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SubjectRe: [PATCH v1 2/2] arm64/mm: Add uffd write-protect support
From
+ Muhammad Usama Anjum <usama.anjum@collabora.com>

Hi Peter, Muhammad,


On 24/04/2024 12:57, Peter Xu wrote:
> Hi, Ryan,
>
> On Wed, Apr 24, 2024 at 12:10:17PM +0100, Ryan Roberts wrote:
>> Let's use the newly-free PTE SW bit (58) to add support for uffd-wp.
>>
>> The standard handlers are implemented for set/test/clear for both pte
>> and pmd. Additionally we must also track the uffd-wp state as a pte swp
>> bit, so use a free swap entry pte bit (3).
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>
> Looks all sane here from userfault perspective, just one comment below.
>
>> ---
>> arch/arm64/Kconfig | 1 +
>> arch/arm64/include/asm/pgtable-prot.h | 8 ++++
>> arch/arm64/include/asm/pgtable.h | 55 +++++++++++++++++++++++++++
>> 3 files changed, 64 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 7b11c98b3e84..763e221f2169 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -255,6 +255,7 @@ config ARM64
>> select SYSCTL_EXCEPTION_TRACE
>> select THREAD_INFO_IN_TASK
>> select HAVE_ARCH_USERFAULTFD_MINOR if USERFAULTFD
>> + select HAVE_ARCH_USERFAULTFD_WP if USERFAULTFD
>> select TRACE_IRQFLAGS_SUPPORT
>> select TRACE_IRQFLAGS_NMI_SUPPORT
>> select HAVE_SOFTIRQ_ON_OWN_STACK
>> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
>> index ef952d69fd04..f1e1f6306e03 100644
>> --- a/arch/arm64/include/asm/pgtable-prot.h
>> +++ b/arch/arm64/include/asm/pgtable-prot.h
>> @@ -20,6 +20,14 @@
>> #define PTE_DEVMAP (_AT(pteval_t, 1) << 57)
>> #define PTE_PROT_NONE (PTE_UXN) /* Reuse PTE_UXN; only when !PTE_VALID */
>>
>> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
>> +#define PTE_UFFD_WP (_AT(pteval_t, 1) << 58) /* uffd-wp tracking */
>> +#define PTE_SWP_UFFD_WP (_AT(pteval_t, 1) << 3) /* only for swp ptes */

I've just noticed code in task_mmu.c:

static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
unsigned long end, struct mm_walk *walk)
{
...

if (!p->arg.category_anyof_mask && !p->arg.category_inverted &&
p->arg.category_mask == PAGE_IS_WRITTEN &&
p->arg.return_mask == PAGE_IS_WRITTEN) {
for (addr = start; addr < end; pte++, addr += PAGE_SIZE) {
unsigned long next = addr + PAGE_SIZE;

if (pte_uffd_wp(ptep_get(pte))) <<<<<<
continue;

...
}
}
}

As far as I can see, you don't know that the pte is present when you do this. So
does this imply that the UFFD-WP bit is expected to be in the same position for
both present ptes and swap ptes? I had assumed pte_uffd_wp() was for present
ptes and pte_swp_uffd_wp() was for swap ptes.

As you can see, the way I've implemented this for arm64 the bit is in a
different position for these 2 cases. I've just done a slightly different
implementation that changes the first patch in this series quite a bit and a
bunch of pagemap_ioctl mm kselftests are now failing. I think this is the root
cause, but haven't proven it definitively yet.

I'm inclined towords thinking the above is a bug and should be fixed so that I
can store the bit in different places. What do you think?

Thanks,
Ryan

>> +#else
>> +#define PTE_UFFD_WP (_AT(pteval_t, 0))
>> +#define PTE_SWP_UFFD_WP (_AT(pteval_t, 0))
>> +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
>> +
>> /*
>> * This bit indicates that the entry is present i.e. pmd_page()
>> * still points to a valid huge page in memory even if the pmd
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 23aabff4fa6f..3f4748741fdb 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -271,6 +271,34 @@ static inline pte_t pte_mkdevmap(pte_t pte)
>> return set_pte_bit(pte, __pgprot(PTE_DEVMAP | PTE_SPECIAL));
>> }
>>
>> +#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
>> +static inline int pte_uffd_wp(pte_t pte)
>> +{
>> + bool wp = !!(pte_val(pte) & PTE_UFFD_WP);
>> +
>> +#ifdef CONFIG_DEBUG_VM
>> + /*
>> + * Having write bit for wr-protect-marked present ptes is fatal, because
>> + * it means the uffd-wp bit will be ignored and write will just go
>> + * through. See comment in x86 implementation.
>> + */
>> + WARN_ON_ONCE(wp && pte_write(pte));
>> +#endif
>
> Feel free to drop this line, see:
>
> https://lore.kernel.org/r/20240417212549.2766883-1-peterx@redhat.com
>
> It's still in mm-unstable only.
>
> AFAICT ARM64 also is supported by check_page_table, I also checked ARM's
> ptep_modify_prot_commit() which uses set_pte_at(), so it should cover
> everything in a superior way already.
>
> With that dropped, feel free to add:
>
> Acked-by: Peter Xu <peterx@redhat.com>
>
> Thanks,
>


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