Re: [PATCH v1 2/2] arm64/mm: Add uffd write-protect support

[Ryan Roberts <ryan.roberts@arm.com>]

+ 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,
>