[PATCH 6.0 0118/1073] lib/fonts: fix undefined behavior in bit shift for get_default_font

Greg Kroah-Hartman
2022-12-28T15:08:33Z

From: Gaosheng Cui <cuigaosheng1@huawei.com>

[ Upstream commit 6fe888c4d2fb174408e4540bb2d5602b9f507f90 ]

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 lib/fonts/fonts.c:139:20
left shift of 1 by 31 places cannot be represented in type 'int'
<TASK>
dump_stack_lvl+0x7d/0xa5
dump_stack+0x15/0x1b
ubsan_epilogue+0xe/0x4e
__ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
get_default_font+0x1c7/0x1f0
fbcon_startup+0x347/0x3a0
do_take_over_console+0xce/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>

Link: https://lkml.kernel.org/r/20221031113829.4183153-1-cuigaosheng1@huawei.com
Fixes: c81f717cb9e0 ("fbcon: Fix typo and bogus logic in get_default_font")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
lib/fonts/fonts.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/fonts/fonts.c b/lib/fonts/fonts.c
index 5f4b07b56cd9..973866438608 100644
--- a/lib/fonts/fonts.c
+++ b/lib/fonts/fonts.c
@@ -135,8 +135,8 @@ const struct font_desc *get_default_font(int xres, int yres, u32 font_w,
if (res > 20)
c += 20 - res;

- if ((font_w & (1 << (f->width - 1))) &&
- (font_h & (1 << (f->height - 1))))
+ if ((font_w & (1U << (f->width - 1))) &&
+ (font_h & (1U << (f->height - 1))))
c += 1000;

if (c > cc) {
--
2.35.1