This patch is a simplified fix to address a use-after-free in 4.14.x and
4.19.x stable kernels. The flaw is already fixed upstream, starting in
5.2, by commit 7dc40713 ("drm/i915: Introduce a mutex for
file_priv->context_idr") as part of a more complex patch series that
isn't appropriate for backporting to stable kernels.

Expand mutex coverage, while destroying the GEM context, to include the
GEM context lookup step. This fixes a use-after-free detected by KASAN:

 ==================================================================
 BUG: KASAN: use-after-free in i915_ppgtt_close+0x2ca/0x2f0
 Write of size 1 at addr ffff8881368a8368 by task i915-poc/3124

 CPU: 0 PID: 3124 Comm: i915-poc Not tainted 4.14.164 #1
 Hardware name: HP HP Elite x2 1012 G1 /80FC, BIOS N85 Ver. 01.20 04/05/2017
 Call Trace:
  dump_stack+0xcd/0x12e
  ? _atomic_dec_and_lock+0x1b2/0x1b2
  ? i915_ppgtt_close+0x2ca/0x2f0
  ? printk+0x8f/0xab
  ? show_regs_print_info+0x53/0x53
  ? i915_ppgtt_close+0x2ca/0x2f0
  print_address_description+0x65/0x270
  ? i915_ppgtt_close+0x2ca/0x2f0
  kasan_report+0x251/0x340
  i915_ppgtt_close+0x2ca/0x2f0
  ? __radix_tree_insert+0x3f0/0x3f0
  ? i915_ppgtt_init_hw+0x7c0/0x7c0
  context_close+0x42e/0x680
  ? i915_gem_context_release+0x230/0x230
  ? kasan_kmalloc+0xa0/0xd0
  ? radix_tree_delete_item+0x1d4/0x250
  ? radix_tree_lookup+0x10/0x10
  ? inet_recvmsg+0x4b0/0x4b0
  ? kasan_slab_free+0x88/0xc0
  i915_gem_context_destroy_ioctl+0x236/0x300
  ? i915_gem_context_create_ioctl+0x360/0x360
  ? drm_dev_printk+0x1d0/0x1d0
  ? memcpy+0x34/0x50
  ? i915_gem_context_create_ioctl+0x360/0x360
  drm_ioctl_kernel+0x1b0/0x2b0
  ? drm_ioctl_permit+0x2a0/0x2a0
  ? avc_ss_reset+0xd0/0xd0
  drm_ioctl+0x6fe/0xa20
  ? i915_gem_context_create_ioctl+0x360/0x360
  ? drm_getstats+0x20/0x20
  ? put_unused_fd+0x260/0x260
  do_vfs_ioctl+0x189/0x12d0
  ? ioctl_preallocate+0x280/0x280
  ? selinux_file_ioctl+0x3a7/0x680
  ? selinux_bprm_set_creds+0xe30/0xe30
  ? security_file_ioctl+0x69/0xa0
  ? selinux_bprm_set_creds+0xe30/0xe30
  SyS_ioctl+0x6f/0x80
  ? __sys_sendmmsg+0x4a0/0x4a0
  ? do_vfs_ioctl+0x12d0/0x12d0
  do_syscall_64+0x214/0x5f0
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x25/0x60
  ? __switch_to_asm+0x31/0x60
  ? syscall_return_slowpath+0x2c0/0x2c0
  ? copy_overflow+0x20/0x20
  ? __switch_to_asm+0x25/0x60
  ? syscall_return_via_sysret+0x2a/0x7a
  ? prepare_exit_to_usermode+0x200/0x200
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x25/0x60
  ? __switch_to_asm+0x25/0x60
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x25/0x60
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x31/0x60
  ? __switch_to_asm+0x25/0x60
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
 RIP: 0033:0x7f7fda5115d7
 RSP: 002b:00007f7eec317ec8 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
 RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f7fda5115d7
 RDX: 000055b306db9188 RSI: 000000004008646e RDI: 0000000000000003
 RBP: 00007f7eec317ef0 R08: 00007f7eec318700 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000286 R12: 00007f7eec317fc0
 R13: 0000000000000000 R14: 0000000000000000 R15: 00007ffd8007ade0

 Allocated by task 2898:
  save_stack+0x32/0xb0
  kasan_kmalloc+0xa0/0xd0
  kmem_cache_alloc_trace+0x5e/0x180
  i915_ppgtt_create+0xab/0x2510
  i915_gem_create_context+0x981/0xf90
  i915_gem_context_create_ioctl+0x1d7/0x360
  drm_ioctl_kernel+0x1b0/0x2b0
  drm_ioctl+0x6fe/0xa20
  do_vfs_ioctl+0x189/0x12d0
  SyS_ioctl+0x6f/0x80
  do_syscall_64+0x214/0x5f0
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

 Freed by task 104:
  save_stack+0x32/0xb0
  kasan_slab_free+0x72/0xc0
  kfree+0x88/0x190
  i915_ppgtt_release+0x24e/0x460
  i915_gem_context_free+0x90/0x480
  contexts_free_worker+0x54/0x80
  process_one_work+0x876/0x14e0
  worker_thread+0x1b8/0xfd0
  kthread+0x2f8/0x3c0
  ret_from_fork+0x35/0x40

 The buggy address belongs to the object at ffff8881368a8000
  which belongs to the cache kmalloc-8192 of size 8192
 The buggy address is located 872 bytes inside of
  8192-byte region [ffff8881368a8000, ffff8881368aa000)
 The buggy address belongs to the page:
 page:ffffea0004da2a00 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
 flags: 0x200000000008100(slab|head)
 raw: 0200000000008100 0000000000000000 0000000000000000 0000000100030003
 raw: dead000000000100 dead000000000200 ffff88822a002280 0000000000000000
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffff8881368a8200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff8881368a8280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 >ffff8881368a8300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                           ^
  ffff8881368a8380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff8881368a8400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ==================================================================

Fixes: 1acfc104 ("drm/i915: Enable rcu-only context lookups")
Reported-by: 罗权 <luoquan@qianxin.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: stable@vger.kernel.org # 4.14.x
Cc: stable@vger.kernel.org # 4.19.x
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>