drm/i915: Avoid dereference past end of page array in gen6_ppgtt_insert_entries()

[   89.237347] BUG: unable to handle kernel paging request at ffff880096326000
[   89.237369] IP: [<ffffffff81347227>] gen6_ppgtt_insert_entries+0x117/0x170
[   89.237382] PGD 2272067 PUD 25df0e067 PMD 25de5c067 PTE 8000000096326060
[   89.237394] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[   89.237404] CPU: 1 PID: 1981 Comm: gem_concurrent_ Not tainted 3.13.0-rc4+ #639
[   89.237411] Hardware name: Intel Corporation 2012 Client Platform/Emerald Lake 2, BIOS ACRVMBY1.86C.0078.P00.1201161002 01/16/2012
[   89.237420] task: ffff88024c038030 ti: ffff88024b130000 task.ti: ffff88024b130000
[   89.237425] RIP: 0010:[<ffffffff81347227>]  [<ffffffff81347227>] gen6_ppgtt_insert_entries+0x117/0x170
[   89.237435] RSP: 0018:ffff88024b131ae0  EFLAGS: 00010286
[   89.237440] RAX: ffff880096325000 RBX: 0000000000000400 RCX: 0000000000001000
[   89.237445] RDX: 0000000000000200 RSI: 0000000000000001 RDI: 0000000000000010
[   89.237451] RBP: ffff88024b131b30 R08: ffff88024cc3aef0 R09: 0000000000000000
[   89.237456] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88024cc3ae00
[   89.237462] R13: ffff88024a578000 R14: 0000000000000001 R15: ffff88024a578ffc
[   89.237469] FS:  00007ff5475d8900(0000) GS:ffff88025d020000(0000) knlGS:0000000000000000
[   89.237475] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   89.237480] CR2: ffff880096326000 CR3: 000000024d531000 CR4: 00000000001407e0
[   89.237485] Stack:
[   89.237488]  ffff880000000000 0000020000000000 ffff88024b23f2c0 0000000100000000
[   89.237499]  0000000000000001 000000000007ffff ffff8801e7bf5ac0 ffff8801e7bf5ac0
[   89.237510]  ffff88024cc3ae00 ffff880248a2ee40 ffff88024b131b58 ffffffff813455ed
[   89.237521] Call Trace:
[   89.237528]  [<ffffffff813455ed>] ppgtt_bind_vma+0x3d/0x60
[   89.237534]  [<ffffffff8133d8dc>] i915_gem_object_pin+0x55c/0x6a0
[   89.237541]  [<ffffffff8134275b>] i915_gem_execbuffer_reserve_vma.isra.14+0x5b/0x110
[   89.237548]  [<ffffffff81342a88>] i915_gem_execbuffer_reserve+0x278/0x2c0
[   89.237555]  [<ffffffff81343d29>] i915_gem_do_execbuffer.isra.22+0x699/0x1250
[   89.237562]  [<ffffffff81344d91>] ? i915_gem_execbuffer2+0x51/0x290
[   89.237569]  [<ffffffff81344de6>] i915_gem_execbuffer2+0xa6/0x290
[   89.237575]  [<ffffffff813014f2>] drm_ioctl+0x4d2/0x610
[   89.237582]  [<ffffffff81080bf1>] ? cpuacct_account_field+0xa1/0xc0
[   89.237588]  [<ffffffff81080b55>] ? cpuacct_account_field+0x5/0xc0
[   89.237597]  [<ffffffff811371c0>] do_vfs_ioctl+0x300/0x520
[   89.237603]  [<ffffffff810757a1>] ? vtime_account_user+0x91/0xa0
[   89.237610]  [<ffffffff810e40eb>] ?  context_tracking_user_exit+0x9b/0xe0
[   89.237617]  [<ffffffff81083d7d>] ? trace_hardirqs_on+0xd/0x10
[   89.237623]  [<ffffffff81137425>] SyS_ioctl+0x45/0x80
[   89.237630]  [<ffffffff815afffa>] tracesys+0xd4/0xd9
[   89.237634] Code: 5b 41 5c 41 5d 41 5e 41 5f 5d c3 66 0f 1f 84 00 00 00 00 00 83 45 bc 01 49 8b 84 24 78 01 00 00 65 ff 0c 25 e0 b8 00 00 8b 55 bc <4c> 8b 2c d0 65 ff 04 25 e0 b8 00 00 49 8b 45 00 48 c1 e8 2d 48
[   89.237741] RIP  [<ffffffff81347227>] gen6_ppgtt_insert_entries+0x117/0x170
[   89.237749]  RSP <ffff88024b131ae0>
[   89.237753] CR2: ffff880096326000
[   89.237758] ---[ end trace 27416ba8b18d496c ]---

This bug dates back to the original introduction of the
gen6_ppgtt_insert_entries()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Dropped cc: stable since without full ppgtt there's no way
we'll access the last page directory with this function since that
range is occupied (only in the allocator) with the ppgtt pdes. Without
aliasing we can start to use that range and blow up.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>