[libgpiod][PATCH v2 1/3] bindings: rust: fix soundness of line_info modeling

[Erik Schilling <erik.schilling@linaro.org>]

While attention was provided to prevent freeing in non-owned use-cases,
the lifetime of these object was not properly modeled.

The line_info from an event may only be used for as long as the event
exists.

This allowed us to write unsafe-free Rust code that causes a
use-after-free:

  let event = chip.read_info_event().unwrap();
  let line_info = event.line_info().unwrap();
  drop(event);
  dbg!(line_info.name().unwrap());

Which makes the AddressSanitizer scream:

  ==90154==ERROR: AddressSanitizer: heap-use-after-free on address 0x50b000005dc4 at pc 0x55a4f883a009 bp 0x7f60ac8fbbc0 sp 0x7f60ac8fb388
  READ of size 2 at 0x50b000005dc4 thread T2
      [...]
      #3 0x55a4f8c3d5f3 in libgpiod::line_info::Info::name::h5ba0bfd360ecb405 libgpiod/bindings/rust/libgpiod/src/line_info.rs:70:18
    	[...]

  0x50b000005dc4 is located 4 bytes inside of 112-byte region [0x50b000005dc0,0x50b000005e30)
  freed by thread T2 here:
      [...]
      #1 0x7f60b07f7e31 in gpiod_info_event_free libgpiod/lib/info-event.c:61:2
      [...]

  previously allocated by thread T2 here:
      #0 0x55a4f88b04be in malloc /rustc/llvm/src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:69:3
      #1 0x7f60b07f8ff0 in gpiod_line_info_from_uapi libgpiod/lib/line-info.c:144:9

The fix is to distinguish between the owned and non-owned variants and
assigning lifetimes to non-owned variants.

For modeling the non-owned type there are a couple of options. The ideal
solution would be using extern_types [1]. But that is still unstable.
Instead, we are defining a #[repr(transparent)] wrapper around the opaque
gpiod_line_info struct and cast the pointer to a reference.

This was recommended on the Rust Discord server as good practise.
(Thanks to Kyuuhachi, shepmaster, pie_flavor and ilyvion! Also thanks to
@epilys for a brainstorming on this on #linaro-virtualization IRC).

Of course, determining the lifetimes and casting across the types
requires some care. So this adds a couple of SAFETY comments that would
probably also have helped the existing code.

[1] https://github.com/rust-lang/rfcs/blob/master/text/1861-extern-types.md

Fixes: 91f9373 ("bindings: rust: Add libgpiod crate")
Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
---
 bindings/rust/libgpiod/src/chip.rs       |   8 +-
 bindings/rust/libgpiod/src/info_event.rs |   8 +-
 bindings/rust/libgpiod/src/line_info.rs  | 130 +++++++++++++++++++++----------
 3 files changed, 100 insertions(+), 46 deletions(-)

diff --git a/bindings/rust/libgpiod/src/chip.rs b/bindings/rust/libgpiod/src/chip.rs
index 81e1be6..9ef8f22 100644
--- a/bindings/rust/libgpiod/src/chip.rs
+++ b/bindings/rust/libgpiod/src/chip.rs
@@ -107,7 +107,9 @@ impl Chip {
             ));
         }
 
-        line::Info::new(info)
+        // SAFETY: We verified that the pointer is valid. We own the pointer and
+        // no longer use it after converting it into a Info instance.
+        Ok(unsafe { line::Info::from_raw_owned(info) })
     }
 
     /// Get the current snapshot of information about the line at given offset and start watching
@@ -123,7 +125,9 @@ impl Chip {
             ));
         }
 
-        line::Info::new_watch(info)
+        // SAFETY: We verified that the pointer is valid. We own the instance and
+        // no longer use it after converting it into a Info instance.
+        Ok(unsafe { line::Info::from_raw_owned(info) })
     }
 
     /// Stop watching a line
diff --git a/bindings/rust/libgpiod/src/info_event.rs b/bindings/rust/libgpiod/src/info_event.rs
index db60600..843f3f7 100644
--- a/bindings/rust/libgpiod/src/info_event.rs
+++ b/bindings/rust/libgpiod/src/info_event.rs
@@ -44,7 +44,7 @@ impl Event {
     }
 
     /// Get the line-info object associated with the event.
-    pub fn line_info(&self) -> Result<line::Info> {
+    pub fn line_info(&self) -> Result<&line::InfoRef> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
         let info = unsafe { gpiod::gpiod_info_event_get_line_info(self.event) };
 
@@ -55,7 +55,11 @@ impl Event {
             ));
         }
 
-        line::Info::new_from_event(info)
+        // SAFETY: The pointer is valid. The returned reference receives the
+        // lifetime '0 - the same as &self. &self also controls lifetime and
+        // ownership of the owning object. Therefore, the borrow prevents moving
+        // of the owning object to another thread.
+        Ok(unsafe { line::InfoRef::from_raw_non_owning(info) })
     }
 }
 
diff --git a/bindings/rust/libgpiod/src/line_info.rs b/bindings/rust/libgpiod/src/line_info.rs
index c4f488c..e3ea5e1 100644
--- a/bindings/rust/libgpiod/src/line_info.rs
+++ b/bindings/rust/libgpiod/src/line_info.rs
@@ -2,9 +2,10 @@
 // SPDX-FileCopyrightText: 2022 Linaro Ltd.
 // SPDX-FileCopyrightText: 2022 Viresh Kumar <viresh.kumar@linaro.org>
 
-use std::ffi::CStr;
+use std::ops::Deref;
 use std::str;
 use std::time::Duration;
+use std::{ffi::CStr, marker::PhantomData};
 
 use super::{
     gpiod,
@@ -12,7 +13,7 @@ use super::{
     Error, Result,
 };
 
-/// Line info
+/// Line info reference
 ///
 /// Exposes functions for retrieving kernel information about both requested and
 /// free lines.  Line info object contains an immutable snapshot of a line's status.
@@ -20,48 +21,57 @@ use super::{
 /// The line info contains all the publicly available information about a
 /// line, which does not include the line value.  The line must be requested
 /// to access the line value.
-
-#[derive(Debug, Eq, PartialEq)]
-pub struct Info {
-    info: *mut gpiod::gpiod_line_info,
-    contained: bool,
+///
+/// [InfoRef] only abstracts a reference to a [gpiod::gpiod_line_info] instance whose lifetime is managed
+/// by a different object instance. The owned counter-part of this type is [Info].
+#[derive(Debug)]
+#[repr(transparent)]
+pub struct InfoRef {
+    _info: gpiod::gpiod_line_info,
+    // Avoid the automatic `Sync` implementation.
+    //
+    // The C lib does not allow parallel invocations of the API. We could model
+    // that by restricting all wrapper functions to `&mut Info` - which would
+    // ensure exclusive access. But that would make the API a bit weird...
+    // So instead we just suppress the `Sync` implementation, which suppresses
+    // the `Send` implementation on `&Info` - disallowing to send it to other
+    // threads, making concurrent use impossible.
+    _not_sync: PhantomData<*mut gpiod::gpiod_line_info>,
 }
 
-impl Info {
-    fn new_internal(info: *mut gpiod::gpiod_line_info, contained: bool) -> Result<Self> {
-        Ok(Self { info, contained })
-    }
-
-    /// Get a snapshot of information about the line.
-    pub(crate) fn new(info: *mut gpiod::gpiod_line_info) -> Result<Self> {
-        Info::new_internal(info, false)
-    }
-
-    /// Get a snapshot of information about the line and start watching it for changes.
-    pub(crate) fn new_watch(info: *mut gpiod::gpiod_line_info) -> Result<Self> {
-        Info::new_internal(info, false)
+impl InfoRef {
+    /// Converts a non-owning pointer to a wrapper reference of a specific
+    /// lifetime
+    ///
+    /// No ownership will be assumed, the pointer must be free'd by the original
+    /// owner.
+    ///
+    /// SAFETY: The pointer must point to an instance that is valid for the
+    /// entire lifetime 'a. The instance must be owned by an object that is
+    /// owned by the thread invoking this method. The owning object may not be
+    /// moved to another thread for the entire lifetime 'a.
+    pub(crate) unsafe fn from_raw_non_owning<'a>(info: *mut gpiod::gpiod_line_info) -> &'a InfoRef {
+        &*(info as *mut _)
     }
 
-    /// Get the Line info object associated with an event.
-    pub(crate) fn new_from_event(info: *mut gpiod::gpiod_line_info) -> Result<Self> {
-        Info::new_internal(info, true)
+    fn as_raw_ptr(&self) -> *mut gpiod::gpiod_line_info {
+        self as *const _ as *mut _
     }
 
     /// Get the offset of the line within the GPIO chip.
     ///
     /// The offset uniquely identifies the line on the chip. The combination of the chip and offset
     /// uniquely identifies the line within the system.
-
     pub fn offset(&self) -> Offset {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        unsafe { gpiod::gpiod_line_info_get_offset(self.info) }
+        unsafe { gpiod::gpiod_line_info_get_offset(self.as_raw_ptr()) }
     }
 
     /// Get GPIO line's name.
     pub fn name(&self) -> Result<&str> {
         // SAFETY: The string returned by libgpiod is guaranteed to live as long
         // as the `struct Info`.
-        let name = unsafe { gpiod::gpiod_line_info_get_name(self.info) };
+        let name = unsafe { gpiod::gpiod_line_info_get_name(self.as_raw_ptr()) };
         if name.is_null() {
             return Err(Error::NullString("GPIO line's name"));
         }
@@ -79,14 +89,14 @@ impl Info {
     /// the line is used and we can't request it.
     pub fn is_used(&self) -> bool {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        unsafe { gpiod::gpiod_line_info_is_used(self.info) }
+        unsafe { gpiod::gpiod_line_info_is_used(self.as_raw_ptr()) }
     }
 
     /// Get the GPIO line's consumer name.
     pub fn consumer(&self) -> Result<&str> {
         // SAFETY: The string returned by libgpiod is guaranteed to live as long
         // as the `struct Info`.
-        let name = unsafe { gpiod::gpiod_line_info_get_consumer(self.info) };
+        let name = unsafe { gpiod::gpiod_line_info_get_consumer(self.as_raw_ptr()) };
         if name.is_null() {
             return Err(Error::NullString("GPIO line's consumer name"));
         }
@@ -100,44 +110,44 @@ impl Info {
     /// Get the GPIO line's direction.
     pub fn direction(&self) -> Result<Direction> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        Direction::new(unsafe { gpiod::gpiod_line_info_get_direction(self.info) })
+        Direction::new(unsafe { gpiod::gpiod_line_info_get_direction(self.as_raw_ptr()) })
     }
 
     /// Returns true if the line is "active-low", false otherwise.
     pub fn is_active_low(&self) -> bool {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        unsafe { gpiod::gpiod_line_info_is_active_low(self.info) }
+        unsafe { gpiod::gpiod_line_info_is_active_low(self.as_raw_ptr()) }
     }
 
     /// Get the GPIO line's bias setting.
     pub fn bias(&self) -> Result<Option<Bias>> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        Bias::new(unsafe { gpiod::gpiod_line_info_get_bias(self.info) })
+        Bias::new(unsafe { gpiod::gpiod_line_info_get_bias(self.as_raw_ptr()) })
     }
 
     /// Get the GPIO line's drive setting.
     pub fn drive(&self) -> Result<Drive> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        Drive::new(unsafe { gpiod::gpiod_line_info_get_drive(self.info) })
+        Drive::new(unsafe { gpiod::gpiod_line_info_get_drive(self.as_raw_ptr()) })
     }
 
     /// Get the current edge detection setting of the line.
     pub fn edge_detection(&self) -> Result<Option<Edge>> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        Edge::new(unsafe { gpiod::gpiod_line_info_get_edge_detection(self.info) })
+        Edge::new(unsafe { gpiod::gpiod_line_info_get_edge_detection(self.as_raw_ptr()) })
     }
 
     /// Get the current event clock setting used for edge event timestamps.
     pub fn event_clock(&self) -> Result<EventClock> {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        EventClock::new(unsafe { gpiod::gpiod_line_info_get_event_clock(self.info) })
+        EventClock::new(unsafe { gpiod::gpiod_line_info_get_event_clock(self.as_raw_ptr()) })
     }
 
     /// Returns true if the line is debounced (either by hardware or by the
     /// kernel software debouncer), false otherwise.
     pub fn is_debounced(&self) -> bool {
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-        unsafe { gpiod::gpiod_line_info_is_debounced(self.info) }
+        unsafe { gpiod::gpiod_line_info_is_debounced(self.as_raw_ptr()) }
     }
 
     /// Get the debounce period of the line.
@@ -147,18 +157,54 @@ impl Info {
         #[allow(clippy::unnecessary_cast)]
         // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
         Duration::from_micros(unsafe {
-            gpiod::gpiod_line_info_get_debounce_period_us(self.info) as u64
+            gpiod::gpiod_line_info_get_debounce_period_us(self.as_raw_ptr()) as u64
         })
     }
 }
 
+/// Line info
+///
+/// This is the owned counterpart to [InfoRef]. Due to a [Deref] implementation,
+/// all functions of [InfoRef] can also be called on this type.
+#[derive(Debug)]
+pub struct Info {
+    info: *mut gpiod::gpiod_line_info,
+}
+
+// SAFETY: Info models a owned instance whose ownership may be safely
+// transferred to other threads.
+unsafe impl Send for Info {}
+
+impl Info {
+    /// Converts a owned pointer into an owned instance
+    ///
+    /// Assumes sole ownership over a [gpiod::gpiod_line_info] instance.
+    ///
+    /// SAFETY: The pointer must point to an instance that is valid. After
+    /// constructing an [Info] the pointer MUST NOT be used for any other
+    /// purpose anymore. All interactions with the libgpiod API have to happen
+    /// through this object.
+    pub(crate) unsafe fn from_raw_owned(info: *mut gpiod::gpiod_line_info) -> Info {
+        Info { info }
+    }
+}
+
+impl Deref for Info {
+    type Target = InfoRef;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The pointer is valid for the entire lifetime '0. Info is not
+        // Sync. Therefore, no &Info may be held by a different thread. Hence,
+        // the current thread owns it. Since we borrow with the lifetime of '0,
+        // no move to a different thread can occur while a reference remains
+        // being hold.
+        unsafe { InfoRef::from_raw_non_owning(self.info) }
+    }
+}
+
 impl Drop for Info {
     fn drop(&mut self) {
-        // We must not free the Line info object created from `struct chip::Event` by calling
-        // libgpiod API.
-        if !self.contained {
-            // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
-            unsafe { gpiod::gpiod_line_info_free(self.info) }
-        }
+        // SAFETY: `gpiod_line_info` is guaranteed to be valid here.
+        unsafe { gpiod::gpiod_line_info_free(self.info) }
     }
 }

-- 
2.41.0