drm/nouveau: fix double-free in nvif_vmm_dtor

In the Linux kernel, the following vulnerability has been resolved:

drm/nouveau: fix double-free in nvif_vmm_dtor

On failure, nouveau_cli_init() calls nouveau_cli_fini() to tear
the client down. Then, nouveau_drm_open() also enters into its
cleanup path and calls nouveau_cli_fini() AGAIN. nouveau_cli_fini()
calls nouveau_vmm_fini():

void
nouveau_vmm_fini(struct nouveau_vmm *vmm)
{
nouveau_svmm_fini(&vmm->svmm);
nvif_vmm_dtor(&vmm->vmm);
vmm->cli = NULL;
}

Inside nvif_vmm_dtor(), vmm->page is freed unconditionally:

void
nvif_vmm_dtor(struct nvif_vmm *vmm)
{
kfree(vmm->page);
nvif_object_dtor(&vmm->object);
}

vmm->page is never cleared after being freed, so the second call of
nvif_vmm_dtor() will cause a double-free.

Found by fuzzing the nouveau driver with a modified Syzkaller:

BUG: KASAN: double-free in nvif_vmm_dtor+0x31/0x50 drivers/gpu/drm/nouveau/nvif/vmm.c:194
Free of addr ffff888010fcdc30 by task syz.0.173/2567

CPU: 1 UID: 0 PID: 2567 Comm: syz.0.173 Not tainted 7.2.0 #24 PREEMPT(lazy)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x95/0xe0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xcb/0x5a0 mm/kasan/report.c:482
kasan_report_invalid_free+0xaa/0xd0 mm/kasan/report.c:557
check_slab_allocation+0xe4/0x110 mm/kasan/common.c:235
kasan_slab_pre_free include/linux/kasan.h:199 [inline]
slab_free_hook mm/slub.c:2622 [inline]
slab_free mm/slub.c:6377 [inline]
kfree+0x192/0x590 mm/slub.c:6692
nvif_vmm_dtor+0x31/0x50 drivers/gpu/drm/nouveau/nvif/vmm.c:194
nouveau_vmm_fini+0x16/0x50 drivers/gpu/drm/nouveau/nouveau_vmm.c:127
nouveau_cli_fini+0x10e/0x210 drivers/gpu/drm/nouveau/nouveau_drm.c:225
nouveau_drm_open+0x24e/0x740 drivers/gpu/drm/nouveau/nouveau_drm.c:1255
drm_file_alloc+0x5f2/0xad0 drivers/gpu/drm/drm_file.c:176
drm_open_helper+0x1d7/0x4a0 drivers/gpu/drm/drm_file.c:335
drm_open+0x190/0x3d0 drivers/gpu/drm/drm_file.c:388
drm_stub_open+0x1f2/0x460 drivers/gpu/drm/drm_drv.c:1211
chrdev_open+0x21c/0x660 fs/char_dev.c:411
do_dentry_open+0x59d/0x12b0 fs/open.c:947
vfs_open+0x82/0x390 fs/open.c:1052
do_open fs/namei.c:4700 [inline]
path_openat+0x2345/0x3420 fs/namei.c:4863
do_file_open+0x207/0x460 fs/namei.c:4892
do_sys_openat2+0xd1/0x1d0 fs/open.c:1368
do_sys_open fs/open.c:1374 [inline]
__do_sys_openat fs/open.c:1390 [inline]
__se_sys_openat fs/open.c:1385 [inline]
__x64_sys_openat+0x144/0x200 fs/open.c:1385
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x690 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc6d687594d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fc6d5295008 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007fc6d6b06180 RCX: 00007fc6d687594d
RDX: 0000000000022501 RSI: 0000200000000000 RDI: ffffffffffffff9c
RBP: 00007fc6d691c303 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc6d6b06218 R14: 00007fc6d6b06180 R15: 00007ffd9451d760
</TASK>

Allocated by task 2567 on cpu 1 at 163.593900s:
kasan_save_stack+0x24/0x50 mm/kasan/common.c:57
kasan_save_track+0x17/0x60 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__do_kmalloc_node mm/slub.c:5334 [inline]
__kmalloc_noprof+0x304/0x7c0 mm/slub.c:5359
_kmalloc_noprof include/linux/slab.h:992 [inline]
nvif_vmm_ctor+0x3c0/0x7e0 drivers/gpu/drm/nouveau/nvif/vmm.c:237
nouveau_vmm_init+0x40/0x90 drivers/gpu/drm/nouveau/nouveau_vmm.c:134
nouveau_cli_init+0x7b9/0
---truncated---