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, 96 insertions, 0 deletions

diff --git a/rust/macros/fmt.rs b/rust/macros/fmt.rs
new file mode 100644
index 000000000..ce6c72493
--- /dev/null
+++ b/rust/macros/fmt.rs
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use std::collections::BTreeSet;
+
+use proc_macro2::{Ident, TokenStream, TokenTree};
+use quote::quote_spanned;
+
+/// Please see [`crate::fmt`] for documentation.
+pub(crate) fn fmt(input: TokenStream) -> TokenStream {
+    let mut input = input.into_iter();
+
+    let first_opt = input.next();
+    let first_owned_str;
+    let mut names = BTreeSet::new();
+    let first_span = {
+        let Some((mut first_str, first_span)) = (match first_opt.as_ref() {
+            Some(TokenTree::Literal(first_lit)) => {
+                first_owned_str = first_lit.to_string();
+                Some(first_owned_str.as_str()).and_then(|first| {
+                    let first = first.strip_prefix('"')?;
+                    let first = first.strip_suffix('"')?;
+                    Some((first, first_lit.span()))
+                })
+            }
+            _ => None,
+        }) else {
+            return first_opt.into_iter().chain(input).collect();
+        };
+
+        // Parse `identifier`s from the format string.
+        //
+        // See https://doc.rust-lang.org/std/fmt/index.html#syntax.
+        while let Some((_, rest)) = first_str.split_once('{') {
+            first_str = rest;
+            if let Some(rest) = first_str.strip_prefix('{') {
+                first_str = rest;
+                continue;
+            }
+            if let Some((name, rest)) = first_str.split_once('}') {
+                first_str = rest;
+                let name = name.split_once(':').map_or(name, |(name, _)| name);
+                if !name.is_empty() && !name.chars().all(|c| c.is_ascii_digit()) {
+                    names.insert(name);
+                }
+            }
+        }
+        first_span
+    };
+
+    let adapter = quote_spanned!(first_span => ::kernel::fmt::Adapter);
+
+    let mut args = TokenStream::from_iter(first_opt);
+    {
+        let mut flush = |args: &mut TokenStream, current: &mut TokenStream| {
+            let current = std::mem::take(current);
+            if !current.is_empty() {
+                let (lhs, rhs) = (|| {
+                    let mut current = current.into_iter();
+                    let mut acc = TokenStream::new();
+                    while let Some(tt) = current.next() {
+                        // Split on `=` only once to handle cases like `a = b = c`.
+                        if matches!(&tt, TokenTree::Punct(p) if p.as_char() == '=') {
+                            names.remove(acc.to_string().as_str());
+                            // Include the `=` itself to keep the handling below uniform.
+                            acc.extend([tt]);
+                            return (Some(acc), current.collect::<TokenStream>());
+                        }
+                        acc.extend([tt]);
+                    }
+                    (None, acc)
+                })();
+                args.extend(quote_spanned!(first_span => #lhs #adapter(&(#rhs))));
+            }
+        };
+
+        let mut current = TokenStream::new();
+        for tt in input {
+            match &tt {
+                TokenTree::Punct(p) if p.as_char() == ',' => {
+                    flush(&mut args, &mut current);
+                    &mut args
+                }
+                _ => &mut current,
+            }
+            .extend([tt]);
+        }
+        flush(&mut args, &mut current);
+    }
+
+    for name in names {
+        let name = Ident::new(name, first_span);
+        args.extend(quote_spanned!(first_span => , #name = #adapter(&#name)));
+    }
+
+    quote_spanned!(first_span => ::core::format_args!(#args))
+}