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, 25 insertions, 0 deletions

diff --git a/rust/macros/export.rs b/rust/macros/export.rs
new file mode 100644
index 000000000..6d53521f6
--- /dev/null
+++ b/rust/macros/export.rs
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use proc_macro2::TokenStream;
+use quote::quote;
+
+/// Please see [`crate::export`] for documentation.
+pub(crate) fn export(f: syn::ItemFn) -> TokenStream {
+    let name = &f.sig.ident;
+
+    quote! {
+        // This verifies that the function has the same signature as the declaration generated by
+        // bindgen. It makes use of the fact that all branches of an if/else must have the same
+        // type.
+        const _: () = {
+            if true {
+                ::kernel::bindings::#name
+            } else {
+                #name
+            };
+        };
+
+        #[no_mangle]
+        #f
+    }
+}