[PATCH 5.10.y] fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font()

[Sasha Levin <sashal@kernel.org>]

From: Mingyu Wang <25181214217@stu.xidian.edu.cn>

[ Upstream commit 8fdc8c2057eea08d40ce2c8eed41ff9e451c65c2 ]

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 <25181214217@stu.xidian.edu.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>
[ Adjust context ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 00cd29a3bddc1b..b72d7bf448793a 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2426,6 +2426,7 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 	struct fbcon_display *p = &fb_display[vc->vc_num];
 	int resize, ret, old_userfont, old_width, old_height, old_charcount;
 	u8 *old_data = vc->vc_font.data;
+	unsigned short old_hi_font_mask = vc->vc_hi_font_mask;
 
 	resize = (w != vc->vc_font.width) || (h != vc->vc_font.height);
 	vc->vc_font.data = (void *)(p->fontdata = data);
@@ -2479,6 +2480,12 @@ 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);
+
 	return ret;
 }
 
-- 
2.53.0