linux-kernel - [PATCH v5.15] drm/ttm: fix undefined behavior in bit shift for TTM_TT_FLAG_PRIV

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Message-Id: <20260116050205.2296956-1-black.hawk@163.com>
Date: Fri, 16 Jan 2026 13:02:05 +0800
From: Rahul Sharma <black.hawk@....com>
To: gregkh@...uxfoundation.org,
	stable@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Gaosheng Cui <cuigaosheng1@...wei.com>,
	Christian König <christian.koenig@....com>,
	Rahul Sharma <black.hawk@....com>
Subject: [PATCH v5.15] drm/ttm: fix undefined behavior in bit shift for TTM_TT_FLAG_PRIV_POPULATED

From: Gaosheng Cui <cuigaosheng1@...wei.com>

[ Upstream 387659939c00156f8d6bab0fbc55b4eaf2b6bc5b commit ]

Shifting signed 32-bit value by 31 bits is undefined, so changing
significant bit to unsigned. The UBSAN warning calltrace like below:

UBSAN: shift-out-of-bounds in ./include/drm/ttm/ttm_tt.h:122:26
left shift of 1 by 31 places cannot be represented in type 'int'
Call Trace:
 <TASK>
 dump_stack_lvl+0x7d/0xa5
 dump_stack+0x15/0x1b
 ubsan_epilogue+0xe/0x4e
 __ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
 ttm_bo_move_memcpy+0x3b4/0x460 [ttm]
 bo_driver_move+0x32/0x40 [drm_vram_helper]
 ttm_bo_handle_move_mem+0x118/0x200 [ttm]
 ttm_bo_validate+0xfa/0x220 [ttm]
 drm_gem_vram_pin_locked+0x70/0x1b0 [drm_vram_helper]
 drm_gem_vram_pin+0x48/0xb0 [drm_vram_helper]
 drm_gem_vram_plane_helper_prepare_fb+0x53/0xe0 [drm_vram_helper]
 drm_gem_vram_simple_display_pipe_prepare_fb+0x26/0x30 [drm_vram_helper]
 drm_simple_kms_plane_prepare_fb+0x4d/0xe0 [drm_kms_helper]
 drm_atomic_helper_prepare_planes+0xda/0x210 [drm_kms_helper]
 drm_atomic_helper_commit+0xc3/0x1e0 [drm_kms_helper]
 drm_atomic_commit+0x9c/0x160 [drm]
 drm_client_modeset_commit_atomic+0x33a/0x380 [drm]
 drm_client_modeset_commit_locked+0x77/0x220 [drm]
 drm_client_modeset_commit+0x31/0x60 [drm]
 __drm_fb_helper_restore_fbdev_mode_unlocked+0xa7/0x170 [drm_kms_helper]
 drm_fb_helper_set_par+0x51/0x90 [drm_kms_helper]
 fbcon_init+0x316/0x790
 visual_init+0x113/0x1d0
 do_bind_con_driver+0x2a3/0x5c0
 do_take_over_console+0xa9/0x270
 do_fbcon_takeover+0xa1/0x170
 do_fb_registered+0x2a8/0x340
 fbcon_fb_registered+0x47/0xe0
 register_framebuffer+0x294/0x4a0
 __drm_fb_helper_initial_config_and_unlock+0x43c/0x880 [drm_kms_helper]
 drm_fb_helper_initial_config+0x52/0x80 [drm_kms_helper]
 drm_fbdev_client_hotplug+0x156/0x1b0 [drm_kms_helper]
 drm_fbdev_generic_setup+0xfc/0x290 [drm_kms_helper]
 bochs_pci_probe+0x6ca/0x772 [bochs]
 local_pci_probe+0x4d/0xb0
 pci_device_probe+0x119/0x320
 really_probe+0x181/0x550
 __driver_probe_device+0xc6/0x220
 driver_probe_device+0x32/0x100
 __driver_attach+0x195/0x200
 bus_for_each_dev+0xbb/0x120
 driver_attach+0x27/0x30
 bus_add_driver+0x22e/0x2f0
 driver_register+0xa9/0x190
 __pci_register_driver+0x90/0xa0
 bochs_pci_driver_init+0x52/0x1000 [bochs]
 do_one_initcall+0x76/0x430
 do_init_module+0x61/0x28a
 load_module+0x1f82/0x2e50
 __do_sys_finit_module+0xf8/0x190
 __x64_sys_finit_module+0x23/0x30
 do_syscall_64+0x58/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 </TASK>

Fixes: 3312be8f6fc8 ("drm/ttm: move populated state into page flags")
Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
Reviewed-by: Christian König <christian.koenig@....com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031113350.4180975-1-cuigaosheng1@huawei.com
Signed-off-by: Christian König <christian.koenig@....com>
[ The context change is due to the commit 43d46f0b78bb
("drm/ttm: s/FLAG_SG/FLAG_EXTERNAL/") in v5.16
which is irrelevant to the logic of this patch.
In addition, v6.1 has included the fix. ]
Signed-off-by: Rahul Sharma <black.hawk@....com>
---
 include/drm/ttm/ttm_tt.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
index b20e89d321b0..4c35802209c4 100644
--- a/include/drm/ttm/ttm_tt.h
+++ b/include/drm/ttm/ttm_tt.h
@@ -43,7 +43,7 @@ struct ttm_operation_ctx;
 #define TTM_PAGE_FLAG_SG              (1 << 8)
 #define TTM_PAGE_FLAG_NO_RETRY	      (1 << 9)
 
-#define TTM_PAGE_FLAG_PRIV_POPULATED  (1 << 31)
+#define TTM_PAGE_FLAG_PRIV_POPULATED  (1U << 31)
 
 /**
  * struct ttm_tt
-- 
2.34.1