FAILED: patch "[PATCH] fbdev: fbcon: fix out-of-bounds read in err_out of" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 8fdc8c2057eea08d40ce2c8eed41ff9e451c65c2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026070231-lash-wick-98ec@gregkh' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8fdc8c2057eea08d40ce2c8eed41ff9e451c65c2 Mon Sep 17 00:00:00 2001
From: Mingyu Wang <[email protected]>
Date: Fri, 26 Jun 2026 00:03:06 +0800
Subject: [PATCH] fbdev: fbcon: fix out-of-bounds read in err_out of
fbcon_do_set_font()

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.

index 9077d3b99357..37beb93045af 100644
@@ -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;

From: Mingyu Wang <[email protected]>

[ 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.

---
 drivers/video/fbdev/core/fbcon.c | 7 +++++++
 1 file changed, 7 insertions(+)

index 00cd29a3bddc1b..b72d7bf448793a 100644
@@ -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