Re: [PATCH v1] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

[David Hildenbrand <david@redhat.com>]

On 08.12.22 12:45, David Hildenbrand wrote:
> On 08.12.22 12:41, David Hildenbrand wrote:
>> Currently, we don't enable writenotify when enabling userfaultfd-wp on
>> a shared writable mapping (for now only shmem and hugetlb). The consequence
>> is that vma->vm_page_prot will still include write permissions, to be set
>> as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
>> page migration, ...).
>>
>> So far, vma->vm_page_prot is assumed to be a safe default, meaning that
>> we only add permissions (e.g., mkwrite) but not remove permissions (e.g.,
>> wrprotect). For example, when enabling softdirty tracking, we enable
>> writenotify. With uffd-wp on shared mappings, that changed. More details
>> on vma->vm_page_prot semantics were summarized in [1].
>>
>> This is problematic for uffd-wp: we'd have to manually check for
>> a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error
>> prone. Prone to such issues is any code that uses vma->vm_page_prot to set
>> PTE permissions: primarily pte_modify() and mk_pte().
>>
>> Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped
>> write-protected as default and we will only allow selected PTEs that are
>> definitely safe to be mapped without write-protection (see
>> can_change_pte_writable()) to be writable. In the future, we might want
>> to enable write-bit recovery -- e.g., can_change_pte_writable() -- at
>> more locations, for example, also when removing uffd-wp protection.
>>
>> This fixes two known cases:
>>
>> (a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting
>>       in uffd-wp not triggering on write access.
>> (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs
>>       writable, resulting in uffd-wp not triggering on write access.
>>
>> Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even
>> without NUMA hinting (which currently doesn't seem to be applicable to
>> shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA.
>> On such a VMA, userfaultfd-wp is currently non-functional.
>>
>> Note that when enabling userfaultfd-wp, there is no need to walk page
>> tables to enforce the new default protection for the PTEs: we know that
>> they cannot be uffd-wp'ed yet, because that can only happen after
>> enabling uffd-wp for the VMA in general.
>>
>> Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not
>> accidentally set the write bit -- which would result in uffd-wp not
>> triggering on later write access. This commit makes uffd-wp on shmem behave
>> just like uffd-wp on anonymous memory (iow, less special) in that regard,
>> even though, mixing mprotect with uffd-wp is controversial.
>>
>> [1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.com
>>
>> Reported-by: Ives van Hoorne <ives@codesandbox.io>
>> Debugged-by: Peter Xu <peterx@redhat.com>
>> Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
>> Cc: stable@vger.kernel.org
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Hugh Dickins <hugh@veritas.com>
> 
> No idea how a wrong mail address from Hugh sneaked in 2 (I assume,
> copy-paste issue from de1ccfb64824). Let's properly cc him and keep the
> full patch.

This time really ;)

> 
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> Cc: Nadav Amit <nadav.amit@gmail.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>
>> As discussed in [2], this is supposed to replace the fix by Peter:
>>     [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover
>>     pte
>>
>> This survives vm/selftests and my reproducers:
>> * migrating pages that are uffd-wp'ed using mbind() on a machine with 2
>>     NUMA nodes
>> * Using a PROT_WRITE mapping with uffd-wp
>> * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and
>>     mprotect()'ing it PROT_WRITE
>> * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and
>>     temporarily mprotect()'ing it PROT_READ
>>
>> uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers
>> fail.
>>
>> It would be good to get some more testing feedback and review.
>>
>> [2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com
>>
>> ---
>>    fs/userfaultfd.c | 28 ++++++++++++++++++++++------
>>    mm/mmap.c        |  4 ++++
>>    2 files changed, 26 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>> index 98ac37e34e3d..fb0733f2e623 100644
>> --- a/fs/userfaultfd.c
>> +++ b/fs/userfaultfd.c
>> @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
>>    	return ctx->features & UFFD_FEATURE_INITIALIZED;
>>    }
>>    
>> +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
>> +				     vm_flags_t flags)
>> +{
>> +	const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
>> +
>> +	vma->vm_flags = flags;
>> +	/*
>> +	 * For shared mappings, we want to enable writenotify while
>> +	 * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
>> +	 * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.
>> +	 */
>> +	if ((vma->vm_flags & VM_SHARED) && uffd_wp)
>> +		vma_set_page_prot(vma);
>> +}
>> +
>>    static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
>>    				     int wake_flags, void *key)
>>    {
>> @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>>    		for_each_vma(vmi, vma) {
>>    			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
>>    				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>> -				vma->vm_flags &= ~__VM_UFFD_FLAGS;
>> +				userfaultfd_set_vm_flags(vma,
>> +							 vma->vm_flags & ~__VM_UFFD_FLAGS);
>>    			}
>>    		}
>>    		mmap_write_unlock(mm);
>> @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>>    	octx = vma->vm_userfaultfd_ctx.ctx;
>>    	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
>>    		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>> -		vma->vm_flags &= ~__VM_UFFD_FLAGS;
>> +		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
>>    		return 0;
>>    	}
>>    
>> @@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>>    	} else {
>>    		/* Drop uffd context if remap feature not enabled */
>>    		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>> -		vma->vm_flags &= ~__VM_UFFD_FLAGS;
>> +		userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
>>    	}
>>    }
>>    
>> @@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
>>    			prev = vma;
>>    		}
>>    
>> -		vma->vm_flags = new_flags;
>> +		userfaultfd_set_vm_flags(vma, new_flags);
>>    		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>>    	}
>>    	mmap_write_unlock(mm);
>> @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>>    		 * the next vma was merged into the current one and
>>    		 * the current one has not been updated yet.
>>    		 */
>> -		vma->vm_flags = new_flags;
>> +		userfaultfd_set_vm_flags(vma, new_flags);
>>    		vma->vm_userfaultfd_ctx.ctx = ctx;
>>    
>>    		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
>> @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>>    		 * the next vma was merged into the current one and
>>    		 * the current one has not been updated yet.
>>    		 */
>> -		vma->vm_flags = new_flags;
>> +		userfaultfd_set_vm_flags(vma, new_flags);
>>    		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>>    
>>    	skip:
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index a5eb2f175da0..6033d20198b0 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
>>    	if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma))
>>    		return 1;
>>    
>> +	/* Do we need write faults for uffd-wp tracking? */
>> +	if (userfaultfd_wp(vma))
>> +		return 1;
>> +
>>    	/* Specialty mapping? */
>>    	if (vm_flags & VM_PFNMAP)
>>    		return 0;
>>
>> base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b
> 

-- 
Thanks,

David / dhildenb