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Subject[PATCH] file capabilities: don't prevent signaling setuid root programs.
This patch is needed to preserve legacy behavior when
CONFIG_SECURITY_FILE_CAPABILITIES=y. Without this patch, xinit can't
kill X, so manually starting X in runlevel 3 then exiting your window
manager will not cause X to exit.


From 81a6d780ad570f9a326fc27912ec0e373f5fa14f Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <>
Date: Tue, 20 Nov 2007 08:47:35 +0000
Subject: [PATCH] file capabilities: don't prevent signaling setuid root programs.

An unprivileged process must be able to kill a setuid root
program started by the same user. This is legacy behavior
needed for instance for xinit to kill X when the window manager

When an unprivileged user runs a setuid root program in !SECURE_NOROOT
mode, fP, fI, and fE are set full on, so pP' and pE' are full on.
Then cap_task_kill() prevents the user from signaling the setuid root
task. This is a change in behavior compared to when

This patch introduces a special check into cap_task_kill() just
to check whether a non-root user is signaling a setuid root
program started by the same user. If so, then signal is allowed.

Nov 26: move test up above CAP_KILL test as per Andrew
Morgan's suggestion.

Signed-off-by: Serge E. Hallyn <>
security/commoncap.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 302e8d0..5bc1895 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -526,6 +526,15 @@ int cap_task_kill(struct task_struct *p, struct siginfo *info,
if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info)))
return 0;

+ /*
+ * Running a setuid root program raises your capabilities.
+ * Killing your own setuid root processes was previously
+ * allowed.
+ * We must preserve legacy signal behavior in this case.
+ */
+ if (p->euid == 0 && p->uid == current->uid)
+ return 0;
/* sigcont is permitted within same session */
if (sig == SIGCONT && (task_session_nr(current) == task_session_nr(p)))
return 0;
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