CVE-2026-74635: Software Security Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gcm2-cr9p-5jc7",
  "modified" : "2026-08-25T06:31:24Z",
  "published" : "2026-08-22T18:30:27Z",
  "aliases" : [ "CVE-2026-74635" ],
  "details" : "In the Linux kernel, the following vulnerability has been resolved:\n\nfbdev: bitblit: bound-check glyph index in bit_cursor()\n\nbit_cursor() fetches the glyph under the cursor with\n\n\tc = scr_readw(vc_pos);\n\tsrc = vc_font.data + ((c & charmask) * w * height);\n\nwhere charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer\nvalue comes directly from scr_readw() and may be larger than the current\nfont's glyph count.\n\nSyzkaller triggers this via vcs_write(). The Call Trace shows\nvcs_write() in vc_screen.c writing an arbitrary 16-bit value with\nwritev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via\nvcs_scr_writew() without checking charcount. The stored value is later\nread in bit_cursor() in bitblit.c.\n\nWhen the font is changed from a font with 512 glyphs to a font with\n256 glyphs, the screen buffer can retain characters with the high\nbit set from the previous mode, which could also produce the same\nout-of-bounds access.\n\n  BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n  Read of size 16 at addr ffff800086c57970\n\n  Call Trace:\n   soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n   bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365\n   fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427\n   hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883\n   update_region+0x100/0x18c drivers/tty/vt/vt.c:669\n   vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685\n\nbit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph\nindex to vc_font.charcount. Apply the same clamp in bit_cursor() after\nextracting the attribute and masking, before indexing fontdata.\n\nThe fix completes the bounds checking started in commit 18c4ef4e765a\n(\"fbdev: bitblit: bound-check glyph index in bit_putcs*\"), which missed\nthe cursor path.\n\nThis change should be safe because the clamp reuses the existing\ncontract from fbcon: charcount is maintained under console_lock in\ncon_font_set() and fbcon_font_set(), and hi_font_mask is cleared when\nswitching from 512 to 256 glyphs. When stale screen data with high bits\nremains after a font switch, or when vcs_write() stores an arbitrary\nvalue, clamping the index to 0 prevents the out-of-bounds read without\nchanging cursor semantics — the same fallback bit_putcs uses.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-74635"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "HIGH",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-08-22T16:16:36Z"
  }
}