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

[David Hildenbrand]

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@xxxxxxxxxx
>
> Reported-by: Ives van Hoorne <ives@xxxxxxxxxxxxxx>
> Debugged-by: Peter Xu <peterx@xxxxxxxxxx>
> Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hugh@xxxxxxxxxxx>

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.

> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> 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@xxxxxxxxxx
>
> ---
>   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