[PATCH v2] fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font()

[Mingyu Wang]

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: sta...@vger.kernel.org
Signed-off-by: Mingyu Wang <25181214...@stu.xidian.edu.cn>
Reviewed-by: Thomas Zimmermann <tzimmerm...@suse.de>
---
 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..37beb93045af 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;
+       unsigned short 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;
-- 
2.34.1