{"id":"DEBIAN-CVE-2026-74635","details":"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  \tc = scr_readw(vc_pos); \tsrc = 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.","modified":"2026-09-19T22:00:12.616224277Z","published":"2026-08-22T16:16:36.590Z","upstream":["CVE-2026-74635"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2026-74635"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:12","purl":"pkg:deb/debian/linux?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.1.187-1"}]}],"versions":["6.1.106-1","6.1.106-2","6.1.106-3","6.1.112-1","6.1.115-1","6.1.119-1","6.1.123-1","6.1.124-1","6.1.128-1","6.1.129-1","6.1.133-1","6.1.135-1","6.1.137-1","6.1.139-1","6.1.140-1","6.1.147-1","6.1.148-1","6.1.153-1","6.1.158-1","6.1.159-1","6.1.162-1","6.1.164-1","6.1.170-1","6.1.170-2","6.1.170-3","6.1.172-1","6.1.174-1","6.1.176-1","6.1.177-1","6.1.180-1","6.1.27-1","6.1.37-1","6.1.38-1","6.1.38-2","6.1.38-2~bpo11+1","6.1.38-3","6.1.38-4","6.1.38-4~bpo11+1","6.1.52-1","6.1.55-1","6.1.55-1~bpo11+1","6.1.64-1","6.1.66-1","6.1.67-1","6.1.69-1","6.1.69-1~bpo11+1","6.1.76-1","6.1.76-1~bpo11+1","6.1.82-1","6.1.85-1","6.1.90-1","6.1.90-1~bpo11+1","6.1.94-1","6.1.94-1~bpo11+1","6.1.98-1","6.1.99-1"],"ecosystem_specific":{"urgency":"not 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