drm: NULL pointer dereference in drm_mode_object_find()

[Alexander Potapenko]

Hello,

the program below triggers a NULL deref in DRM code when ran on QEMU:

===================================================
BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<     inline     >] __list_add ./include/linux/list.h:44
IP: [<     inline     >] list_add_tail ./include/linux/list.h:77
IP: [<     inline     >] __mutex_lock_common kernel/locking/mutex.c:543
IP: [<ffffffff818e850f>] __mutex_lock_slowpath+0x6f/0x100
kernel/locking/mutex.c:824
PGD 1c555067 PUD 1c554067 PMD 0
Oops: 0002 [#1] SMP
Modules linked in:
CPU: 0 PID: 2517 Comm: crash_drm_mode_ Not tainted 4.8.0-rc2+ #1157
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff88001c40a700 task.stack: ffff88001c984000
RIP: 0010:[<ffffffff818e850f>]  [<ffffffff818e850f>]
__mutex_lock_slowpath+0x6f/0x100
RSP: 0018:ffff88001c987cb0  EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff88001d5212a0 RCX: 00000000c0000100
RDX: 0000000000000001 RSI: ffff88001c40a700 RDI: ffff88001d5212a4
RBP: ffff88001c987cf8 R08: ffff88001c984000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88001c40a700
R13: ffff88001d5212a4 R14: 00000000ffffffff R15: ffff88001d5212a8
FS:  0000000000dc9880(0000) GS:ffff88001f000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000001c8a9000 CR4: 00000000000406f0
Stack:
 ffff88001d5212a8 0000000000000000 0000000000000000 ffffffff811a398f
 ffff88001d5212a0 ffff88001d5212a0 0000000000000000 00000000cccccccc
 ffffffff81a6eb20 ffff88001c987d10 ffffffff818e85ba ffff88001d521000
Call Trace:
 [<     inline     >] __mutex_fastpath_lock ./arch/x86/include/asm/mutex_64.h:28
 [<ffffffff818e85ba>] mutex_lock+0x1a/0x30 kernel/locking/mutex.c:102
 [<ffffffff8142dd23>] _object_find+0x23/0xc0 drivers/gpu/drm/drm_crtc.c:329
 [<     inline     >] drm_mode_object_find drivers/gpu/drm/drm_crtc.c:360
 [<     inline     >] drm_crtc_find ./include/drm/drm_crtc.h:2999
 [<ffffffff8143502e>] drm_mode_page_flip_ioctl+0x4e/0x300
drivers/gpu/drm/drm_crtc.c:5414
 [<ffffffff81426bb2>] drm_ioctl+0x2a2/0x460 drivers/gpu/drm/drm_ioctl.c:721
 [<     inline     >] vfs_ioctl fs/ioctl.c:43
 [<ffffffff8119700d>] do_vfs_ioctl+0x8d/0x580 fs/ioctl.c:675
 [<     inline     >] SYSC_ioctl fs/ioctl.c:690
 [<ffffffff81197574>] SyS_ioctl+0x74/0x80 fs/ioctl.c:681
 [<ffffffff818ea9db>] entry_SYSCALL_64_fastpath+0x13/0x8f
arch/x86/entry/entry_64.S:207
Code: e8 37 23 00 00 8b 03 83 f8 01 0f 84 95 00 00 00 48 8b 43 10 4c
8d 7b 08 48 89 63 10 41 be ff ff ff ff 4c 89 3c 24 48 89 44 24 08 <48>
89 20 4c 89 64 24 10 eb 19 49 c7 04 24 02 00 00 00 c6 43 04
RIP  [<     inline     >] __list_add ./include/linux/list.h:44
RIP  [<     inline     >] list_add_tail ./include/linux/list.h:77
RIP  [<     inline     >] __mutex_lock_common kernel/locking/mutex.c:543
RIP  [<ffffffff818e850f>] __mutex_lock_slowpath+0x6f/0x100
kernel/locking/mutex.c:824
 RSP <ffff88001c987cb0>
CR2: 0000000000000000
---[ end trace 3cef4eb618ac6bb6 ]---
===================================================

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <stdint.h>
#include <string.h>
#include <unistd.h>

int main()
{
  int fd = open("/dev/dri/card0", 0);
  mmap(0x20036000ul, 0x1000ul, 0x3ul, 0x32ul, 0xfffffffffffffffful, 0x0ul);
  memcpy((void*)0x20036ad7,
         "\x1e\xa4\x45\xdc\xca\x11\xff\x25\x72\x65\x7e\x4a\x56\x54\x35"
         "\x67\xe3\x8b\x41\x5c\x6d\x69\xa5\xf9\x88\x29\xb8\xc9\x6a\x45"
         "\x76\xa9\xe7\x14\xd1\xf6\xa3\x59\x07\x4d\xe5\xc8\x39\xbf\x33"
         "\xb9\x3d\x21\xd1\xaf\x16\x4d\xbc\xbf\xb1\x0a\x92\x97\xd9\x91"
         "\x4d\xd8\xf8\xa1\xa6\xa3\x20\x02\x2c\x5e\x8f\x87\x05\x8b\xeb"
         "\x9a\xb9\xbc\xa6\x60\x45\x8d\x19\x01\x7d\xb7\xef\x64\x62\x2e"
         "\x5e\x3d\xfe\x65\xbf\xe2\x80\x89\x36\x48\x73\xc6\xa2\x6e\xe2"
         "\x1a\x8f\x1b\x11\x6f\x49\x20\xeb\x74\x2d\x41\xb9\x8b\xb4\x8e"
         "\x8b\xf5\x6d\xb7\xb1\xa3\xcb\xc4\xc2\x7f\x6d\xef\x32\xef\xa7"
         "\x1c\x17\x03\x60\x7b\x31\x1f\x66",
         143);
  ioctl(fd, 0xbb0ul, 0x20036ad7ul, 0, 0, 0);
  return 0;
}

I build the ToT kernel (commit
952b159f2919a8d514f13999f9f463bddcc1dae7, Aug 18) with defconfig and
CONFIG_DRM_VGEM=y.

-- 
Alexander Potapenko
Software Engineer

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