{"id":"CVE-2026-74635","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-74635","generatedAt":"2026-10-27T23:55:51.331Z","title":"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);…","description":"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":"2026-08-22T16:16:37.000Z","lastModified":"2026-10-16T20:17:49.000Z","status":"Received","sourceIdentifier":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","cvss":{"version":"3.1","score":7.8,"severity":"HIGH","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"},"cwe":[],"affected":[],"epss":{"score":0.00129,"percentile":0.02543,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (0.13%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[],"references":[{"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","source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-74635","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}