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From:   Miguel Ojeda <ojeda@kernel.org>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= <bjorn3_gh@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Miguel Ojeda <ojeda@kernel.org>,
        Domen Puncer Kugler <domen.puncerkugler@nccgroup.com>
Subject: [PATCH] rust: print: avoid evaluating arguments in `pr_*` macros in `unsafe` blocks
Date:   Mon,  9 Jan 2023 21:49:12 +0100
Message-Id: <20230109204912.539790-1-ojeda@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

At the moment it is possible to perform unsafe operations in
the arguments of `pr_*` macros since they are evaluated inside
an `unsafe` block:

    let x = &10u32 as *const u32;
    pr_info!("{}", *x);

In other words, this is a soundness issue.

Fix it so that it requires an explicit `unsafe` block.

Reported-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reported-by: Domen Puncer Kugler <domen.puncerkugler@nccgroup.com>
Link: https://github.com/Rust-for-Linux/linux/issues/479
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/print.rs | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
index 29bf9c2e8aee..30103325696d 100644
--- a/rust/kernel/print.rs
+++ b/rust/kernel/print.rs
@@ -142,17 +142,24 @@ pub fn call_printk_cont(args: fmt::Arguments<'_>) {
 macro_rules! print_macro (
     // The non-continuation cases (most of them, e.g. `INFO`).
     ($format_string:path, false, $($arg:tt)+) => (
-        // SAFETY: This hidden macro should only be called by the documented
-        // printing macros which ensure the format string is one of the fixed
-        // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
-        // by the `module!` proc macro or fixed values defined in a kernel
-        // crate.
-        unsafe {
-            $crate::print::call_printk(
-                &$format_string,
-                crate::__LOG_PREFIX,
-                format_args!($($arg)+),
-            );
+        // To remain sound, `arg`s must be expanded outside the `unsafe` block.
+        // Typically one would use a `let` binding for that; however, `format_args!`
+        // takes borrows on the arguments, but does not extend the scope of temporaries.
+        // Therefore, a `match` expression is used to keep them around, since
+        // the scrutinee is kept until the end of the `match`.
+        match format_args!($($arg)+) {
+            // SAFETY: This hidden macro should only be called by the documented
+            // printing macros which ensure the format string is one of the fixed
+            // ones. All `__LOG_PREFIX`s are null-terminated as they are generated
+            // by the `module!` proc macro or fixed values defined in a kernel
+            // crate.
+            args => unsafe {
+                $crate::print::call_printk(
+                    &$format_string,
+                    crate::__LOG_PREFIX,
+                    args,
+                );
+            }
         }
     );
 

base-commit: b7bfaa761d760e72a969d116517eaa12e404c262
-- 
2.39.0