[PATCH] rust: file: handle fd table teardown in file descriptor APIs

[Georgios Androutsopoulos <georgeandrout13@gmail.com>]

Several Rust file descriptor APIs rely on `current->files` being
available. However, `exit_files()` clears it while execution may still
continue on the same task.

This affects `LocalFile::fget()` and the
`FileDescriptorReservation` operations that call
`get_unused_fd_flags()`, `fd_install()`, and `put_unused_fd()`.
`FileDescriptorReservation` cannot cross task boundaries, but remaining
on the same task does not guarantee that `current->files` is still
available when these operations are performed.

Guard the affected operations against a missing `current->files`.
`LocalFile::fget()` returns `EBADF` and
`FileDescriptorReservation::get_unused_fd_flags()` returns `EMFILE`.
For the infallible `fd_install()` and drop paths, warn once and avoid
calling the corresponding C helper when the fd table is already gone.

This prevents NULL dereferences through these safe Rust APIs after
`exit_files()`.

Fixes: 851849824bb5 ("rust: file: add Rust abstraction for `struct file`")
Fixes: 5da9857b127e ("rust: file: add `FileDescriptorReservation`")
Closes: https://github.com/Rust-for-Linux/linux/issues/1256
Signed-off-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
---
 rust/kernel/fs/file.rs | 69 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 60 insertions(+), 9 deletions(-)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 23ee689bd240..559f0985b12c 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -260,7 +260,17 @@ impl LocalFile {
     /// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
     #[inline]
     pub fn fget(fd: u32) -> Result<ARef<LocalFile>, BadFdError> {
-        // SAFETY: FFI call, there are no requirements on `fd`.
+        let current = crate::current!();
+
+        // SAFETY: `current` points to the currently executing task, so it is
+        // valid to read its `files` pointer. The pointer may be null during
+        // task teardown.
+        if unsafe { (*current.as_ptr()).files.is_null() } {
+            return Err(BadFdError);
+        }
+
+        // SAFETY: There are no requirements on `fd`. We checked above that the
+        // current task still has a file descriptor table, which `fget` accesses.
         let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
 
         // SAFETY: `bindings::fget` created a refcount, and we pass ownership of it to the `ARef`.
@@ -403,7 +413,18 @@ impl FileDescriptorReservation {
     /// Creates a new file descriptor reservation.
     #[inline]
     pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
-        // SAFETY: FFI call, there are no safety requirements on `flags`.
+        let current = crate::current!();
+
+        // SAFETY: `current` points to the currently executing task, so it is
+        // valid to read its `files` pointer. The pointer may be null during
+        // task teardown.
+        if unsafe { (*current.as_ptr()).files.is_null() } {
+            return Err(EMFILE);
+        }
+
+        // SAFETY: There are no safety requirements on `flags`. We checked above
+        // that the current task still has a file descriptor table, which
+        // `get_unused_fd_flags` accesses.
         let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
         to_result(fd)?;
 
@@ -421,13 +442,30 @@ pub fn reserved_fd(&self) -> u32 {
 
     /// Commits the reservation.
     ///
-    /// The previously reserved file descriptor is bound to `file`. This method consumes the
-    /// [`FileDescriptorReservation`], so it will not be usable after this call.
+    /// The previously reserved file descriptor is bound to `file`. If the current task no longer
+    /// has a file descriptor table, the reservation is abandoned instead. This method consumes the
+    /// [`FileDescriptorReservation`] in either case.
     #[inline]
     pub fn fd_install(self, file: ARef<File>) {
-        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
-        // the fd, so it is still valid, and `current` still refers to the same task, as this type
-        // cannot be moved across task boundaries.
+        let current = crate::current!();
+
+        // SAFETY: `current` points to the currently executing task, so it is
+        // valid to read its `files` pointer. The pointer may be null during
+        // task teardown.
+        if unsafe { (*current.as_ptr()).files.is_null() } {
+            crate::pr_warn_once!(
+                "FileDescriptorReservation::fd_install called with current->files == NULL\n"
+            );
+
+            // `put_unused_fd` also requires `current->files` to be valid, so do not run
+            // the reservation's destructor after the current task has lost its fd table.
+            core::mem::forget(self);
+            return;
+        }
+
+        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags` and has not yet been
+        // used. This type cannot be moved across task boundaries, so `current` still refers to the
+        // same task, and we checked above that it still has an fd table.
         //
         // Furthermore, the file pointer is guaranteed to own a refcount by its type invariants,
         // and we take ownership of that refcount by not running the destructor below.
@@ -446,9 +484,22 @@ pub fn fd_install(self, file: ARef<File>) {
 impl Drop for FileDescriptorReservation {
     #[inline]
     fn drop(&mut self) {
+        let current = crate::current!();
+
+        // SAFETY: `current` points to the currently executing task, so it is
+        // valid to read its `files` pointer. The pointer may be null during
+        // task teardown.
+        if unsafe { (*current.as_ptr()).files.is_null() } {
+            crate::pr_warn_once!(
+                "FileDescriptorReservation dropped with current->files == NULL\n"
+            );
+            return;
+        }
+
         // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
-        // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
-        // still refers to the same task, as this type cannot be moved across task boundaries.
+        // `get_unused_fd_flags` and has not yet been used. This type cannot be moved across task
+        // boundaries, so `current` still refers to the same task, and we checked above that it
+        // still has an fd table.
         unsafe { bindings::put_unused_fd(self.fd) };
     }
 }
-- 
2.47.3