The patch titled Subject: mm/userfaultfd: fix uninitialized output field for -EAGAIN race has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-userfaultfd-fix-uninitialized-output-field-for-eagain-race.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Peter Xu peterx@redhat.com Subject: mm/userfaultfd: fix uninitialized output field for -EAGAIN race Date: Thu, 24 Apr 2025 17:57:28 -0400

While discussing some userfaultfd relevant issues recently, Andrea noticed a potential ABI breakage with -EAGAIN on almost all userfaultfd ioctl()s.

Quote from Andrea, explaining how -EAGAIN was processed, and how this should fix it (taking example of UFFDIO_COPY ioctl):

The "mmap_changing" and "stale pmd" conditions are already reported as -EAGAIN written in the copy field, this does not change it. This change removes the subnormal case that left copy.copy uninitialized and required apps to explicitly set the copy field to get deterministic behavior (which is a requirement contrary to the documentation in both the manpage and source code). In turn there's no alteration to backwards compatibility as result of this change because userland will find the copy field consistently set to -EAGAIN, and not anymore sometime -EAGAIN and sometime uninitialized.

Even then the change only can make a difference to non cooperative users of userfaultfd, so when UFFD_FEATURE_EVENT_* is enabled, which is not true for the vast majority of apps using userfaultfd or this unintended uninitialized field may have been noticed sooner.

Meanwhile, since this bug existed for years, it also almost affects all ioctl()s that was introduced later. Besides UFFDIO_ZEROPAGE, these also get affected in the same way:

- UFFDIO_CONTINUE - UFFDIO_POISON - UFFDIO_MOVE

This patch should have fixed all of them.

Link: https://lkml.kernel.org/r/20250424215729.194656-2-peterx@redhat.com Fixes: df2cc96e7701 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races") Fixes: f619147104c8 ("userfaultfd: add UFFDIO_CONTINUE ioctl") Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl") Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Signed-off-by: Peter Xu peterx@redhat.com Reported-by: Andrea Arcangeli aarcange@redhat.com Suggested-by: Andrea Arcangeli aarcange@redhat.com Cc: Mike Rapoport rppt@kernel.org Cc: Axel Rasmussen axelrasmussen@google.com Cc: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/userfaultfd.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-)

--- a/fs/userfaultfd.c~mm-userfaultfd-fix-uninitialized-output-field-for-eagain-race +++ a/fs/userfaultfd.c @@ -1585,8 +1585,11 @@ static int userfaultfd_copy(struct userf user_uffdio_copy = (struct uffdio_copy __user *) arg;

ret = -EAGAIN; - if (atomic_read(&ctx->mmap_changing)) + if (unlikely(atomic_read(&ctx->mmap_changing))) { + if (unlikely(put_user(ret, &user_uffdio_copy->copy))) + return -EFAULT; goto out; + }

ret = -EFAULT; if (copy_from_user(&uffdio_copy, user_uffdio_copy, @@ -1641,8 +1644,11 @@ static int userfaultfd_zeropage(struct u user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;

ret = -EAGAIN; - if (atomic_read(&ctx->mmap_changing)) + if (unlikely(atomic_read(&ctx->mmap_changing))) { + if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage))) + return -EFAULT; goto out; + }

ret = -EFAULT; if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage, @@ -1744,8 +1750,11 @@ static int userfaultfd_continue(struct u user_uffdio_continue = (struct uffdio_continue __user *)arg;

ret = -EAGAIN; - if (atomic_read(&ctx->mmap_changing)) + if (unlikely(atomic_read(&ctx->mmap_changing))) { + if (unlikely(put_user(ret, &user_uffdio_continue->mapped))) + return -EFAULT; goto out; + }

ret = -EFAULT; if (copy_from_user(&uffdio_continue, user_uffdio_continue, @@ -1801,8 +1810,11 @@ static inline int userfaultfd_poison(str user_uffdio_poison = (struct uffdio_poison __user *)arg;

ret = -EAGAIN; - if (atomic_read(&ctx->mmap_changing)) + if (unlikely(atomic_read(&ctx->mmap_changing))) { + if (unlikely(put_user(ret, &user_uffdio_poison->updated))) + return -EFAULT; goto out; + }

ret = -EFAULT; if (copy_from_user(&uffdio_poison, user_uffdio_poison, @@ -1870,8 +1882,12 @@ static int userfaultfd_move(struct userf

user_uffdio_move = (struct uffdio_move __user *) arg;

- if (atomic_read(&ctx->mmap_changing)) - return -EAGAIN; + ret = -EAGAIN; + if (unlikely(atomic_read(&ctx->mmap_changing))) { + if (unlikely(put_user(ret, &user_uffdio_move->move))) + return -EFAULT; + goto out; + }

if (copy_from_user(&uffdio_move, user_uffdio_move, /* don't copy "move" last field */ _

Patches currently in -mm which might be from peterx@redhat.com are

mm-userfaultfd-fix-uninitialized-output-field-for-eagain-race.patch mm-selftests-add-a-test-to-verify-mmap_changing-race-with-eagain.patch