KASLR vs. KASAN on x86

[Dave Hansen]

Hi KASAN folks,

Currently, x86 disables (most) KASLR when KASAN is enabled:

> /*
>  * Apply no randomization if KASLR was disabled at boot or if KASAN
>  * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
>  */
> static inline bool kaslr_memory_enabled(void)
> {
>         return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
> }

I'm a bit confused by this, though.  This code predates 5-level paging
so a PGD should be assumed to be 512G.  The kernel_randomize_memory()
granularity seems to be 1 TB, which *is* PGD-aligned.

Are KASAN and kernel_randomize_memory()/KASLR (modules and
cpu_entry_area randomization is separate) really incompatible?  Does
anyone have a more thorough explanation than that comment?

This isn't a big deal since KASAN is a debugging option after all.  But,
I'm trying to unravel why this:

>         if (kaslr_enabled()) {
>                 pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
>                          kaslr_offset(),
>                          __START_KERNEL,
>                          __START_KERNEL_map,
>                          MODULES_VADDR-1);

for instance uses kaslr_enabled() which includes just randomizing
module_load_offset, but *not* __START_KERNEL.  I think this case should
be using kaslr_memory_enabled() to match up with the check in
kernel_randomize_memory().  But this really boils down to what the
difference is between kaslr_memory_enabled() and kaslr_enabled().