Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jan 02, 2026 at 05:26:22PM +0100, Ben Hutchings wrote:
> 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

All now queued up, thanks!

greg k-h