From 914eedcb9ba0ff53c3380829a024b7cef16accfb Mon Sep 17 00:00:00 2001 From: Axel Rasmussen Date: Fri, 22 Jul 2022 13:15:13 -0700 Subject: userfaultfd: don't fail on unrecognized features The basic interaction for setting up a userfaultfd is, userspace issues a UFFDIO_API ioctl, and passes in a set of zero or more feature flags, indicating the features they would prefer to use. Of course, different kernels may support different sets of features (depending on kernel version, kconfig options, architecture, etc). Userspace's expectations may also not match: perhaps it was built against newer kernel headers, which defined some features the kernel it's running on doesn't support. Currently, if userspace passes in a flag we don't recognize, the initialization fails and we return -EINVAL. This isn't great, though. Userspace doesn't have an obvious way to react to this; sure, one of the features I asked for was unavailable, but which one? The only option it has is to turn off things "at random" and hope something works. Instead, modify UFFDIO_API to just ignore any unrecognized feature flags. The interaction is now that the initialization will succeed, and as always we return the *subset* of feature flags that can actually be used back to userspace. Now userspace has an obvious way to react: it checks if any flags it asked for are missing. If so, it can conclude this kernel doesn't support those, and it can either resign itself to not using them, or fail with an error on its own, or whatever else. Link: https://lkml.kernel.org/r/20220722201513.1624158-1-axelrasmussen@google.com Signed-off-by: Axel Rasmussen Cc: Peter Xu Cc: Axel Rasmussen Signed-off-by: Andrew Morton --- fs/userfaultfd.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) (limited to 'fs/userfaultfd.c') diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index e943370107d0..4974da1f620c 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1923,10 +1923,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx, ret = -EFAULT; if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api))) goto out; - features = uffdio_api.features; - ret = -EINVAL; - if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) - goto err_out; + /* Ignore unsupported features (userspace built against newer kernel) */ + features = uffdio_api.features & UFFD_API_FEATURES; ret = -EPERM; if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE)) goto err_out; -- cgit