Linux Kernel fbdev bit_cursor Does Not Clamp Glyph Index to Font charcount, Enabling Out-of-Bounds Read via Stale Screen Buffer Entries or vcs_write

Status: NEW  |  Advisory ID: CVE-2026-74635

Key Details

What to Know

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. (NVD)

References