FKIE_CVE-2026-74635

Vulnerability from fkie_nvd - Published: 2026-08-22 16:16 - Updated: 2026-10-14 13:10
Severity
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "drivers/video/fbdev/core/bitblit.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "250159ace2dc53c1bdad267aa8da51b638748700",
              "status": "affected",
              "version": "0998a6cb232674408a03e8561dc15aa266b2f53b",
              "versionType": "git"
            },
            {
              "lessThan": "46336f476484f36145e5117e72d7b590f47433ee",
              "status": "affected",
              "version": "db5c9a162d2f42bcc842b76b3d935dcc050a0eec",
              "versionType": "git"
            },
            {
              "lessThan": "bf750cfeacf4e47ac72dadc7f05839696efb8576",
              "status": "affected",
              "version": "c12003bf91fdff381c55ef54fef3e961a5af2545",
              "versionType": "git"
            },
            {
              "lessThan": "94134d70abf9273b70499d97d0adc9185ef21091",
              "status": "affected",
              "version": "9ba1a7802ca9a2590cef95b253e6526f4364477f",
              "versionType": "git"
            },
            {
              "lessThan": "c1e7351767dd30fc574395c82121e4c67b882da3",
              "status": "affected",
              "version": "901f44227072be60812fe8083e83e1533c04eed1",
              "versionType": "git"
            },
            {
              "lessThan": "bc9db0d879c655d5dfd8add32fd60f13e65d132c",
              "status": "affected",
              "version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
              "versionType": "git"
            },
            {
              "lessThan": "9ea879862e66e354e616028c31b39aa3eb6d35d8",
              "status": "affected",
              "version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
              "versionType": "git"
            },
            {
              "lessThan": "e033cbf3975a8465f879ebd5989dc35b04423a4d",
              "status": "affected",
              "version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "a10cede006f9614b465cf25609a8753efbfd45cc",
              "versionType": "git"
            },
            {
              "status": "affected",
              "version": "efaf89a75a29b2d179bf4fe63ca62852e93ad620",
              "versionType": "git"
            },
            {
              "lessThan": "5.10.265",
              "status": "affected",
              "version": "5.10.247",
              "versionType": "semver"
            },
            {
              "lessThan": "5.15.216",
              "status": "affected",
              "version": "5.15.197",
              "versionType": "semver"
            },
            {
              "lessThan": "6.1.183",
              "status": "affected",
              "version": "6.1.159",
              "versionType": "semver"
            },
            {
              "lessThan": "6.6.152",
              "status": "affected",
              "version": "6.6.117",
              "versionType": "semver"
            },
            {
              "lessThan": "6.12.104",
              "status": "affected",
              "version": "6.12.58",
              "versionType": "semver"
            },
            {
              "lessThan": "5.5",
              "status": "affected",
              "version": "5.4.302",
              "versionType": "semver"
            },
            {
              "lessThan": "6.18",
              "status": "affected",
              "version": "6.17.8",
              "versionType": "semver"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "drivers/video/fbdev/core/bitblit.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.18"
            },
            {
              "lessThan": "6.18",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.10.*",
              "status": "unaffected",
              "version": "5.10.265",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.15.*",
              "status": "unaffected",
              "version": "5.15.216",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.1.*",
              "status": "unaffected",
              "version": "6.1.183",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.6.*",
              "status": "unaffected",
              "version": "6.6.152",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.104",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.45",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.1.*",
              "status": "unaffected",
              "version": "7.1.9",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.2",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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 \u0026 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\u0027s 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 \u2014 the same fallback bit_putcs uses."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nfbdev: bitblit: comprobaci\u00f3n de l\u00edmites del \u00edndice de glifos en bit_cursor()\n\nbit_cursor() obtiene el glifo bajo el cursor con\n\n\tc = scr_readw(vc_pos);\n\tsrc = vc_font.data + ((c \u0026 charmask) * w * height);\n\ndonde charmask es 0x1ff cuando vc_hi_font_mask est\u00e1 establecido. El valor del b\u00fafer de pantalla proviene directamente de scr_readw() y puede ser mayor que el recuento de glifos de la fuente actual.\n\nSyzkaller activa esto a trav\u00e9s de vcs_write(). El Call Trace muestra a vcs_write() en vc_screen.c escribiendo un valor arbitrario de 16 bits con writev() en /dev/vcsa, el cual vcs_write_buf() en vc_screen.c almacena a trav\u00e9s de vcs_scr_writew() sin comprobar charcount. El valor almacenado se lee posteriormente en bit_cursor() en bitblit.c.\n\nCuando la fuente se cambia de una fuente con 512 glifos a una fuente con 256 glifos, el b\u00fafer de pantalla puede retener caracteres con el bit alto establecido del modo anterior, lo que tambi\u00e9n podr\u00eda producir el mismo acceso fuera de l\u00edmites.\n\n  BUG: KASAN: global-fuera-de-l\u00edmites en soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n  Lectura de tama\u00f1o 16 en la direcci\u00f3n 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() y bit_putcs_unaligned() ya limitan el \u00edndice de glifos a vc_font.charcount. Aplique la misma limitaci\u00f3n en bit_cursor() despu\u00e9s de extraer el atributo y enmascarar, antes de indexar fontdata.\n\nLa correcci\u00f3n completa la comprobaci\u00f3n de l\u00edmites iniciada en el commit 18c4ef4e765a (\u0027fbdev: bitblit: comprobaci\u00f3n de l\u00edmites del \u00edndice de glifos en bit_putcs\u0027), que omiti\u00f3 la ruta del cursor.\n\nEste cambio deber\u00eda ser seguro porque la limitaci\u00f3n reutiliza el contrato existente de fbcon: charcount se mantiene bajo console_lock en con_font_set() y fbcon_font_set(), y hi_font_mask se borra al cambiar de 512 a 256 glifos. Cuando los datos de pantalla obsoletos con bits altos permanecen despu\u00e9s de un cambio de fuente, o cuando vcs_write() almacena un valor arbitrario, limitar el \u00edndice a 0 evita la lectura fuera de l\u00edmites sin cambiar la sem\u00e1ntica del cursor - el mismo mecanismo de reserva que usa bit_putcs."
    }
  ],
  "id": "CVE-2026-74635",
  "lastModified": "2026-10-14T13:10:00.187",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 7.8,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 1.8,
        "impactScore": 5.9,
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-08-22T16:16:36.590",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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