Re: [PATCH v1] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA
From: David Hildenbrand
Date: Thu Dec 08 2022 - 11:45:33 EST
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@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>
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>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
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@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);
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