dri: WARNING in idr_remove

[Dmitry Vyukov]

Hello,

The following program causes a WARNING in idr_remove:

------------[ cut here ]------------
WARNING: CPU: 3 PID: 26766 at lib/idr.c:505
idr_remove called for id=1 which is not allocated.
CPU: 3 PID: 26766 Comm: syz-executor Not tainted 4.8.0-rc3+ #33
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffffffff884b8280 ffff8800634878d8 ffffffff82d1b239 ffffffff01fc0000
 fffffbfff1097050 ffffffff86e8eec0 ffff8800634879b0 ffffffff8723b400
 dffffc0000000000 0000000000000009 ffff8800634879a0 ffffffff816ab4e3
Call Trace:
 [<ffffffff813809bc>] warn_slowpath_fmt+0xac/0xd0 kernel/panic.c:532
 [<     inline     >] idr_remove_warning lib/idr.c:505
 [<ffffffff82d1e5c7>] idr_remove+0x617/0x830 lib/idr.c:559
 [<ffffffff8340761d>] drm_legacy_ctxbitmap_free+0x9d/0xd0
drivers/gpu/drm/drm_context.c:61
 [<ffffffff834090ae>] drm_legacy_rmctx+0x30e/0x410
drivers/gpu/drm/drm_context.c:496
 [<ffffffff83410a5c>] drm_ioctl+0x7bc/0xc60 drivers/gpu/drm/drm_ioctl.c:724
 [<     inline     >] vfs_ioctl fs/ioctl.c:43
 [<ffffffff8186f1fc>] do_vfs_ioctl+0x18c/0x1080 fs/ioctl.c:675
 [<     inline     >] SYSC_ioctl fs/ioctl.c:690
 [<ffffffff8187017f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:681
 [<ffffffff810088ff>] do_syscall_64+0x1df/0x640 arch/x86/entry/common.c:288
 [<ffffffff86c25543>] entry_SYSCALL64_slow_path+0x25/0x25
arch/x86/entry/entry_64.S:248


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

int main()
{
  syscall(__NR_mmap, 0x20000000ul, 0xc000ul, 0x3ul,
                         0x32ul, -1, 0x0ul, 0, 0, 0);
  int fd = open("/dev/dri/card0", 0x101102ul, 0);
  *(uint32_t*)0x2000b000 = (uint32_t)0x2;
  *(uint64_t*)0x2000b008 = (uint64_t)0x2000b000;
  syscall(__NR_ioctl, fd, 0xc0106426ul, 0x2000b000ul);
  int res = *(uint32_t*)0x2000b000;
  *(uint32_t*)0x2000bff8 = res;
  *(uint32_t*)0x2000bffc = (uint32_t)0x2;
  syscall(__NR_ioctl, fd, 0xc0086421ul, 0x2000bff8ul);
  return 0;
}

On commit 61c04572de404e52a655a36752e696bbcb483cf5 (Aug 25).