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Subject[PATCH 6/7] Allow users to require UFFD_SECURE
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This change adds 2 as an allowable value for
unprivileged_userfaultfd. (Previously, this sysctl could be either 0
or 1.) When unprivileged_userfaultfd is 2, users with CAP_SYS_PTRACE
may create userfaultfd with or without UFFD_SECURE, but users without
CAP_SYS_PTRACE must pass UFFD_SECURE to userfaultfd in order for the
system call to succeed, effectively forcing them to opt into
additional security checks.

Signed-off-by: Daniel Colascione <dancol@google.com>
---
Documentation/admin-guide/sysctl/vm.rst | 6 ++++--
fs/userfaultfd.c | 4 +++-
kernel/sysctl.c | 2 +-
3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 64aeee1009ca..6664eec7bd35 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -842,8 +842,10 @@ 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).
+userfaultfd system calls, or set this to 0 to restrict userfaultfd to
+only privileged users (with SYS_CAP_PTRACE capability). If set to 2,
+unprivileged (non-SYS_CAP_PTRACE) users may use userfaultfd only if
+they pass the UFFD_SECURE, enabling MAC security checks.

The default value is 1.

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 986d23b2cd33..aaed9347973e 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1963,8 +1963,10 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
struct userfaultfd_ctx *ctx;
int fd;
static const int uffd_flags = UFFD_SECURE | UFFD_USER_MODE_ONLY;
+ bool need_cap_check = sysctl_unprivileged_userfaultfd == 0 ||
+ (sysctl_unprivileged_userfaultfd == 2 && !(flags & UFFD_SECURE));

- if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
+ if (need_cap_check && !capable(CAP_SYS_PTRACE))
return -EPERM;

BUG_ON(!current->mm);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 00fcea236eba..fc98d5df344e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1738,7 +1738,7 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec_minmax,
.extra1 = SYSCTL_ZERO,
- .extra2 = SYSCTL_ONE,
+ .extra2 = &two,
},
#endif
{ }
--
2.23.0.700.g56cf767bdb-goog
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