userfaultfd/sysctl: add vm.unprivileged_userfaultfd

Userfaultfd can be misued to make it easier to exploit existing
use-after-free (and similar) bugs that might otherwise only make a
short window or race condition available.  By using userfaultfd to
stall a kernel thread, a malicious program can keep some state that it
wrote, stable for an extended period, which it can then access using an
existing exploit.  While it doesn't cause the exploit itself, and while
it's not the only thing that can stall a kernel thread when accessing a
memory location, it's one of the few that never needs privilege.

We can add a flag, allowing userfaultfd to be restricted, so that in
general it won't be useable by arbitrary user programs, but in
environments that require userfaultfd it can be turned back on.

Add a global sysctl knob "vm.unprivileged_userfaultfd" to control
whether userfaultfd is allowed by unprivileged users.  When this is
set to zero, only privileged users (root user, or users with the
CAP_SYS_PTRACE capability) will be able to use the userfaultfd
syscalls.

Andrea said:

: The only difference between the bpf sysctl and the userfaultfd sysctl
: this way is that the bpf sysctl adds the CAP_SYS_ADMIN capability
: requirement, while userfaultfd adds the CAP_SYS_PTRACE requirement,
: because the userfaultfd monitor is more likely to need CAP_SYS_PTRACE
: already if it's doing other kind of tracking on processes runtime, in
: addition of userfaultfd.  In other words both syscalls works only for
: root, when the two sysctl are opt-in set to 1.

[[email protected]: changelog additions]
[[email protected]: documentation tweak, per Mike]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Suggested-by: Andrea Arcangeli <[email protected]>
Suggested-by: Mike Rapoport <[email protected]>
Reviewed-by: Mike Rapoport <[email protected]>
Reviewed-by: Andrea Arcangeli <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Maxime Coquelin <[email protected]>
Cc: Maya Gokhale <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Martin Cracauer <[email protected]>
Cc: Denis Plotnikov <[email protected]>
Cc: Marty McFadden <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

4 files changed, 31 insertions, 0 deletions

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 3f13d8599337..749322060f10 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -61,6 +61,7 @@ Currently, these files are in /proc/sys/vm:
 - stat_refresh
 - numa_stat
 - swappiness
+- unprivileged_userfaultfd
 - user_reserve_kbytes
 - vfs_cache_pressure
 - watermark_boost_factor
@@ -818,6 +819,17 @@ The default value is 60.
 
 ==============================================================
 
+unprivileged_userfaultfd
+
+This flag controls whether unprivileged users can use the userfaultfd
+system calls.  Set this to 1 to allow unprivileged users to use the
+userfaultfd system calls, or set this to 0 to restrict userfaultfd to only
+privileged users (with SYS_CAP_PTRACE capability).
+
+The default value is 1.
+
+==============================================================
+
 - user_reserve_kbytes
 
 When overcommit_memory is set to 2, "never overcommit" mode, reserve
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f5de1e726356..3b30301c90ec 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -30,6 +30,8 @@
 #include <linux/security.h>
 #include <linux/hugetlb.h>
 
+int sysctl_unprivileged_userfaultfd __read_mostly = 1;
+
 static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
 
 enum userfaultfd_state {
@@ -1930,6 +1932,9 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	struct userfaultfd_ctx *ctx;
 	int fd;
 
+	if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
+		return -EPERM;
+
 	BUG_ON(!current->mm);
 
 	/* Check the UFFD_* constants for consistency.  */
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 37c9eba75c98..ac9d71e24b81 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -28,6 +28,8 @@
 #define UFFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
 #define UFFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS)
 
+extern int sysctl_unprivileged_userfaultfd;
+
 extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
 
 extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 599510a3355e..ba158f61aab4 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -66,6 +66,7 @@
 #include <linux/kexec.h>
 #include <linux/bpf.h>
 #include <linux/mount.h>
+#include <linux/userfaultfd_k.h>
 
 #include "../lib/kstrtox.h"
 
@@ -1720,6 +1721,17 @@ static struct ctl_table vm_table[] = {
 		.extra2		= (void *)&mmap_rnd_compat_bits_max,
 	},
 #endif
+#ifdef CONFIG_USERFAULTFD
+	{
+		.procname	= "unprivileged_userfaultfd",
+		.data		= &sysctl_unprivileged_userfaultfd,
+		.maxlen		= sizeof(sysctl_unprivileged_userfaultfd),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+#endif
 	{ }
 };