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 `fd_install()`, warn and abandon the reservation when the fd table is already gone. In the drop path, skip `put_unused_fd()` after the fd table has been torn down. 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 --- Changes in v2: - Use `warn_on!()` for `FileDescriptorReservation::fd_install()` and remove the warning from the drop path, following feedback from Gary Guo. - Link to v1: https://lore.kernel.org/rust-for-linux/20260920200154.1194983-1-georgeandrout13@gmail.com/ --- rust/kernel/fs/file.rs | 64 ++++++++++++++++++++++++++++++++++++------ 1 file changed, 55 insertions(+), 9 deletions(-) diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs index 23ee689bd240..8b5b5ec4cd04 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, 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 { - // 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 +442,26 @@ 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) { - // 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 crate::warn_on!(unsafe { (*current.as_ptr()).files.is_null() }) { + // `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 +480,21 @@ pub fn fd_install(self, file: ARef) { 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() } { + // `put_unused_fd` uses `current->files`, so it cannot be called + // after the current task has torn down its fd table. + 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