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fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font()

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[ Upstream commit 8fdc8c2057 ]

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: a5a923038d ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Mingyu Wang
2026-07-18 16:53:05 +02:00
committed by Greg Kroah-Hartman
parent 406c28af75
commit 076b1aa65f
1 changed files with 7 additions and 0 deletions
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drivers/video/fbdev/core/fbcon.c
+7
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@@ -2422,6 +2422,7 @@ 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);
@@ -2475,6 +2476,12 @@ err_out:
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;
}