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Subject[PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.
From
The new secure flag makes userfaultfd use a new "secure" anonymous
file object instead of the default one, letting security modules
supervise userfaultfd use.

Requiring that users pass a new flag lets us avoid changing the
semantics for existing callers.

Signed-off-by: Daniel Colascione <dancol@google.com>
---
fs/userfaultfd.c | 28 +++++++++++++++++++++++++---
include/uapi/linux/userfaultfd.h | 8 ++++++++
2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f9fd18670e22..29f920fb236e 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1022,6 +1022,13 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
{
int fd;

+ /*
+ * Using a secure-mode UFFD to monitor forks isn't supported
+ * right now.
+ */
+ if (new->flags & UFFD_SECURE)
+ return -EOPNOTSUPP;
+
fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
if (fd < 0)
@@ -1841,6 +1848,18 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
ret = -EINVAL;
goto out;
}
+ if ((ctx->flags & UFFD_SECURE) &&
+ (features & UFFD_FEATURE_EVENT_FORK)) {
+ /*
+ * We don't support UFFD_FEATURE_EVENT_FORK on a
+ * secure-mode UFFD: doing so would need us to
+ * construct the new file object in the context of the
+ * fork child, and it's not worth it right now.
+ */
+ ret = -EINVAL;
+ goto out;
+ }
+
/* report all available features and ioctls to userland */
uffdio_api.features = UFFD_API_FEATURES;
uffdio_api.ioctls = UFFD_API_IOCTLS;
@@ -1942,6 +1961,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
{
struct userfaultfd_ctx *ctx;
int fd;
+ static const int uffd_flags = UFFD_SECURE;

if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
return -EPERM;
@@ -1951,8 +1971,9 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
/* Check the UFFD_* constants for consistency. */
BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
+ BUILD_BUG_ON(UFFD_SHARED_FCNTL_FLAGS & uffd_flags);

- if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
+ if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | uffd_flags))
return -EINVAL;

ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
@@ -1969,8 +1990,9 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
/* prevent the mm struct to be freed */
mmgrab(ctx->mm);

- fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
- O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+ fd = anon_inode_getfd2("[userfaultfd]", &userfaultfd_fops, ctx,
+ O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
+ ((flags & UFFD_SECURE) ? ANON_INODE_SECURE : 0));
if (fd < 0) {
mmdrop(ctx->mm);
kmem_cache_free(userfaultfd_ctx_cachep, ctx);
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 48f1a7c2f1f0..12d7d40d7f25 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -231,4 +231,12 @@ struct uffdio_zeropage {
__s64 zeropage;
};

+/*
+ * Flags for the userfaultfd(2) system call itself.
+ */
+
+/*
+ * Create a userfaultfd with MAC security checks enabled.
+ */
+#define UFFD_SECURE 1
#endif /* _LINUX_USERFAULTFD_H */
--
2.23.0.700.g56cf767bdb-goog
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