[PATCH v2] exec: clarify reasoning for euid/egid reset

[Kees Cook <keescook@chromium.org>]

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 <keescook@chromium.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
---
v2:
- clarified example as a setuid script, dhowells
---
 fs/exec.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index c4010b8207a1..a98b21d47385 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 setuid script then again for its interpreter).
+	 */
 	bprm->cred->euid = current_euid();
 	bprm->cred->egid = current_egid();
 
-- 
2.6.3


-- 
Kees Cook
Chrome OS & Brillo Security