{"id":"CVE-2026-74635","aliases":[],"url":"https://o3.security/vulnerability/CVE-2026-74635","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 — the same fallback bit_putcs uses.","published":"2026-08-22T15:32:15.280Z","modified":"2026-08-24T11:47:19.067229363Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Linux","name":"Kernel","fixedVersion":"5.10.265"}],"fix":null,"references":[{"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"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/74xxx/CVE-2026-74635.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-74635"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-24T11:47:19.067229363Z"}}