Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46 kfence_protect

[Dave Hansen <dave.hansen@xxxxxxxxx>]

On 11/17/22 05:58, Marco Elver wrote:
> [    0.663761] WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46 kfence_protect+0x7b/0x120
> [    0.664033] WARNING: CPU: 0 PID: 0 at mm/kfence/core.c:234 kfence_protect+0x7d/0x120
> [    0.664465] kfence: kfence_init failed

Any chance you could add some debugging and figure out what actually
made kfence call over?  Was it the pte or the level?

        if (WARN_ON(!pte || level != PG_LEVEL_4K))
                return false;

I can see how the thing you bisected to might lead to a page table not
being split, which could mess with the 'level' check.

Also, is there a reason this code is mucking with the page tables
directly?  It seems, uh, rather wonky.  This, for instance:

>         if (protect)
>                 set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
>         else
>                 set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
> 
>         /*
>          * Flush this CPU's TLB, assuming whoever did the allocation/free is
>          * likely to continue running on this CPU.
>          */
>         preempt_disable();
>         flush_tlb_one_kernel(addr);
>         preempt_enable();

Seems rather broken.  I assume the preempt_disable() is there to get rid
of some warnings.  But, there is nothing I can see to *keep* the CPU
that did the free from being different from the one where the TLB flush
is performed until the preempt_disable().  That makes the
flush_tlb_one_kernel() mostly useless.

Is there a reason this code isn't using the existing page table
manipulation functions and tries to code its own?  What prevents it from
using something like the attached patch?

diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h
index ff5c7134a37a..5cdb3a1f3995 100644
--- a/arch/x86/include/asm/kfence.h
+++ b/arch/x86/include/asm/kfence.h
@@ -37,34 +37,13 @@ static inline bool arch_kfence_init_pool(void)
 	return true;
 }
 
-/* Protect the given page and flush TLB. */
 static inline bool kfence_protect_page(unsigned long addr, bool protect)
 {
-	unsigned int level;
-	pte_t *pte = lookup_address(addr, &level);
-
-	if (WARN_ON(!pte || level != PG_LEVEL_4K))
-		return false;
-
-	/*
-	 * We need to avoid IPIs, as we may get KFENCE allocations or faults
-	 * with interrupts disabled. Therefore, the below is best-effort, and
-	 * does not flush TLBs on all CPUs. We can tolerate some inaccuracy;
-	 * lazy fault handling takes care of faults after the page is PRESENT.
-	 */
-
 	if (protect)
-		set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
+		set_memory_np(addr, addr + PAGE_SIZE);
 	else
-		set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
+		set_memory_p(addr, addr + PAGE_SIZE);
 
-	/*
-	 * Flush this CPU's TLB, assuming whoever did the allocation/free is
-	 * likely to continue running on this CPU.
-	 */
-	preempt_disable();
-	flush_tlb_one_kernel(addr);
-	preempt_enable();
 	return true;
 }