From: Jianyong Wu <Jianyong.Wu@arm.com>
To: David Hildenbrand <david@redhat.com>,
Ard Biesheuvel <ardb@kernel.org>,
Catalin Marinas <Catalin.Marinas@arm.com>
Cc: Justin He <Justin.He@arm.com>,
"will@kernel.org" <will@kernel.org>,
Anshuman Khandual <Anshuman.Khandual@arm.com>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"quic_qiancai@quicinc.com" <quic_qiancai@quicinc.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"gshan@redhat.com" <gshan@redhat.com>, nd <nd@arm.com>
Subject: RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping
Date: Thu, 27 Jan 2022 06:24:50 +0000 [thread overview]
Message-ID: <AM9PR08MB7276062BC7B474174FFFA11BF4219@AM9PR08MB7276.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <7eb4bc77-c1db-99c4-4c77-ae9ddd159abb@redhat.com>
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
next prev parent reply other threads:[~2022-01-27 6:25 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-16 8:28 [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping Jianyong Wu
2021-12-16 15:19 ` David Hildenbrand
2021-12-17 9:30 ` Mark Rutland
2021-12-17 10:09 ` Jianyong Wu
2022-01-05 18:03 ` Catalin Marinas
2022-01-06 10:13 ` Jianyong Wu
2022-01-06 15:56 ` Catalin Marinas
2022-01-07 9:10 ` Jianyong Wu
2022-01-07 10:42 ` Catalin Marinas
2022-01-26 4:20 ` Justin He
2022-01-26 8:36 ` Ard Biesheuvel
2022-01-26 10:09 ` Jianyong Wu
2022-01-26 10:12 ` Ard Biesheuvel
2022-01-26 10:17 ` David Hildenbrand
2022-01-26 10:28 ` Jianyong Wu
2022-01-26 10:30 ` David Hildenbrand
2022-01-26 10:31 ` David Hildenbrand
2022-01-27 6:24 ` Jianyong Wu [this message]
2022-01-27 12:22 ` David Hildenbrand
2022-01-27 12:34 ` Catalin Marinas
2022-01-31 8:13 ` Jianyong Wu
2022-01-31 8:10 ` Jianyong Wu
2022-01-27 1:31 ` Justin He
2022-01-07 10:53 ` Catalin Marinas
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