@@ -389,6 +389,9 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
if (ctx->features & UFFD_FEATURE_SIGBUS)
goto out;
+ if ((vmf->flags & FAULT_FLAG_USER) == 0 &&
+ ctx->flags & UFFD_USER_MODE_ONLY)
+ goto out;
/*
* If it's already released don't get it. This avoids to loop
@@ -1945,6 +1948,7 @@ static void init_once_userfaultfd_ctx(void *mem)
SYSCALL_DEFINE1(userfaultfd, int, flags)
{
+ static const int uffd_flags = UFFD_USER_MODE_ONLY;
struct userfaultfd_ctx *ctx;
int fd;
@@ -1954,10 +1958,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
BUG_ON(!current->mm);
/* Check the UFFD_* constants for consistency. */
+ BUILD_BUG_ON(uffd_flags & UFFD_SHARED_FCNTL_FLAGS);
BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
- 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);
@@ -231,4 +231,13 @@ struct uffdio_zeropage {
__s64 zeropage;
};
+/*
+ * Flags for the userfaultfd(2) system call itself.
+ */
+
+/*
+ * Create a userfaultfd that can handle page faults only in user mode.
+ */
+#define UFFD_USER_MODE_ONLY 1
+
#endif /* _LINUX_USERFAULTFD_H */
userfaultfd handles page faults from both user and kernel code. Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes the resulting userfaultfd object refuse to handle faults from kernel mode, treating these faults as if SIGBUS were always raised, causing the kernel code to fail with EFAULT. A future patch adds a knob allowing administrators to give some processes the ability to create userfaultfd file objects only if they pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes will exploit userfaultfd's ability to delay kernel page faults to open timing windows for future exploits. Signed-off-by: Daniel Colascione <dancol@google.com> --- fs/userfaultfd.c | 7 ++++++- include/uapi/linux/userfaultfd.h | 9 +++++++++ 2 files changed, 15 insertions(+), 1 deletion(-)