From e033cbf3975a8465f879ebd5989dc35b04423a4d Mon Sep 17 00:00:00 2001 From: Rik van Riel Date: Fri, 7 Aug 2026 22:19:56 -0400 Subject: [PATCH] fbdev: bitblit: bound-check glyph index in bit_cursor() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit bit_cursor() fetches the glyph under the cursor with c = scr_readw(vc_pos); src = vc_font.data + ((c & charmask) * w * height); where charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer value comes directly from scr_readw() and may be larger than the current font's glyph count. Syzkaller triggers this via vcs_write(). The Call Trace shows vcs_write() in vc_screen.c writing an arbitrary 16-bit value with writev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via vcs_scr_writew() without checking charcount. The stored value is later read in bit_cursor() in bitblit.c. When the font is changed from a font with 512 glyphs to a font with 256 glyphs, the screen buffer can retain characters with the high bit set from the previous mode, which could also produce the same out-of-bounds access. BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 Read of size 16 at addr ffff800086c57970 Call Trace: soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365 fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427 hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883 update_region+0x100/0x18c drivers/tty/vt/vt.c:669 vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685 bit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph index to vc_font.charcount. Apply the same clamp in bit_cursor() after extracting the attribute and masking, before indexing fontdata. The fix completes the bounds checking started in commit 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*"), which missed the cursor path. This change should be safe because the clamp reuses the existing contract from fbcon: charcount is maintained under console_lock in con_font_set() and fbcon_font_set(), and hi_font_mask is cleared when switching from 512 to 256 glyphs. When stale screen data with high bits remains after a font switch, or when vcs_write() stores an arbitrary value, clamping the index to 0 prevents the out-of-bounds read without changing cursor semantics — the same fallback bit_putcs uses. Reported-by: syzbot+61b1db46218109869c14@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=61b1db46218109869c14 Link: https://lore.kernel.org/all/6a75205c.01d0871a.3a0d52.0032.GAE@google.com/ Fixes: 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*") Cc: stable@vger.kernel.org Assisted-by: Hermes:muse-spark-1.2 syzkaller Signed-off-by: Rik van Riel Signed-off-by: Helge Deller --- drivers/video/fbdev/core/bitblit.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c index 65681dcc5930..39f44258d793 100644 --- a/drivers/video/fbdev/core/bitblit.c +++ b/drivers/video/fbdev/core/bitblit.c @@ -273,9 +273,14 @@ static void bit_cursor(struct vc_data *vc, struct fb_info *info, bool enable, if (!vc->vc_font.data) return; - c = scr_readw((u16 *) vc->vc_pos); + c = scr_readw((u16 *) vc->vc_pos); attribute = get_attribute(info, c); - src = vc->vc_font.data + ((c & charmask) * (w * vc->vc_font.height)); + c &= charmask; + + /* Clamp to font size, same as bit_putcs_aligned() */ + if (c >= vc->vc_font.charcount) + c = 0; + src = vc->vc_font.data + (c * (w * vc->vc_font.height)); if (par->cursor_state.image.data != (const char *)src || par->cursor_reset) { -- 2.47.3