From: Kees Cook
> Sent: 30 November 2024 04:49
>
> Instead of adding a new use of the ambiguous strncpy(), we'd want to
> use memtostr_pad() which enforces being able to check at compile time
> that sizes are sensible, but this requires being able to see string
> buffer lengths. Instead of trying to inline __set_task_comm() (which
> needs to call trace and perf functions), just open-code it. But to
> make sure we're always safe, add compile-time checking like we already
> do for get_task_comm().
...
> Here's what I'd prefer to use to clean up set_task_comm(). I merged
> Linus and Eric's suggestions and open-coded memtostr_pad().
> ---
> fs/exec.c | 12 ++++++------
> include/linux/sched.h | 9 ++++-----
> io_uring/io-wq.c | 2 +-
> io_uring/sqpoll.c | 2 +-
> kernel/kthread.c | 3 ++-
> 5 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index e0435b31a811..5f16500ac325 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1200,16 +1200,16 @@ char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> EXPORT_SYMBOL_GPL(__get_task_comm);
>
> /*
> - * These functions flushes out all traces of the currently running executable
> - * so that a new one can be started
> + * This is unlocked -- the string will always be NUL-terminated, but
> + * may show overlapping contents if racing concurrent reads.
> */
> -
> void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
> {
> - task_lock(tsk);
> + size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
> +
> trace_task_rename(tsk, buf);
> - strscpy_pad(tsk->comm, buf, sizeof(tsk->comm));
> - task_unlock(tsk);
> + memcpy(tsk->comm, buf, len);
> + memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
> perf_event_comm(tsk, exec);
Why not do strscpy_pad() into a local char[16] and then do a 16 byte
memcpy() into the target buffer?
Then non-constant input data will always give a valid '\0' terminated string
regardless of how strscpy_pad() is implemented.
David
-
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