Linux kernel nouveau driver: failed re-pin leaves a stale pin count

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

drm/nouveau: don't bump pin count on failed re-pin in nouveau_bo_pin_locked()

nouveau_bo_pin_locked() checks whether an already pinned BO is in a memory domain compatible with a new pin request. When the domains are incompatible, it sets -EBUSY but still calls ttm_bo_pin() before returning.

Callers treat a failed nouveau_bo_pin() as not having acquired a new pin, so the extra pin count is never decreased by a matching unpin. This triggers the warning in ttm_bo_release():

WARN_ON_ONCE(bo->pin_count);

Found when fuzzing the nouveau driver with a modified Syzkaller:

WARNING: drivers/gpu/drm/ttm/ttm_bo.c:256 at ttm_bo_release+0x827/0x9e0 drivers/gpu/drm/ttm/ttm_bo.c:256, CPU#1: syz.3.24/2212
Modules linked in:
CPU: 1 UID: 0 PID: 2212 Comm: syz.3.24 Not tainted 7.2.0 #24 PREEMPT(lazy)
nouveau 0000:01:00.0: gsp:msg fn:103 len:0x40/0x20 res:0x19 resp:0x19
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:ttm_bo_release+0x827/0x9e0 drivers/gpu/drm/ttm/ttm_bo.c:256
Code: 02 00 0f 85 51 01 00 00 48 8b 7b 08 e8 d2 20 01 00 e9 80 fd ff ff e8 d8 15 c0 fe 90 0f 0b 90 e9 e1 f8 ff ff e8 ca 15 c0 fe 90 <0f> 0b 90 e9 a4 f8 ff ff e8 bc 15 c0 fe be 03 00 00 00 4c 89 e7 e8
msg: 00000000: 05 00 d0 c1 04 00 f0 f1 01 30 00 00 2d 90 00 00  .........0..-...
RSP: 0018:ffffc9000f5cf710 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff888018e5d2a8 RCX: ffffffff82bb1b36
RDX: ffff888017b68000 RSI: 0000000000000004 RDI: ffff888018e5d2a8
msg: 00000010: 19 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
RBP: ffff88801261c720 R08: 0000000000000001 R09: ffffed10031cba55
R10: ffff888018e5d2ab R11: 00000000000000f3 R12: ffff888018e5d290
R13: ffff888018e5d2d4 R14: ffff88801b219c18 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ffff8880e0f6f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b31223ffc CR3: 0000000028e00005 CR4: 0000000000770ef0
PKRU: 80000000
Call Trace:
<TASK>
kref_put include/linux/kref.h:65 [inline]
ttm_bo_put drivers/gpu/drm/ttm/ttm_bo.c:325 [inline]
ttm_bo_fini+0x55/0x80 drivers/gpu/drm/ttm/ttm_bo.c:330
nouveau_gem_object_del+0xb2/0x1b0 drivers/gpu/drm/nouveau/nouveau_gem.c:90
drm_gem_object_free+0x5f/0x90 drivers/gpu/drm/drm_gem.c:1165
kref_put include/linux/kref.h:65 [inline]
__drm_gem_object_put include/drm/drm_gem.h:562 [inline]
drm_gem_object_put include/drm/drm_gem.h:575 [inline]
nouveau_abi16_chan_fini.constprop.0+0x44f/0x5a0 drivers/gpu/drm/nouveau/nouveau_abi16.c:195
nouveau 0000:01:00.0: syz.2.23[2209]: Unknown handle 0x00000000
nouveau_abi16_fini+0x1d0/0x340 drivers/gpu/drm/nouveau/nouveau_abi16.c:225
nouveau_drm_postclose+0x18b/0x3e0 drivers/gpu/drm/nouveau/nouveau_drm.c:1284
nouveau 0000:01:00.0: syz.2.23[2209]: validate_init
drm_file_free.part.0+0x6d6/0xb60 drivers/gpu/drm/drm_file.c:267
drm_file_free drivers/gpu/drm/drm_file.c:237 [inline]
drm_close_helper.isra.0+0x11a/0x160 drivers/gpu/drm/drm_file.c:290
drm_release+0x1ab/0x330 drivers/gpu/drm/drm_file.c:438
__fput+0x39c/0xa60 fs/file_table.c:512
nouveau 0000:01:00.0: syz.2.23[2209]: validate: -2
task_work_run+0x15a/0x230 kernel/task_work.c:233
exit_task_work include/linux/task_work.h:40 [inline]
do_exit+0x82b/0x25a0 kernel/exit.c:1009
do_group_exit+0xc2/0x280 kernel/exit.c:1152
get_signal+0x1d6e/0x1f30 kernel/signal.c:3046
arch_do_signal_or_restart+0x7d/0x6e0 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:66 [inline]
exit_to_user_mode_loop+0xdf/0x440 kernel/entry/common.c:101
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x4f8/0x690 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RI

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