RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping

[Jianyong Wu]

Hi David,

> >>>>>> Thanks for tracking that down.
> >>>>>>
> >>>>>> Note that clearing the BSS twice is not the root problem here.
> >>>>>> The root problem is that we set global state while the kernel
> >>>>>> runs at the default link time address, and then refer to it again
> >>>>>> after the entire kernel has been shifted in the kernel VA space.
> >>>>>> Such global state could consist of mutable pointers to statically
> >>>>>> allocated data (which would be reset to their default values
> >>>>>> after the relocation code
> >>> runs again), or global pointer variables in BSS.
> >>>>>> In either case, relying on such a global variable after the
> >>>>>> second relocation performed by KASLR would be risky, and so we
> >>>>>> should avoid manipulating global state at all if it might involve
> >>>>>> pointer to statically allocated data structures.
> >>>>>>
> >>>>>>> In other ways, if we invoke mutex_lock/unlock in such a early
> >>>>>>> booting
> >>> stage.
> >>>>>>> It might be unsafe because lockdep inserts lock_acquire/release
> >>>>>>> as the complex hooks.
> >>>>>>>
> >>>>>>> In summary, would it better if Jianyong splits these early boot
> >>>>>>> and late boot case? e.g. introduce a nolock version for
> >>>>>> create_mapping_noalloc().
> >>>>>>>
> >>>>>>> What do you think of it?
> >>>>>>>
> >>>>>>
> >>>>>> The pre-KASLR case definitely doesn't need a lock. But given that
> >>>>>> create_mapping_noalloc() is only used to map the FDT, which
> >>>>>> happens very early one way or the other, wouldn't it be better to
> >>>>>> move the lock/unlock into other callers of
> >>>>>> __create_pgd_mapping()? (and make sure no other users of the
> >>>>>> fixmap slots exist)
> >>>>>
> >>>>> There are server callers of __create_pgd_mapping. I think some of
> >>>>> them
> >>> need no fixmap lock as they are called so early. I figure out all of them
> here:
> >>>>> create_mapping_noalloc:   no lock
> >>>>> create_pgd_mapping:   no lock
> >>>>> __map_memblock:    no lock
> >>>>> map_kernel_segment:  no lock
> >>>>> map_entry_trampoline: no lock
> >>>>> update_mapping_prot:    need lock
> >>>>> arch_add_memory:  need lock
> >>>>>
> >>>>> WDYT?
> >>>>>
> >>>>
> >>>> That seems reasonable, but it needs to be documented clearly in the
> code.
> >>>>
> >>>
> >>> Just a random thought, could we rely on system_state to do the
> >>> locking conditionally?
> >>
> >> I can't see the point. At the early stages of kernel boot, we definitely
> need no lock. Also, I think we should keep it simple.
> >>
> >
> > Is e.g.,
> >
> > if (system_state < SYSTEM_RUNNING)
> > 	/* lock */
> >
> > if (system_state < SYSTEM_RUNNING)
> > 	/* unlock */
> 
> of course, inverting the conditions ;)

Yes, system_state can roughly separate these callers of __create_pgd_mapping. When system_state > SYSTEM_BOOTING we can add the lock.
Thus, I have the following change:

static DEFINE_SPINLOCK(swapper_pgdir_lock);
+static DEFINE_MUTEX(fixmap_lock);

 void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
 {
@@ -329,6 +330,8 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
        }
        BUG_ON(p4d_bad(p4d));

+       if (system_state > SYSTEM_BOOTING)
+               mutex_lock(&fixmap_lock);
        pudp = pud_set_fixmap_offset(p4dp, addr);
        do {
                pud_t old_pud = READ_ONCE(*pudp);
@@ -359,6 +362,8 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
        } while (pudp++, addr = next, addr != end);

        pud_clear_fixmap();
+       if (system_state > SYSTEM_BOOTING)
+               mutex_unlock(&fixmap_lock);
 }

It seems work and somehow simper. But I don't know if it is reasonable to do this. So, any idea? @Ard Biesheuvel  @Catalin Marinas 

Thanks
Jianyong

> 
> 
> --
> Thanks,
> 
> David / dhildenb