CVE-2026-74635: fbdev: bitblit: bound-check glyph index in bit_cursor()

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

fbdev: bitblit: bound-check glyph index in bit_cursor()

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.

The Linux kernel CVE team has assigned CVE-2026-74635 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.10.247 with commit 0998a6cb232674408a03e8561dc15aa266b2f53b and fixed in 5.10.265 with commit 250159ace2dc53c1bdad267aa8da51b638748700
	Issue introduced in 5.15.197 with commit db5c9a162d2f42bcc842b76b3d935dcc050a0eec and fixed in 5.15.216 with commit 46336f476484f36145e5117e72d7b590f47433ee
	Issue introduced in 6.1.159 with commit c12003bf91fdff381c55ef54fef3e961a5af2545 and fixed in 6.1.183 with commit bf750cfeacf4e47ac72dadc7f05839696efb8576
	Issue introduced in 6.6.117 with commit 9ba1a7802ca9a2590cef95b253e6526f4364477f and fixed in 6.6.152 with commit 94134d70abf9273b70499d97d0adc9185ef21091
	Issue introduced in 6.12.58 with commit 901f44227072be60812fe8083e83e1533c04eed1 and fixed in 6.12.104 with commit c1e7351767dd30fc574395c82121e4c67b882da3
	Issue introduced in 6.18 with commit 18c4ef4e765a798b47980555ed665d78b71aeadf and fixed in 6.18.45 with commit bc9db0d879c655d5dfd8add32fd60f13e65d132c
	Issue introduced in 6.18 with commit 18c4ef4e765a798b47980555ed665d78b71aeadf and fixed in 7.1.9 with commit 9ea879862e66e354e616028c31b39aa3eb6d35d8
	Issue introduced in 6.18 with commit 18c4ef4e765a798b47980555ed665d78b71aeadf and fixed in 7.2 with commit e033cbf3975a8465f879ebd5989dc35b04423a4d
	Issue introduced in 5.4.302 with commit a10cede006f9614b465cf25609a8753efbfd45cc
	Issue introduced in 6.17.8 with commit efaf89a75a29b2d179bf4fe63ca62852e93ad620

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74635
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/video/fbdev/core/bitblit.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
	https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
	https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
	https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
	https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
	https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
	https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
	https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d