ARM: 9485/1: mm: acquire mmap write lock around show_pte() for user faultsgrafted

When CONFIG_DEBUG_USER=y, and cmdline "user_debug=31" is set,
a user fault may trigger show_pte() without any lock.
If another thread in the same process concurrently calls munmap(),
the page table pages may be freed while show_pte() is still traversing
them, causing a use-after-free in show_pte().

If CONFIG_ARM_LPAE=y, this may cause a kernel panic if the pages table
of PMD are freed when show_pte() is running.

Acquire mmap_write_lock() around show_pte() for user faults to fix the
contention.

For user faults, additionally restrict that show_pte() is called only
when the addr is a user-space address (addr < TASK_SIZE). This is because
the lock of tsk->mm only protects the virtual memory of user address space,
furthermore, dumping the page tables of a kernel-space address for user
faults is unnecessary and may have security implications.

Keep everything unchanged for kernel faults, because the kernel is
already in the "oops" state, acquiring a lock may risk a deadlock.

Co-developed-by: Qi Xi <xiqi2@huawei.com>

Fixes: 6d021b724481 ("ARM: dump pgd, pmd and pte states on unhandled data abort faults")
Link: https://lore.kernel.org/20260716014022.2823-1-xieyuanbin1@huawei.com
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 18 insertions, 0 deletions

diff --git a/rust/syn/print.rs b/rust/syn/print.rs
new file mode 100644
index 000000000..d73262cda
--- /dev/null
+++ b/rust/syn/print.rs
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+
+pub(crate) struct TokensOrDefault<'a, T: 'a>(pub &'a Option<T>);
+
+impl<'a, T> ToTokens for TokensOrDefault<'a, T>
+where
+    T: ToTokens + Default,
+{
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self.0 {
+            Some(t) => t.to_tokens(tokens),
+            None => T::default().to_tokens(tokens),
+        }
+    }
+}