2016-04-12 16:44:32

by Kees Cook

Subject: [PATCH] exec: clarify reasoning for euid/egid reset

This section of code initially looks redundant, but is required. This
improves the comment to explain more clearly why the reset is needed.

Signed-off-by: Kees Cook <[email protected]>
---
fs/exec.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index c4010b8207a1..889221bbfdb3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1387,7 +1387,12 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
kuid_t uid;
kgid_t gid;

- /* clear any previous set[ug]id data from a previous binary */
+ /*
+ * Since this can be called multiple times (via prepare_binprm),
+ * we must clear any previous work done when setting set[ug]id
+ * bits from any earlier bprm->file uses (for example when run
+ * first for a script then for its interpreter).
+ */
bprm->cred->euid = current_euid();
bprm->cred->egid = current_egid();

--
2.6.3


--
Kees Cook
Chrome OS & Brillo Security

2016-04-12 16:55:20

by Serge Hallyn

Subject: Re: [PATCH] exec: clarify reasoning for euid/egid reset

Quoting Kees Cook ([email protected]):
> This section of code initially looks redundant, but is required. This
> improves the comment to explain more clearly why the reset is needed.
>
> Signed-off-by: Kees Cook <[email protected]>

Thanks, Kees.

Acked-by: Serge E. Hallyn <[email protected]>

> ---
> fs/exec.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index c4010b8207a1..889221bbfdb3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1387,7 +1387,12 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
> kuid_t uid;
> kgid_t gid;
>
> - /* clear any previous set[ug]id data from a previous binary */
> + /*
> + * Since this can be called multiple times (via prepare_binprm),
> + * we must clear any previous work done when setting set[ug]id
> + * bits from any earlier bprm->file uses (for example when run
> + * first for a script then for its interpreter).
> + */
> bprm->cred->euid = current_euid();
> bprm->cred->egid = current_egid();
>
> --
> 2.6.3
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security

2016-04-12 17:18:29

by David Howells

Subject: Re: [PATCH] exec: clarify reasoning for euid/egid reset

Kees Cook <[email protected]> wrote:

> - /* clear any previous set[ug]id data from a previous binary */
> + /*
> + * Since this can be called multiple times (via prepare_binprm),
> + * we must clear any previous work done when setting set[ug]id
> + * bits from any earlier bprm->file uses (for example when run
> + * first for a script then for its interpreter).
> + */
> bprm->cred->euid = current_euid();
> bprm->cred->egid = current_egid();

I would further add that this is where we deal with setuid scripts.

David