Re: [5.10] regression: virtual consoles 2-12 unusable

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

On Thu, Jan 15, 2026 at 12:41:10AM -0800, Barry K. Nathan wrote:
> On 1/8/26 5:23 AM, Greg Kroah-Hartman wrote:
> > 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
> 
> For what it's worth, now that the above commits are queued up for 5.10.y:
> There are two more commits, which were previously applied to 5.15.y, that
> now apply to 5.10.y without merge conflicts (also without fuzz, if you apply
> the 5.15.y versions of the patches):
> 
> 
> a5a923038d70
> fbdev: fbcon: Properly revert changes when vc_resize() failed
> (previously applied to 5.15.64 and 5.19.6)
> 
> 3c3bfb8586f8
> fbdev: fbcon: release buffer when fbcon_do_set_font() failed
> (previously applied to 5.15.86, 6.0.16, and 6.1.2)
> 
> 
> After looking at these two commits, it seems to me that they are now
> applicable to 5.10.y, and I think they probably should be applied (unless
> I'm overlooking or missing something).

Both now applied, thanks!

greg k-h