing the node. Thus, only
initialize kernel_alloc_node's address and range when the overflow check
succeeds.

Note that this fixes unsoundness in the Rust GpuVm::new() abstraction:
passing a backwards reserve_range (start > end) when overflow checks are
disabled causes `end - start` to wrap around, triggering the overflow in
drm_gpuvm_init() and dereferencing NULL when the GpuVm is dropped.

Cc: stable@vger.kernel.org
Fixes: e6303f323b1a ("drm: manager to keep track of GPUs VA mappings")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Collapse the two if statements into one.
- Add Cc: stable.
- Link to v1: https://lore.kernel.org/r/20261011-gpuvm-reserve-overflow-v1-1-9c0cb23d7409@google.com
---
 drivers/gpu/drm/drm_gpuvm.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
index d1c80ad3dead..8e74d171125b 100644
--- a/drivers/gpu/drm/drm_gpuvm.c
+++ b/drivers/gpu/drm/drm_gpuvm.c
@@ -1126,13 +1126,12 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const char *name,
 	gpuvm->mm_range = range;
 
 	memset(&gpuvm->kernel_alloc_node, 0, sizeof(struct drm_gpuva));
-	if (reserve_range) {
+	if (reserve_range &&
+	    likely(!drm_gpuvm_warn_check_overflow(gpuvm, reserve_offset,
+						  reserve_range))) {
 		gpuvm->kernel_alloc_node.va.addr = reserve_offset;
 		gpuvm->kernel_alloc_node.va.range = reserve_range;
-
-		if (likely(!drm_gpuvm_warn_check_overflow(gpuvm, reserve_offset,
-							  reserve_range)))
-			__drm_gpuva_insert(gpuvm, &gpuvm->kernel_alloc_node);
+		__drm_gpuva_insert(gpuvm, &gpuvm->kernel_alloc_node);
 	}
 }
 EXPORT_SYMBOL_GPL(drm_gpuvm_init);

---
base-commit: a90ee4305c4a5df72c11b31dacfdc76e00fcf78a
change-id: 20261011-gpuvm-reserve-overflow-5b520d76889e

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>