{"id":"CVE-2026-74635","revision":"aeb15d6ea3e44113ac50a97adc8754aa","records":[{"revision":"aeb15d6ea3e44113ac50a97adc8754aa","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-74635","aliases":[],"title":"fbdev: bitblit: bound-check glyph index in bit_cursor()","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.","state":0,"publishedAt":"2026-08-22T15:32:15.28+00:00","modifiedAt":"2026-10-16T19:06:04.057+00:00","affected":[{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"unaffected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"drivers/video/fbdev/core/bitblit.c\"\n ],\n \"versions\": [\n {\n \"version\": \"0998a6cb232674408a03e8561dc15aa266b2f53b\",\n \"lessThan\": \"250159ace2dc53c1bdad267aa8da51b638748700\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"db5c9a162d2f42bcc842b76b3d935dcc050a0eec\",\n \"lessThan\": \"46336f476484f36145e5117e72d7b590f47433ee\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"c12003bf91fdff381c55ef54fef3e961a5af2545\",\n \"lessThan\": \"bf750cfeacf4e47ac72dadc7f05839696efb8576\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"9ba1a7802ca9a2590cef95b253e6526f4364477f\",\n \"lessThan\": \"94134d70abf9273b70499d97d0adc9185ef21091\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"901f44227072be60812fe8083e83e1533c04eed1\",\n \"lessThan\": \"c1e7351767dd30fc574395c82121e4c67b882da3\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"18c4ef4e765a798b47980555ed665d78b71aeadf\",\n \"lessThan\": \"bc9db0d879c655d5dfd8add32fd60f13e65d132c\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"18c4ef4e765a798b47980555ed665d78b71aeadf\",\n \"lessThan\": \"9ea879862e66e354e616028c31b39aa3eb6d35d8\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"18c4ef4e765a798b47980555ed665d78b71aeadf\",\n \"lessThan\": \"e033cbf3975a8465f879ebd5989dc35b04423a4d\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"a10cede006f9614b465cf25609a8753efbfd45cc\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"efaf89a75a29b2d179bf4fe63ca62852e93ad620\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"5.10.247\",\n \"lessThan\": \"5.10.265\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.15.197\",\n \"lessThan\": \"5.15.216\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.1.159\",\n \"lessThan\": \"6.1.183\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.6.117\",\n \"lessThan\": \"6.6.152\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.12.58\",\n \"lessThan\": \"6.12.104\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.4.302\",\n \"lessThan\": \"5.5\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.17.8\",\n \"lessThan\": \"6.18\",\n \"status\": \"affected\",\n \"versionType\": \"semver\"\n }\n ]\n }"},{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"affected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"drivers/video/fbdev/core/bitblit.c\"\n ],\n \"versions\": [\n {\n \"version\": \"6.18\",\n \"status\": \"affected\"\n },\n {\n \"version\": \"0\",\n \"lessThan\": \"6.18\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.10.265\",\n \"lessThanOrEqual\": \"5.10.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"5.15.216\",\n \"lessThanOrEqual\": \"5.15.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.1.183\",\n \"lessThanOrEqual\": \"6.1.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.6.152\",\n \"lessThanOrEqual\": \"6.6.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.12.104\",\n \"lessThanOrEqual\": \"6.12.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.18.45\",\n \"lessThanOrEqual\": \"6.18.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.1.9\",\n \"lessThanOrEqual\": \"7.1.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.2\",\n \"lessThanOrEqual\": \"*\",\n \"status\": \"unaffected\",\n \"versionType\": \"original_commit_for_fix\"\n }\n ]\n }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","score":7.8,"severity":"HIGH"}],"references":["https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700","https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee","https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576","https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091","https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3","https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c","https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8","https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-74635","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-74635","identifiers":["CVE-2026-74635"]},"retained":false,"historicalAffected":[]}],"coverage":[]}