XPost: linux.debian.bugs.dist   
   From: ben@decadent.org.uk   
      
   Hello stable maintainers,   
      
   Several Debian users reported a regression after updating to kernel   
   version 5.10.247.   
      
   Commit f0982400648a ("fbdev: Add bounds checking in bit_putcs to fix   
   vmalloc-out-of-bounds"), a backport of upstream commit 3637d34b35b2,   
   depends on vc_data::vc_font.charcount being initialised correctly.   
      
   However, before commit a1ac250a82a5 ("fbcon: Avoid using FNTCHARCNT()   
   and hard-coded built-in font charcount") in 5.11, this member was set   
   to 256 for VTs initially created with a built-in font and 0 for VTs   
   initially created with a user font.   
      
   Since Debian normally sets a user font before creating VTs 2 and up,   
   those additional VTs became unusable.  VT 1 also doesn't work correctly   
   if the user font has > 256 characters, and the bounds check is   
   ineffective if it has < 256 characters.   
      
   This can be fixed by backporting the following commits from 5.11:   
      
   7a089ec7d77f console: Delete unused con_font_copy() callback implementations   
   259a252c1f4e console: Delete dummy con_font_set() and con_font_default()   
   callback implementations   
   4ee573086bd8 Fonts: Add charcount field to font_desc   
   4497364e5f61 parisc/sticore: Avoid hard-coding built-in font charcount   
   a1ac250a82a5 fbcon: Avoid using FNTCHARCNT() and hard-coded built-in font   
   charcount   
      
   These all apply without fuzz and builds cleanly for x86_64 and parisc64.   
      
   I tested on x86_64 that:   
      
   - VT 2 works again   
   - bit_putcs_aligned() is setting charcnt = 256   
   - After loading a font with 512 characters, bit_putcs_aligned() sets   
     charcnt = 512 and is able to display characters at positions >= 256   
      
   Ben.   
      
   --    
   Ben Hutchings   
   Man invented language to satisfy his deep need to complain.   
                                                             - Lily Tomlin   
      
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