[PATCH v16 2/2] rust: fmt: route {:p} through HashedPtr to prevent address leaks

[Ke Sun <sunke@kylinos.cn>]

Define a custom `kernel::fmt::Pointer` trait and `HashedPtr` wrapper
so that `{:p}` formatting uses the kernel's `%p` hashed format instead
of printing raw pointer values, preventing kernel address space leaks.

Tested-by: Link Mauve <linkmauve@linkmauve.fr>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Ke Sun <sunke@kylinos.cn>
---
 rust/kernel/fmt.rs | 141 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 139 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/fmt.rs b/rust/kernel/fmt.rs
index cd7d9664ff5b9..29454c83dc6b7 100644
--- a/rust/kernel/fmt.rs
+++ b/rust/kernel/fmt.rs
@@ -4,6 +4,8 @@
 //!
 //! This module is intended to be used in place of `core::fmt` in kernel code.
 
+use kernel::prelude::*;
+
 pub use core::fmt::{
     Arguments,
     Debug,
@@ -39,13 +41,110 @@ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
     LowerExp,
     LowerHex,
     Octal,
-    Pointer,
     UpperExp,
     UpperHex, //
 };
+use core::ptr::NonNull;
 impl_fmt_adapter_forward!(Debug, LowerHex, UpperHex, Octal, Binary, LowerExp, UpperExp);
 
-impl<T: ?Sized + Pointer> Pointer for Adapter<&T> {
+/// A copy of [`core::fmt::Pointer`] that allows implementing pointer formatting for foreign types.
+///
+/// Together with the [`Adapter`] type and [`fmt!`] macro, it enables raw pointer formatting to be
+/// intercepted and routed to [`HashedPtr`] (kernel's `%p` hashed format), preventing kernel address
+/// leaks.
+///
+/// [`fmt!`]: crate::prelude::fmt!
+pub trait Pointer {
+    /// Same as [`core::fmt::Pointer::fmt`].
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
+}
+
+/// A wrapper for pointers that formats them using kernel's `%p` format specifier.
+///
+/// By default, `%p` prints a hashed representation of the pointer address to prevent kernel address
+/// leaks. When the `no_hash_pointers` kernel command-line parameter is enabled, the real address is
+/// printed instead (for debugging purposes).
+pub struct HashedPtr<T: ?Sized>(pub *const T);
+
+impl<T: ?Sized> Pointer for HashedPtr<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        use crate::str::CStrExt as _;
+
+        let mut buf = [0u8; 32];
+
+        // Use `%#0*p` for the `0x` prefix and zero-padding; `+2` compensates for
+        // the prefix counting toward the field width.
+        let default_width = (2 * size_of::<usize>() + 2) as c_int;
+        let width = match (f.sign_aware_zero_pad(), f.width()) {
+            (true, Some(w)) if w > 0 => w.min(buf.len() - 1) as c_int,
+            _ => default_width,
+        };
+
+        // SAFETY: `buf` is a valid, writable 32-byte buffer, sufficient for
+        // all architectures (max 19 bytes for 64-bit under the default width).
+        // The format string is null-terminated; `width` (c_int) and pointer
+        // match the `%*` and `%p` specifiers.
+        let len = unsafe {
+            crate::bindings::scnprintf(
+                buf.as_mut_ptr().cast(),
+                buf.len(),
+                c"%#0*p".as_char_ptr(),
+                width,
+                self.0.cast::<c_void>(),
+            )
+        };
+
+        // SAFETY: `%#0*p` produces only ASCII, which is valid UTF-8.
+        let s = unsafe { core::str::from_utf8_unchecked(&buf[..len as usize]) };
+
+        if f.sign_aware_zero_pad() {
+            // `scnprintf` already applied the width and zero-padding via `%#0*p`.
+            f.write_str(s)
+        } else {
+            f.pad(s)
+        }
+    }
+}
+
+// Raw pointers are formatted via `HashedPtr` (kernel `%p`: hashed by default, plain with
+// `no_hash_pointers`).
+impl<T: ?Sized> Pointer for *const T {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&HashedPtr(*self), f)
+    }
+}
+
+impl<T: ?Sized> Pointer for *mut T {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&HashedPtr(*self), f)
+    }
+}
+
+impl<T: ?Sized> Pointer for &T {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&HashedPtr(*self), f)
+    }
+}
+
+impl<T: ?Sized> Pointer for &mut T {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&HashedPtr(core::ptr::from_ref(*self)), f)
+    }
+}
+
+impl<T: ?Sized> Pointer for NonNull<T> {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        Pointer::fmt(&HashedPtr(self.as_ptr()), f)
+    }
+}
+
+// `Adapter<&T>` bridges our `Pointer` trait to `core::fmt::Pointer`
+impl<T: Pointer> core::fmt::Pointer for Adapter<&T> {
     #[inline]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
         Pointer::fmt(self.0, f)
@@ -112,3 +211,41 @@ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
     {<T: ?Sized>} crate::sync::Arc<T> {where crate::sync::Arc<T>: core::fmt::Display},
     {<T: ?Sized>} crate::sync::UniqueArc<T> {where crate::sync::UniqueArc<T>: core::fmt::Display},
 );
+
+#[macros::kunit_tests(rust_kernel_fmt)]
+mod tests {
+    use crate::{
+        prelude::fmt,
+        str::CString, //
+    };
+
+    #[cfg(CONFIG_64BIT)]
+    const PTR_VALUE: usize = 0xffffffffdeadbeef;
+
+    #[cfg(not(CONFIG_64BIT))]
+    const PTR_VALUE: usize = 0xdeadbeef;
+
+    #[test]
+    fn test_ptr_formatting() -> core::result::Result<(), crate::error::Error> {
+        let ptr: *const u8 = core::ptr::without_provenance(PTR_VALUE);
+
+        let cstr = CString::try_from_fmt(fmt!("{:p}", ptr))?;
+        let formatted = cstr.to_str()?;
+        // If the RNG is not yet ready, `"%p"` falls back to `"(ptrval)"` / `"(____ptrval____)"`.
+        let formatted = formatted.strip_prefix("0x").unwrap_or(formatted);
+
+        let cstr = CString::try_from_fmt(fmt!("{:>24p}", ptr))?;
+        let padded = cstr.to_str()?;
+        assert!(padded.ends_with(formatted));
+
+        let cstr = CString::try_from_fmt(fmt!("{:024p}", ptr))?;
+        let zero_padded = cstr.to_str()?;
+        assert!(zero_padded.ends_with(formatted));
+
+        let cstr = CString::try_from_fmt(fmt!("{:0100p}", ptr))?;
+        let clamped = cstr.to_str()?;
+        assert!(clamped.ends_with(formatted));
+
+        Ok(())
+    }
+}

-- 
2.43.0