On 30.03.23 17:56, Peter Xu wrote:
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.
Agreed.
- 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.
Agreed.
I understand the motivation of the original commit, but it should not have changed existing checks/functionality. Introducing a different way to enable such functionality on explicit request would be better. But maybe simple feature probing (is X support? is Y supported? is Z supported) might be easier without requiring ABI changes.
I assume we better add
Fixes: 914eedcb9ba0 ("userfaultfd: don't fail on unrecognized features")
Acked-by: David Hildenbrand david@redhat.com
Link: https://lore.kernel.org/r/20220722201513.1624158-1-axelrasmussen@google.com Cc: Axel Rasmussen axelrasmussen@google.com Cc: linux-stable stable@vger.kernel.org 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;