Medium · published September 18, 2025
In the Linux kernel, the following vulnerability has been resolved:
drm/ttm: fix undefined behavior in bit shift for TTM_TT_FLAG_PRIV_POPULATED
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>