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

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.

Timeline

Scores

EPSS (exploitation in 30 days)

0.13% · top 97% of all CVEs (8 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: no · Technical impact: total

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.8.

Affected products

• Linux · Linux

  • affected 0998a6cb232674408a03e8561dc15aa266b2f53b – < 250159ace2dc53c1bdad267aa8da51b638748700
  • affected db5c9a162d2f42bcc842b76b3d935dcc050a0eec – < 46336f476484f36145e5117e72d7b590f47433ee
  • affected c12003bf91fdff381c55ef54fef3e961a5af2545 – < bf750cfeacf4e47ac72dadc7f05839696efb8576
  • affected 9ba1a7802ca9a2590cef95b253e6526f4364477f – < 94134d70abf9273b70499d97d0adc9185ef21091
  • affected 901f44227072be60812fe8083e83e1533c04eed1 – < c1e7351767dd30fc574395c82121e4c67b882da3
  • affected 18c4ef4e765a798b47980555ed665d78b71aeadf – < bc9db0d879c655d5dfd8add32fd60f13e65d132c
  • affected 18c4ef4e765a798b47980555ed665d78b71aeadf – < 9ea879862e66e354e616028c31b39aa3eb6d35d8
  • affected 18c4ef4e765a798b47980555ed665d78b71aeadf – < e033cbf3975a8465f879ebd5989dc35b04423a4d
  • affected a10cede006f9614b465cf25609a8753efbfd45cc
  • affected efaf89a75a29b2d179bf4fe63ca62852e93ad620
  • affected 5.10.247 – < 5.10.265
  • affected 5.15.197 – < 5.15.216
  • affected 6.1.159 – < 6.1.183
  • affected 6.6.117 – < 6.6.152
  • affected 6.12.58 – < 6.12.104
  • affected 5.4.302 – < 5.5
  • affected 6.17.8 – < 6.18

• Linux · Linux

  • affected 6.18
  • unaffected < 6.18
  • unaffected 5.10.265 – ≤ 5.10.*
  • unaffected 5.15.216 – ≤ 5.15.*
  • unaffected 6.1.183 – ≤ 6.1.*
  • unaffected 6.6.152 – ≤ 6.6.*
  • unaffected 6.12.104 – ≤ 6.12.*
  • unaffected 6.18.45 – ≤ 6.18.*
  • unaffected 7.1.9 – ≤ 7.1.*
  • unaffected 7.2 – ≤ *

References (8)

• https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700 (opens in a new tab)
• https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee (opens in a new tab)
• https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576 (opens in a new tab)
• https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091 (opens in a new tab)
• https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3 (opens in a new tab)
• https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c (opens in a new tab)
• https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8 (opens in a new tab)
• https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d (opens in a new tab)

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-74635](https://cveledger.app/badge/CVE-2026-74635.svg)](https://cveledger.app/cve/CVE-2026-74635)

Record last updated 16 Oct 2026, 19:06 UTC.