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e033cbf397
bit_cursor() fetches the glyph under the cursor with c = scr_readw(vc_pos); src = 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 |
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| .. | ||
| bitblit.c | ||
| cfbcopyarea.c | ||
| cfbfillrect.c | ||
| cfbimgblt.c | ||
| cfbmem.h | ||
| fb_backlight.c | ||
| fb_chrdev.c | ||
| fb_cmdline.c | ||
| fb_copyarea.h | ||
| fb_ddc.c | ||
| fb_defio.c | ||
| fb_draw.h | ||
| fb_fillrect.h | ||
| fb_imageblit.h | ||
| fb_info.c | ||
| fb_internal.h | ||
| fb_io_fops.c | ||
| fb_logo.c | ||
| fb_notify.c | ||
| fb_procfs.c | ||
| fb_sys_fops.c | ||
| fbcmap.c | ||
| fbcon_ccw.c | ||
| fbcon_cw.c | ||
| fbcon_rotate.c | ||
| fbcon_rotate.h | ||
| fbcon_ud.c | ||
| fbcon.c | ||
| fbcon.h | ||
| fbcvt.c | ||
| fbmem.c | ||
| fbmon.c | ||
| fbsysfs.c | ||
| Kconfig | ||
| Makefile | ||
| modedb.c | ||
| softcursor.c | ||
| svgalib.c | ||
| syscopyarea.c | ||
| sysfillrect.c | ||
| sysimgblt.c | ||
| sysmem.h | ||
| tileblit.c | ||