Re: [PATCH] fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font()
From: Thomas Zimmermann <tzimmermann@suse.de>
Date: 2026-06-25 15:04:59
Also in:
dri-devel, lkml, stable
Hi Am 24.06.26 um 10:33 schrieb w15303746062@163.com:
From: Mingyu Wang <redacted>
When fbcon_do_set_font() fails (e.g., due to a memory allocation failure
inside vc_resize() under heavy memory pressure), it jumps to the `err_out`
label to roll back the console state. However, the current rollback logic
forgets to restore the `hi_font` state, leading to a severe state machine
corruption.
Earlier in the function, `set_vc_hi_font()` might be called to change
`vc->vc_hi_font_mask` and mutate the screen buffer. If `vc_resize()`
subsequently fails, the `err_out` path restores `vc_font.charcount`
but entirely skips rolling back the `vc_hi_font_mask` and the screen
buffer.
This mismatch leaves the terminal in a desynchronized state. Because
`vc_hi_font_mask` remains set, the VT subsystem will still accept
character indices greater than 255 from userspace and write them to the
screen buffer. Subsequent rendering calls (e.g., `fbcon_putcs()`) will
then use these inflated indices to access the reverted, 256-character
font array, leading to a deterministic out-of-bounds read and potential
kernel memory disclosure.
Fix this by adding the missing rollback logic for the `hi_font` mask
and screen buffer in the error path.
Fixes: a5a923038d70 ("fbdev: fbcon: Properly revert changes when vc_resize() failed")
Cc: stable@vger.kernel.org
Signed-off-by: Mingyu Wang <redacted>The email in your S-o-b tag differs from the one in the mail's From: line. I think this is not accepted in the kernel. Can you please resubmit with the email addresses synchronized. Apart from that: Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de> Thanks for fixing this problem. Best regards Thomas
quoted hunk ↗ jump to hunk
--- drivers/video/fbdev/core/fbcon.c | 7 +++++++ 1 file changed, 7 insertions(+)diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c index 9077d3b99357..5880ab9f3cde 100644 --- a/drivers/video/fbdev/core/fbcon.c +++ b/drivers/video/fbdev/core/fbcon.c@@ -2405,6 +2405,7 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount, int resize, ret, old_width, old_height, old_charcount; font_data_t *old_fontdata = p->fontdata; const u8 *old_data = vc->vc_font.data; + int old_hi_font_mask = vc->vc_hi_font_mask; font_data_get(data);@@ -2451,6 +2452,12 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount, vc->vc_font.height = old_height; vc->vc_font.charcount = old_charcount; + /* Restore the hi_font state and screen buffer */ + if (old_hi_font_mask && !vc->vc_hi_font_mask) + set_vc_hi_font(vc, true); + else if (!old_hi_font_mask && vc->vc_hi_font_mask) + set_vc_hi_font(vc, false); + font_data_put(data); return ret;
-- -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)