linux-kernel - [PATCH] fbdev: Delay the setting of fbcon

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Message-Id: <20250905024340.337521-1-wozizhi@huaweicloud.com>
Date: Fri,  5 Sep 2025 10:43:40 +0800
From: Zizhi Wo <wozizhi@...weicloud.com>
To: deller@....de,
	tzimmermann@...e.de,
	lee@...nel.org,
	jani.nikula@...el.com,
	oushixiong@...inos.cn,
	soci@....rulez.org
Cc: linux-kernel@...r.kernel.org,
	linux-fbdev@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	yangerkun@...wei.com,
	wozizhi@...weicloud.com
Subject: [PATCH] fbdev: Delay the setting of fbcon_ops to fix KASAN issues

[BUG]
Recently, we encountered a KASAN warning as follows:

kasan_report+0xaf/0xe0 mm/kasan/report.c:588
fb_pad_aligned_buffer+0x12f/0x150 drivers/video/fbdev/core/fbmem.c:116
ccw_putcs_aligned drivers/video/fbdev/core/fbcon_ccw.c:119 [inline]
ccw_putcs+0x9ac/0xbb0 drivers/video/fbdev/core/fbcon_ccw.c:175
fbcon_putcs+0x329/0x3f0 drivers/video/fbdev/core/fbcon.c:1297
do_update_region+0x3de/0x670 drivers/tty/vt/vt.c:623
invert_screen+0x1de/0x600 drivers/tty/vt/vt.c:748
highlight drivers/tty/vt/selection.c:57 [inline]
clear_selection+0x5e/0x70 drivers/tty/vt/selection.c:81
vc_do_resize+0xc8e/0xf40 drivers/tty/vt/vt.c:1206
fbcon_modechanged+0x489/0x7a0 drivers/video/fbdev/core/fbcon.c:2705
fbcon_set_all_vcs+0x1e0/0x600 drivers/video/fbdev/core/fbcon.c:2752
fbcon_rotate_all drivers/video/fbdev/core/fbcon.c:250 [inline]
...

reproduce[probabilistic, depending on the width and height of vc_font, as
well as the value of "p" in do_update_region()]:
1) echo 2 > /sys/devices/virtual/graphics/fbcon/rotate_all
2) echo 3 > /sys/devices/virtual/graphics/fbcon/rotate_all

[CAUSE]
The root cause is that fbcon_modechanged() first sets the current rotate's
corresponding ops. Subsequently, during vc_resize(), it may trigger
clear_selection(), and in fbcon_putcs->ccw_putcs[rotate=3], this can result
in an out-of-bounds access to "src". This happens because ops->fontbuffer
is reallocated in fbcon_rotate_font():
1) When rotate=2, its size is (width + 7) / 8 * height
2) When rotate=3, its size is (height + 7) / 8 * width

And the call to fbcon_rotate_font() occurs after clear_selection(). In
other words, the fontbuffer is allocated using the size calculated from the
previous rotation[2], but before reallocating it with the new size,
con_putcs is already using the new rotation[3]:

rotate_all_store
 fbcon_rotate_all
  fbcon_set_all_vcs
   fbcon_modechanged
   ...
    fbcon_set_rotate
     fbcon_rotate_ccw
      ops->putcs = ccw_putcs // set rotate 3 ops
    vc_resize
    ...
     clear_selection
      highlight
      ...
       do_update_region
	fbcon_putcs
	 ccw_putcs_aligned
	  src = ops->fontbuffer + (scr_readw(s--) & charmask)*cellsize
	  fb_pad_aligned_buffer----[src KASAN!!!]
       update_screen
        redraw_screen
	 fbcon_switch
	  fbcon_rotate_font
	   dst = kmalloc_array(len, d_cellsize, GFP_KERNEL)
	   ops->fontbuffer = dst

[FIX]
Considering that when the rotation changes, clear_selection() should clear
the previously selected region and not consider the new rotation yet.
Therefore, the assignment to fbcon_ops for the newly set rotate can be
postponed to fbcon_rotate_font(), since the fontbuffer is regenerated
there. To avoid affecting other code paths, fbcon_set_rotate() will
temporarily continue assigning fbcon_ops based on cur_rotate not rotate.

Signed-off-by: Zizhi Wo <wozizhi@...weicloud.com>
---
 drivers/video/fbdev/core/fbcon_rotate.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbcon_rotate.c b/drivers/video/fbdev/core/fbcon_rotate.c
index ec3c883400f7..d76446da24d4 100644
--- a/drivers/video/fbdev/core/fbcon_rotate.c
+++ b/drivers/video/fbdev/core/fbcon_rotate.c
@@ -70,6 +70,7 @@ static int fbcon_rotate_font(struct fb_info *info, struct vc_data *vc)
 			src += s_cellsize;
 			dst += d_cellsize;
 		}
+		fbcon_rotate_ud(ops);
 		break;
 	case FB_ROTATE_CW:
 		for (i = len; i--; ) {
@@ -78,6 +79,7 @@ static int fbcon_rotate_font(struct fb_info *info, struct vc_data *vc)
 			src += s_cellsize;
 			dst += d_cellsize;
 		}
+		fbcon_rotate_cw(ops);
 		break;
 	case FB_ROTATE_CCW:
 		for (i = len; i--; ) {
@@ -86,6 +88,7 @@ static int fbcon_rotate_font(struct fb_info *info, struct vc_data *vc)
 			src += s_cellsize;
 			dst += d_cellsize;
 		}
+		fbcon_rotate_ccw(ops);
 		break;
 	}

@@ -97,7 +100,7 @@ void fbcon_set_rotate(struct fbcon_ops *ops)
 {
 	ops->rotate_font = fbcon_rotate_font;

-	switch(ops->rotate) {
+	switch (ops->cur_rotate) {
 	case FB_ROTATE_CW:
 		fbcon_rotate_cw(ops);
 		break;
-- 
2.39.2