On Fri, Nov 29, 2024 at 08:54:38PM -0800, Kees Cook wrote:
> Zbigniew mentioned at Linux Plumber's that systemd is interested in
> switching to execveat() for service execution, but can't, because the
> contents of /proc/pid/comm are the file descriptor which was used,
> instead of the path to the binary. This makes the output of tools like
> top and ps useless, especially in a world where most fds are opened
> CLOEXEC so the number is truly meaningless.
>
> When the filename passed in is empty (e.g. with AT_EMPTY_PATH), use the
> dentry's filename for "comm" instead of using the useless numeral from
> the synthetic fdpath construction. This way the actual exec machinery
> is unchanged, but cosmetically the comm looks reasonable to admins
> investigating things.
>
> Instead of adding TASK_COMM_LEN more bytes to bprm, use one of the unused
> flag bits to indicate that we need to set "comm" from the dentry.
>
> Suggested-by: Zbigniew Jędrzejewski-Szmek <[email protected]>
> Suggested-by: Tycho Andersen <[email protected]>
> Suggested-by: Al Viro <[email protected]>
> Suggested-by: Linus Torvalds <[email protected]>
> CC: Aleksa Sarai <[email protected]>
> Link: https://github.com/uapi-group/kernel-features#set-comm-field-before-exec
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Al Viro <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Cc: Eric Biederman <[email protected]>
> Cc: Alexander Viro <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Jan Kara <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
>
> Here's what I've put together from the various suggestions. I didn't
> want to needlessly grow bprm, so I just added a flag instead. Otherwise,
> this is very similar to what Linus and Al suggested.
> ---
> fs/exec.c | 22 +++++++++++++++++++---
> include/linux/binfmts.h | 4 +++-
> 2 files changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 5f16500ac325..d897d60ca5c2 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1347,7 +1347,21 @@ int begin_new_exec(struct linux_binprm * bprm)
> set_dumpable(current->mm, SUID_DUMP_USER);
>
> perf_event_exec();
> - __set_task_comm(me, kbasename(bprm->filename), true);
> +
> + /*
> + * If the original filename was empty, alloc_bprm() made up a path
> + * that will probably not be useful to admins running ps or similar.
> + * Let's fix it up to be something reasonable.
> + */
> + if (bprm->comm_from_dentry) {
> + rcu_read_lock();
> + /* The dentry name won't change while we hold the rcu read
> lock. */
> + __set_task_comm(me,
> smp_load_acquire(&bprm->file->f_path.dentry->d_name.name),
> + true);
This does not sound legit whatsoever as it would indicate all renames
wait for rcu grace periods to end, which would be prettye weird.
Even commentary above dentry_cmp states:
* Be careful about RCU walk racing with rename:
* use 'READ_ONCE' to fetch the name pointer.
*
* NOTE! Even if a rename will mean that the length
* was not loaded atomically, we don't care.
It may be this is considered tolerable, but there should be no
difficulty getting a real name there?
Regardless, the comment looks bogus.
