On 5/27/23 12:18, Sam Ravnborg wrote:
Hi Helge,
On Sat, May 27, 2023 at 11:19:02AM +0200, Helge Deller wrote:
syzbot repored this bug in the softcursor code:
BUG: KASAN: null-ptr-deref in soft_cursor+0x384/0x6b4 drivers/video/fbdev/core/softcursor.c:70
Read of size 16 at addr 0000000000000200 by task kworker/u4:1/12
CPU: 0 PID: 12 Comm: kworker/u4:1 Not tainted 6.4.0-rc3-syzkaller-geb0f1697d729 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/28/2023
Workqueue: events_power_efficient fb_flashcursor
Call trace:
dump_backtrace+0x1b8/0x1e4 arch/arm64/kernel/stacktrace.c:233
show_stack+0x2c/0x44 arch/arm64/kernel/stacktrace.c:240
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd0/0x124 lib/dump_stack.c:106
print_report+0xe4/0x514 mm/kasan/report.c:465
kasan_report+0xd4/0x130 mm/kasan/report.c:572
kasan_check_range+0x264/0x2a4 mm/kasan/generic.c:187
__asan_memcpy+0x3c/0x84 mm/kasan/shadow.c:105
soft_cursor+0x384/0x6b4 drivers/video/fbdev/core/softcursor.c:70
bit_cursor+0x113c/0x1a64 drivers/video/fbdev/core/bitblit.c:377
fb_flashcursor+0x35c/0x54c drivers/video/fbdev/core/fbcon.c:380
process_one_work+0x788/0x12d4 kernel/workqueue.c:2405
worker_thread+0x8e0/0xfe8 kernel/workqueue.c:2552
kthread+0x288/0x310 kernel/kthread.c:379
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:853
This fix let bit_cursor() bail out early when a font bitmap
isn't available yet.
Signed-off-by: Helge Deller <deller@xxxxxx>
Cc: stable@xxxxxxxxxx
Reported-by: syzbot+d910bd780e6efac35869@xxxxxxxxxxxxxxxxxxxxxxxxx
---
drivers/video/fbdev/core/bitblit.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index f98e8f298bc1..8587c9da0670 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -247,6 +247,9 @@ static void bit_cursor(struct vc_data *vc, struct fb_info *info, int mode,
cursor.set = 0;
+ if (!vc->vc_font.data)
+ return;
+
c = scr_readw((u16 *) vc->vc_pos);
attribute = get_attribute(info, c);
src = vc->vc_font.data + ((c & charmask) * (w * vc->vc_font.height));
When browsing the code I noticed another use of vc->vc_font.data.
bit_putcs_{un}aligned() access vc->vc_font.data too, so I wonder if
bit_putcs() should also check for vc->vc_font.data?
bit_putcs() is the only user of bit_putcs_{un}aligned.
I think the font is set before any char is printed, so we should
probably be safe there.
The actual patch here looks good from a quick browsing around, but I
cannot claim I did a review. But you have my:
Acked-by: Sam Ravnborg <sam@xxxxxxxxxxxx>
Thank you!
Helge
