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From:   Shigeru Yoshida <syoshida@redhat.com>
To:     deller@gmx.de
Cc:     linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org,
        Shigeru Yoshida <syoshida@redhat.com>,
        syzbot+a168dbeaaa7778273c1b@syzkaller.appspotmail.com
Subject: [PATCH] fbcon: Properly revert changes when vc_resize() failed
Date:   Fri, 19 Aug 2022 03:13:36 +0900
Message-Id: <20220818181336.3504010-1-syoshida@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

fbcon_do_set_font() calls vc_resize() when font size is changed.
However, if if vc_resize() failed, current implementation doesn't
revert changes for font size, and this causes inconsistent state.

syzbot reported unable to handle page fault due to this issue [1].
syzbot's repro uses fault injection which cause failure for memory
allocation, so vc_resize() failed.

This patch fixes this issue by properly revert changes for font
related date when vc_resize() failed.

Link: https://syzkaller.appspot.com/bug?id=3443d3a1fa6d964dd7310a0cb1696d165a3e07c4 [1]
Reported-by: syzbot+a168dbeaaa7778273c1b@syzkaller.appspotmail.com
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
---
 drivers/video/fbdev/core/fbcon.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index cf9ac4da0a82..825b012debe7 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2401,15 +2401,21 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 	struct fb_info *info = fbcon_info_from_console(vc->vc_num);
 	struct fbcon_ops *ops = info->fbcon_par;
 	struct fbcon_display *p = &fb_display[vc->vc_num];
-	int resize;
+	int resize, ret, old_userfont, old_width, old_height, old_charcount;
 	char *old_data = NULL;
 
 	resize = (w != vc->vc_font.width) || (h != vc->vc_font.height);
 	if (p->userfont)
 		old_data = vc->vc_font.data;
 	vc->vc_font.data = (void *)(p->fontdata = data);
+	old_userfont = p->userfont;
 	if ((p->userfont = userfont))
 		REFCOUNT(data)++;
+
+	old_width = vc->vc_font.width;
+	old_height = vc->vc_font.height;
+	old_charcount = vc->vc_font.charcount;
+
 	vc->vc_font.width = w;
 	vc->vc_font.height = h;
 	vc->vc_font.charcount = charcount;
@@ -2425,7 +2431,9 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 		rows = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
 		cols /= w;
 		rows /= h;
-		vc_resize(vc, cols, rows);
+		ret = vc_resize(vc, cols, rows);
+		if (ret)
+			goto err_out;
 	} else if (con_is_visible(vc)
 		   && vc->vc_mode == KD_TEXT) {
 		fbcon_clear_margins(vc, 0);
@@ -2435,6 +2443,21 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 	if (old_data && (--REFCOUNT(old_data) == 0))
 		kfree(old_data - FONT_EXTRA_WORDS * sizeof(int));
 	return 0;
+
+err_out:
+	p->fontdata = old_data;
+	vc->vc_font.data = (void *)old_data;
+
+	if (userfont) {
+		p->userfont = old_userfont;
+		REFCOUNT(data)--;
+	}
+
+	vc->vc_font.width = old_width;
+	vc->vc_font.height = old_height;
+	vc->vc_font.charcount = old_charcount;
+
+	return ret;
 }
 
 /*
-- 
2.37.1