drm/amdkfd: Fix use-after-free of HMM range in svm_range_validate_and_map()

The function svm_range_validate_and_map() was freeing `range` when
amdgpu_hmm_range_get_pages() failed. But later, the code still used the
same `range` pointer and freed it again. This could cause a
use-after-free and double-free issue.

The fix sets `range = NULL` right after it is freed and checks for
`range` before using or freeing it again.

v2: Removed duplicate !r check in the condition for clarity.

v3: In amdgpu_hmm_range_get_pages(), when hmm_range_fault() fails, we
kvfree(pfns) but leave the pointer in hmm_range->hmm_pfns still pointing
to freed memory. The caller (or amdgpu_hmm_range_free(range)) may try to
free range->hmm_range.hmm_pfns again, causing a double free, Setting
hmm_range->hmm_pfns = NULL immediately after kvfree(pfns) prevents both
double free. (Philip)

In svm_range_validate_and_map(), When r == 0, it means success → range
is not NULL. When r != 0, it means failure → already made range = NULL.
So checking both (!r && range) is unnecessary because the moment r == 0,
we automatically know range exists and is safe to use. (Philip)

Fixes: 737da5363cc0 ("drm/amdgpu: update the functions to use amdgpu version of hmm")
Reported by: Dan Carpenter <[e-mail]>
Cc: Philip Yang <[e-mail]>
Cc: Sunil Khatri <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Srinivasan Shanmugam <[e-mail]>
Reviewed-by: Philip Yang<[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c

···
 
 out_free_pfns:
 	kvfree(pfns);
+	hmm_range->hmm_pfns = NULL;
 out_free_range:
 	if (r == -EBUSY)
 		r = -EAGAIN;

drivers/gpu/drm/amd/amdkfd/kfd_svm.c

···
 			WRITE_ONCE(p->svms.faulting_task, NULL);
 			if (r) {
 				amdgpu_hmm_range_free(range);
+				range = NULL;
 				pr_debug("failed %d to get svm range pages\n", r);
 			}
 		} else {
···
 		svm_range_lock(prange);
 
 		/* Free backing memory of hmm_range if it was initialized
-		 * Overrride return value to TRY AGAIN only if prior returns
+		 * Override return value to TRY AGAIN only if prior returns
 		 * were successful
 		 */
 		if (range && !amdgpu_hmm_range_valid(range) && !r) {
···
 			r = -EAGAIN;
 		}
 		/* Free the hmm range */
-		amdgpu_hmm_range_free(range);
+		if (range)
+			amdgpu_hmm_range_free(range);
 
 
 		if (!r && !list_empty(&prange->child_list)) {