This is a proposal to revert commit 914eedcb9ba0ff53c33808.

I found this when writting a simple UFFDIO_API test to be the first unit test in this set. Two things breaks with the commit:

- UFFDIO_API check was lost and missing. According to man page, the kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa. This check is needed if the api version will be extended in the future, or user app won't be able to identify which is a new kernel.

- Feature flags checks were removed, which means UFFDIO_API with a feature that does not exist will also succeed. According to the man page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if unknown features passed in.

Link: https://lore.kernel.org/r/20220722201513.1624158-1-axelrasmussen@google.com Fixes: 914eedcb9ba0 ("userfaultfd: don't fail on unrecognized features") Cc: Axel Rasmussen axelrasmussen@google.com Cc: Dmitry Safonov 0x7f454c46@gmail.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: linux-stable stable@vger.kernel.org Acked-by: David Hildenbrand david@redhat.com Signed-off-by: Peter Xu peterx@redhat.com --- fs/userfaultfd.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 8395605790f6..3b2a41c330e6 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1977,8 +1977,10 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, ret = -EFAULT; if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api))) goto out; - /* Ignore unsupported features (userspace built against newer kernel) */ - features = uffdio_api.features & UFFD_API_FEATURES; + features = uffdio_api.features; + ret = -EINVAL; + if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) + goto err_out; ret = -EPERM; if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE)) goto err_out;