openrisc: fix arbitrary kernel memory access via or1k_atomic syscallgrafted

sys_or1k_atomic() (syscall 244 in the "or1k" ABI) takes two user
pointers, v1 and v2, and swaps the words they point to in hand-written
assembly.

    l.lwz   r29,0(r4)
    l.lwz   r27,0(r5)
    l.sw    0(r4),r27
    l.sw    0(r5),r29

The pointers are not checked with access_ok(). The four memory
accesses also have no exception table entries.

A caller passes a kernel address as either pointer, and the syscall
reads from and writes to it directly.

This gives an unprivileged process a kernel read/write primitive. It
overwrites kernel data such as the sys_call_table, gaining code
execution in kernel context.

Check both pointers before entering the critical section. Add fixups
for the four memory accesses so faults on valid but unmapped user
addresses return -EFAULT.

[shorne@gmail.com: fix comment style]
Fixes: 9d02a4283e9c ("OpenRISC: Boot code")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>

1 files changed, 62 insertions, 0 deletions

diff --git a/rust/syn/thread.rs b/rust/syn/thread.rs
new file mode 100644
index 000000000..4165a173d
--- /dev/null
+++ b/rust/syn/thread.rs
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use std::fmt::{self, Debug};
+use std::thread::{self, ThreadId};
+
+/// ThreadBound is a Sync-maker and Send-maker that allows accessing a value
+/// of type T only from the original thread on which the ThreadBound was
+/// constructed.
+pub(crate) struct ThreadBound<T> {
+    value: T,
+    thread_id: ThreadId,
+}
+
+unsafe impl<T> Sync for ThreadBound<T> {}
+
+// Send bound requires Copy, as otherwise Drop could run in the wrong place.
+//
+// Today Copy and Drop are mutually exclusive so `T: Copy` implies `T: !Drop`.
+// This impl needs to be revisited if that restriction is relaxed in the future.
+unsafe impl<T: Copy> Send for ThreadBound<T> {}
+
+impl<T> ThreadBound<T> {
+    pub(crate) fn new(value: T) -> Self {
+        ThreadBound {
+            value,
+            thread_id: thread::current().id(),
+        }
+    }
+
+    pub(crate) fn get(&self) -> Option<&T> {
+        if thread::current().id() == self.thread_id {
+            Some(&self.value)
+        } else {
+            None
+        }
+    }
+}
+
+impl<T: Debug> Debug for ThreadBound<T> {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        match self.get() {
+            Some(value) => Debug::fmt(value, formatter),
+            None => formatter.write_str("unknown"),
+        }
+    }
+}
+
+// Copy the bytes of T, even if the currently running thread is the "wrong"
+// thread. This is fine as long as the original thread is not simultaneously
+// mutating this value via interior mutability, which would be a data race.
+//
+// Currently `T: Copy` is sufficient to guarantee that T contains no interior
+// mutability, because _all_ interior mutability in Rust is built on
+// std::cell::UnsafeCell, which has no Copy impl. This impl needs to be
+// revisited if that restriction is relaxed in the future.
+impl<T: Copy> Copy for ThreadBound<T> {}
+
+impl<T: Copy> Clone for ThreadBound<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}