drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup Protect vga_switcheroo_client_fb_set() with console lock. Avoids OOB access in fbcon_remap_all(). Without holding the console lock the call races with switching outputs. VGA switcheroo calls fbcon_remap_all() when switching clients. The fbcon function uses struct fb_info.node, which is set by register_framebuffer(). As the fb-helper code currently sets up VGA switcheroo before registering the framebuffer, the value of node is -1 and therefore not a legal value. For example, fbcon uses the value within set_con2fb_map() [1] as an index into an array. Moving vga_switcheroo_client_fb_set() after register_framebuffer() can result in VGA switching that does not switch fbcon correctly. Therefore move vga_switcheroo_client_fb_set() under fbcon_fb_registered(), which already holds the console lock. Fbdev calls fbcon_fb_registered() from within register_framebuffer(). Serializes the helper with VGA switcheroo's call to fbcon_remap_all(). Although vga_switcheroo_client_fb_set() takes an instance of struct fb_info as parameter, it really only needs the contained fbcon state. Moving the call to fbcon initialization is therefore cleaner than before. Only amdgpu, i915, nouveau and radeon support vga_switcheroo. For all other drivers, this change does nothing.

References

• https://git.kernel.org/stable/c/711ebd961190def4c69ea24b2f0be75e995af24a
• https://git.kernel.org/stable/c/482330f8261b4bea8146d9bd69c1199e5dfcbb5c
• https://git.kernel.org/stable/c/05814c389b53d2f3a0b9eeb90ba7a05ba77c4c2a
• https://git.kernel.org/stable/c/eb76d0f5553575599561010f24c277cc5b31d003