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

[David Hildenbrand <david@redhat.com>]

On 08.12.22 17:29, Peter Xu wrote:
> On Thu, Dec 08, 2022 at 12:41:37PM +0100, 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>
>> 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>
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> 
> One trivial nit.
> 
>> ---
>>
>> 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);
> 
> IIUC this can be "uffd_wp_changed" then switch "|" to "^".  But not a hot
> path at all, so shouldn't matter a lot.

Yes, let's do that (we can also remove the !! here):

This hunk will be:


diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 98ac37e34e3d..a988485ada05 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_changed = (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 changes.
+	 */
+	if ((vma->vm_flags & VM_SHARED) && uffd_wp_changed)
+		vma_set_page_prot(vma);
+}
+


I'll wait for some more (+retest) before I resend tomorrow.

Thanks!


-- 
Thanks,

David / dhildenb