Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm
On Mon, Sep 30, 2024 at 03:10:29PM -0500, Eric W. Biederman wrote: > "Eric W. Biederman" writes: > > > Kees Cook writes: > > >> I'm not super comfortable doing this regardless of bprm->fdpath; that > >> seems like too many cases getting changed. Can we just leave it as > >> depending on bprm->fdpath? > > I was recommending that because I did not expect that there was any > widespread usage of aliasing of binary names using symlinks. > > I realized today that on debian there are many aliases > of binaries created with the /etc/alternatives mechanism. > So there is much wider exposure to problems than I would have > supposed. > > So I remove any objections to making the new code conditional on bprm->fdpath. Yep, and it looks like Alpine distributes busybox with symlinks instead of hard links. I will respin with a fixed subject line shortly. Thanks, Tycho
Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm
"Eric W. Biederman" writes: > Kees Cook writes: >> I'm not super comfortable doing this regardless of bprm->fdpath; that >> seems like too many cases getting changed. Can we just leave it as >> depending on bprm->fdpath? I was recommending that because I did not expect that there was any widespread usage of aliasing of binary names using symlinks. I realized today that on debian there are many aliases of binaries created with the /etc/alternatives mechanism. So there is much wider exposure to problems than I would have supposed. So I remove any objections to making the new code conditional on bprm->fdpath. Eric
Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm
On Fri, Sep 27, 2024 at 10:45:58AM -0500, Eric W. Biederman wrote:
> Tycho Andersen writes:
>
> > From: Tycho Andersen
> >
> > 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.
> >
> > Change exec path to fix up /proc/pid/comm in the case where we have
> > allocated one of these synthetic paths in bprm_init(). This way the actual
> > exec machinery is unchanged, but cosmetically the comm looks reasonable to
> > admins investigating things.
>
> Perhaps change the subject to match the code.
>
> > Signed-off-by: Tycho Andersen
> > Suggested-by: Zbigniew Jędrzejewski-Szmek
> > CC: Aleksa Sarai
> > Link:
> > https://github.com/uapi-group/kernel-features#set-comm-field-before-exec
> > ---
> > v2: * drop the flag, everyone :)
> > * change the rendered value to f_path.dentry->d_name.name instead of
> > argv[0], Eric
> > ---
> > fs/exec.c | 13 -
> > 1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index dad402d55681..9520359a8dcc 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1416,7 +1416,18 @@ 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 fdpath was set, execveat() 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->fdpath) {
> > + BUILD_BUG_ON(TASK_COMM_LEN > DNAME_INLINE_LEN);
> > + __set_task_comm(me, bprm->file->f_path.dentry->d_name.name,
> > true);
>
> We can just do this regardless of bprm->fdpath.
>
> It will be a change of behavior on when executing symlinks and possibly
> mount points but I don't think we care. If we do then we can add make
> it conditional with "if (bprm->fdpath)"
>
> At the very least using the above version unconditionally ought to flush
> out any bugs.
I'm not super comfortable doing this regardless of bprm->fdpath; that
seems like too many cases getting changed. Can we just leave it as
depending on bprm->fdpath?
Also, is d_name.name always going to be set? e.g. what about memfd, etc?
--
Kees Cook
Re: [PATCH v2 1/2] exec: add a flag for "reasonable" execveat() comm
Tycho Andersen writes:
> From: Tycho Andersen
>
> 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.
>
> Change exec path to fix up /proc/pid/comm in the case where we have
> allocated one of these synthetic paths in bprm_init(). This way the actual
> exec machinery is unchanged, but cosmetically the comm looks reasonable to
> admins investigating things.
Perhaps change the subject to match the code.
> Signed-off-by: Tycho Andersen
> Suggested-by: Zbigniew Jędrzejewski-Szmek
> CC: Aleksa Sarai
> Link: https://github.com/uapi-group/kernel-features#set-comm-field-before-exec
> ---
> v2: * drop the flag, everyone :)
> * change the rendered value to f_path.dentry->d_name.name instead of
> argv[0], Eric
> ---
> fs/exec.c | 13 -
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index dad402d55681..9520359a8dcc 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1416,7 +1416,18 @@ 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 fdpath was set, execveat() 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->fdpath) {
> + BUILD_BUG_ON(TASK_COMM_LEN > DNAME_INLINE_LEN);
> + __set_task_comm(me, bprm->file->f_path.dentry->d_name.name,
> true);
We can just do this regardless of bprm->fdpath.
It will be a change of behavior on when executing symlinks and possibly
mount points but I don't think we care. If we do then we can add make
it conditional with "if (bprm->fdpath)"
At the very least using the above version unconditionally ought to flush
out any bugs.
It should be 99% application invisible as all an application can see
is argv0. So it is only ps and friends where the comm value is visible.
Eric
