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

[Naresh Kamboju]

On Thu, 17 Nov 2022 at 20:04, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> 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?

I have applied this patch and found build warnings / errors.

In file included from mm/kfence/core.c:34:
arch/x86/include/asm/kfence.h: In function 'kfence_protect_page':
arch/x86/include/asm/kfence.h:45:17: error: implicit declaration of
function 'set_memory_p'; did you mean 'set_memory_np'?
[-Werror=implicit-function-declaration]
   45 |                 set_memory_p(addr, addr + PAGE_SIZE);
      |                 ^~~~~~~~~~~~
      |                 set_memory_np
cc1: all warnings being treated as errors
make[4]: *** [scripts/Makefile.build:250: mm/kfence/core.o] Error 1
In file included from mm/kfence/report.c:20:
arch/x86/include/asm/kfence.h: In function 'kfence_protect_page':
arch/x86/include/asm/kfence.h:45:17: error: implicit declaration of
function 'set_memory_p'; did you mean 'set_memory_np'?
[-Werror=implicit-function-declaration]
   45 |                 set_memory_p(addr, addr + PAGE_SIZE);
      |                 ^~~~~~~~~~~~
      |                 set_memory_np
cc1: all warnings being treated as errors
make[4]: *** [scripts/Makefile.build:250: mm/kfence/report.o] Error 1
In file included from mm/kfence/kfence_test.c:26:
arch/x86/include/asm/kfence.h: In function 'kfence_protect_page':
arch/x86/include/asm/kfence.h:45:17: error: implicit declaration of
function 'set_memory_p'; did you mean 'set_memory_np'?
[-Werror=implicit-function-declaration]
   45 |                 set_memory_p(addr, addr + PAGE_SIZE);
      |                 ^~~~~~~~~~~~
      |                 set_memory_np
cc1: all warnings being treated as errors
make[4]: *** [scripts/Makefile.build:250: mm/kfence/kfence_test.o] Error 1

ref:
https://builds.tuxbuild.com/2HqMWcweeInju7rqVgGdNge7gby/

- Naresh