drm/msm: Fix pgtable prealloc error path

The following splat was reported:

    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
    Mem abort info:
      ESR = 0x0000000096000004
      EC = 0x25: DABT (current EL), IL = 32 bits
      SET = 0, FnV = 0
      EA = 0, S1PTW = 0
      FSC = 0x04: level 0 translation fault
    Data abort info:
      ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
      CM = 0, WnR = 0, TnD = 0, TagAccess = 0
      GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
    user pgtable: 4k pages, 48-bit VAs, pgdp=00000008d0fd8000
    [0000000000000010] pgd=0000000000000000, p4d=0000000000000000
    Internal error: Oops: 0000000096000004 [#1]  SMP
    CPU: 5 UID: 1000 PID: 149076 Comm: Xwayland Tainted: G S                  6.16.0-rc2-00809-g0b6974bb4134-dirty #367 PREEMPT
    Tainted: [S]=CPU_OUT_OF_SPEC
    Hardware name: Qualcomm Technologies, Inc. SM8650 HDK (DT)
    pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
    pc : build_detached_freelist+0x28/0x224
    lr : kmem_cache_free_bulk.part.0+0x38/0x244
    sp : ffff000a508c7a20
    x29: ffff000a508c7a20 x28: ffff000a508c7d50 x27: ffffc4e49d16f350
    x26: 0000000000000058 x25: 00000000fffffffc x24: 0000000000000000
    x23: ffff00098c4e1450 x22: 00000000fffffffc x21: 0000000000000000
    x20: ffff000a508c7af8 x19: 0000000000000002 x18: 00000000000003e8
    x17: ffff000809523850 x16: ffff000809523820 x15: 0000000000401640
    x14: ffff000809371140 x13: 0000000000000130 x12: ffff0008b5711e30
    x11: 00000000001058fa x10: 0000000000000a80 x9 : ffff000a508c7940
    x8 : ffff000809371ba0 x7 : 781fffe033087fff x6 : 0000000000000000
    x5 : ffff0008003cd000 x4 : 781fffe033083fff x3 : ffff000a508c7af8
    x2 : fffffdffc0000000 x1 : 0001000000000000 x0 : ffff0008001a6a00
    Call trace:
     build_detached_freelist+0x28/0x224 (P)
     kmem_cache_free_bulk.part.0+0x38/0x244
     kmem_cache_free_bulk+0x10/0x1c
     msm_iommu_pagetable_prealloc_cleanup+0x3c/0xd0
     msm_vma_job_free+0x30/0x240
     msm_ioctl_vm_bind+0x1d0/0x9a0
     drm_ioctl_kernel+0x84/0x104
     drm_ioctl+0x358/0x4d4
     __arm64_sys_ioctl+0x8c/0xe0
     invoke_syscall+0x44/0x100
     el0_svc_common.constprop.0+0x3c/0xe0
     do_el0_svc+0x18/0x20
     el0_svc+0x30/0x100
     el0t_64_sync_handler+0x104/0x130
     el0t_64_sync+0x170/0x174
    Code: aa0203f5 b26287e2 f2dfbfe2 aa0303f4 (f8737ab6)
    ---[ end trace 0000000000000000 ]---

Since msm_vma_job_free() is called directly from the ioctl, this looks
like an error path cleanup issue.  Which I think results from
prealloc_cleanup() called without a preceding successful
prealloc_allocate() call.  So handle that case better.

Reported-by: Connor Abbott <cwabbott0@gmail.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/678677/
Message-ID: <20251006153542.419998-1-robin.clark@oss.qualcomm.com>

Author: Rob Clark

drivers/gpu/drm/msm/msm_iommu.c

@@ -338,6 +338,8 @@ msm_iommu_pagetable_prealloc_allocate(struct msm_mmu *mmu, struct msm_mmu_preall
 
 	ret = kmem_cache_alloc_bulk(pt_cache, GFP_KERNEL, p->count, p->pages);
 	if (ret != p->count) {
+		kfree(p->pages);
+		p->pages = NULL;
 		p->count = ret;
 		return -ENOMEM;
 	}
@@ -351,6 +353,9 @@ msm_iommu_pagetable_prealloc_cleanup(struct msm_mmu *mmu, struct msm_mmu_preallo
 	struct kmem_cache *pt_cache = get_pt_cache(mmu);
 	uint32_t remaining_pt_count = p->count - p->ptr;
 
+	if (!p->pages)
+		return;
+
 	if (p->count > 0)
 		trace_msm_mmu_prealloc_cleanup(p->count, remaining_pt_count);