{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2026-74635",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2026-74635"
  ],
  "published": "2026-08-22T15:32:12Z",
  "summary": "fbdev: bitblit: bound-check glyph index in bit_cursor()",
  "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 \u2014 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": [
    {
      "package": {
        "ecosystem": "Raspberry Pi OS:12",
        "name": "linux"
      },
      "versions": [
        "1:6.12.62-1+rpt1~bookworm",
        "1:6.12.75-1+rpt1~bookworm",
        "1:6.12.87-1+rpt1~bookworm",
        "1:6.12.93-1+rpt1",
        "1:6.12.96-1+rpt1"
      ],
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-2712",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v6",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v7",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v7l",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v8",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.62-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-2712",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v6",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v7",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v7l",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v8",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.75-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-2712",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-v6",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-v7",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-v7l",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-v8",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.87+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.87-1+rpt1~bookworm"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-2712",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-v6",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-v7",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-v7l",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-v8",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.93+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.93-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-2712",
            "binary_version": "1:6.12.96-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-v6",
            "binary_version": "1:6.12.96-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-v7",
            "binary_version": "1:6.12.96-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-v7l",
            "binary_version": "1:6.12.96-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-v8",
            "binary_version": "1:6.12.96-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.96+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.96-1+rpt1"
          }
        ]
      },
      "database_specific": {
        "per_version": {
          "1:6.12.62-1+rpt1~bookworm": {
            "linux_commit": "a1073743767f9e7fdc7017ababd2a07ea0c97c1c",
            "upstream_version": "6.12.62",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.12.75-1+rpt1~bookworm": {
            "linux_commit": "89050b1059997d38d55462b323b099a6436dc10d",
            "upstream_version": "6.12.75",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.12.87-1+rpt1~bookworm": {
            "linux_commit": "ae4d75fb36deba0fe1a986d9bfae65755e082dd0",
            "upstream_version": "6.12.87",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.12.93-1+rpt1": {
            "linux_commit": "d8ab4e908235da7727f22dd36ad5af224671677d",
            "upstream_version": "6.12.93",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.12.96-1+rpt1": {
            "linux_commit": "ae161617d7a4552fa91d03626b8d9f3696d17481",
            "upstream_version": "6.12.96",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          }
        }
      }
    },
    {
      "package": {
        "ecosystem": "Raspberry Pi OS:13",
        "name": "linux"
      },
      "versions": [
        "1:6.12.62-1+rpt1",
        "1:6.12.75-1+rpt1",
        "1:6.18.29-1+rpt1",
        "1:6.18.33-1+rpt1",
        "1:6.18.34-1+rpt1",
        "1:6.18.39-1+rpt1"
      ],
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-2712",
            "binary_version": "1:6.12.62-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v6",
            "binary_version": "1:6.12.62-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v7",
            "binary_version": "1:6.12.62-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v8",
            "binary_version": "1:6.12.62-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.62+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.62-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-2712",
            "binary_version": "1:6.12.75-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v6",
            "binary_version": "1:6.12.75-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v7",
            "binary_version": "1:6.12.75-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v8",
            "binary_version": "1:6.12.75-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.12.75+rpt-rpi-v8-rt",
            "binary_version": "1:6.12.75-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.29+rpt-rpi-2712",
            "binary_version": "1:6.18.29-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.29+rpt-rpi-v6",
            "binary_version": "1:6.18.29-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.29+rpt-rpi-v7",
            "binary_version": "1:6.18.29-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.29+rpt-rpi-v8",
            "binary_version": "1:6.18.29-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.29+rpt-rpi-v8-rt",
            "binary_version": "1:6.18.29-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.33+rpt-rpi-2712",
            "binary_version": "1:6.18.33-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.33+rpt-rpi-v6",
            "binary_version": "1:6.18.33-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.33+rpt-rpi-v7",
            "binary_version": "1:6.18.33-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.33+rpt-rpi-v8",
            "binary_version": "1:6.18.33-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.33+rpt-rpi-v8-rt",
            "binary_version": "1:6.18.33-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.34+rpt-rpi-2712",
            "binary_version": "1:6.18.34-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.34+rpt-rpi-v6",
            "binary_version": "1:6.18.34-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.34+rpt-rpi-v7",
            "binary_version": "1:6.18.34-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.34+rpt-rpi-v8",
            "binary_version": "1:6.18.34-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.34+rpt-rpi-v8-rt",
            "binary_version": "1:6.18.34-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.39+rpt-rpi-2712",
            "binary_version": "1:6.18.39-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.39+rpt-rpi-v6",
            "binary_version": "1:6.18.39-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.39+rpt-rpi-v7",
            "binary_version": "1:6.18.39-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.39+rpt-rpi-v8",
            "binary_version": "1:6.18.39-1+rpt1"
          },
          {
            "binary_name": "linux-image-6.18.39+rpt-rpi-v8-rt",
            "binary_version": "1:6.18.39-1+rpt1"
          }
        ]
      },
      "database_specific": {
        "per_version": {
          "1:6.12.62-1+rpt1": {
            "linux_commit": "a1073743767f9e7fdc7017ababd2a07ea0c97c1c",
            "upstream_version": "6.12.62",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.12.75-1+rpt1": {
            "linux_commit": "89050b1059997d38d55462b323b099a6436dc10d",
            "upstream_version": "6.12.75",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.12.104"
          },
          "1:6.18.29-1+rpt1": {
            "linux_commit": "66318c5b5cd8145a662303af12ed39ac07eb27cc",
            "upstream_version": "6.18.29",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.18.45"
          },
          "1:6.18.33-1+rpt1": {
            "linux_commit": "95b85bebbedcaedfa7ca79116ed38b7376fba412",
            "upstream_version": "6.18.33",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.18.45"
          },
          "1:6.18.34-1+rpt1": {
            "linux_commit": "c8c7494100e99ee05b11aaa4f0588a223a63d1af",
            "upstream_version": "6.18.34",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.18.45"
          },
          "1:6.18.39-1+rpt1": {
            "linux_commit": "60ea684a8ace97bb0db1a16e20753bdd6ab371ff",
            "upstream_version": "6.18.39",
            "assessment": "commit ancestry (strak rule)",
            "upstream_fixed": "6.18.45"
          }
        }
      }
    }
  ],
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-74635"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d"
    }
  ],
  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-08-25T05:42:41Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - Exploitation requires local write() to /dev/vcsa (or ioctl on an owned virtual console) to poison the VT screen buffer; the bug is in fbcon cursor rendering, not in any network-facing protocol handler.\nAC:L - Syzkaller reproduces the fault reliably by writing arbitrary screen-buffer values to /dev/vcsa; the attacker fully controls the glyph index and can repeat the trigger on every console refresh without races or victim-specific timing.\nPR:L - Triggering needs only a local account with write access to /dev/vcsa* (typically membership in the tty group) or a session owning the target console for KDFONTOP/font-switch paths; no CAP_SYS_ADMIN or init-namespace root is required.\nUI:N - Once the attacker has the needed local device access, exploitation is fully self-contained and does not require any additional action from another user or administrator.\nS:U - Impact is confined to kernel framebuffer-console memory within the same kernel security domain; it does not cross VM, container, or IOMMU boundaries to affect a separate authority.\nC:H - An out-of-bounds read past the font glyph table copies adjacent kernel memory (global or heap font data) into cursor buffers and framebuffer blit paths, disclosing more than a few bytes per trigger.\nI:H - The out-of-bounds glyph pointer feeds update_attr() and soft_cursor() memcpy paths that copy attacker-influenced kernel memory into kmalloc cursor buffers and framebuffer state, constituting kernel memory corruption with integrity impact.\nA:H - KASAN reports a kernel BUG on the out-of-bounds read in soft_cursor(), and repeated malicious /dev/vcsa writes force console refresh through hide_cursor(), enabling kernel oops/panic and console denial of service."
    }
  }
}