Distributions face a backward compatibility problem with starting to use
file capabilities. For instance, removing setuid root from ping and
doing setcap cap_net_raw=pe means that booting with an older kernel
or one compiled without file capabilities means ping won't work for
non-root users.
In order to replace the setuid root bit on a capability-unaware
program, one has to set the effective, or legacy, file capability,
which makes the capability effective immediately. This patch
uses the legacy bit as a queue to not automatically add full
privilege to a setuid-root program.
So, with this patch, an ordinary setuid-root program will run with
privilege. But if /bin/ping has both setuid-root and cap_net_raw in
fP and fE, then ping (when run by non-root user) will not run
with only cap_net_raw.
Changelog:
Apr 2 2009: Print a message once when such a binary is loaded,
as per James Morris' suggestion.
Apr 2 2009: Fix the condition to only catch uid!=0 && euid==0.
Signed-off-by: Serge E. Hallyn <[email protected]>
Acked-by: Casey Schaufler <[email protected]>
---
security/commoncap.c | 32 ++++++++++++++++++++++++++++++++
1 files changed, 32 insertions(+), 0 deletions(-)
diff --git a/security/commoncap.c b/security/commoncap.c
index 7cd61a5..97ac1f1 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -28,6 +28,28 @@
#include <linux/prctl.h>
#include <linux/securebits.h>
+/*
+ * If a non-root user executes a setuid-root binary in
+ * !secure(SECURE_NOROOT) mode, then we raise capabilities.
+ * However if fE is also set, then the intent is for only
+ * the file capabilities to be applied, and the setuid-root
+ * bit is left on either to change the uid (plausible) or
+ * to get full privilege on a kernel without file capabilities
+ * support. So in that case we do not raise capabilities.
+ *
+ * Warn if that happens, once per boot.
+ */
+static void warn_setuid_and_fcaps_mixed(char *fname)
+{
+ static int warned;
+ if (!warned) {
+ printk(KERN_INFO "warning: `%s' has both setuid-root and"
+ " effective capabilities. Therefore not raising all"
+ " capabilities.\n", fname);
+ warned = 1;
+ }
+}
+
int cap_netlink_send(struct sock *sk, struct sk_buff *skb)
{
NETLINK_CB(skb).eff_cap = current_cap();
@@ -464,6 +486,15 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
if (!issecure(SECURE_NOROOT)) {
/*
+ * If the legacy file capability is set, then don't set privs
+ * for a setuid root binary run by a non-root user. Do set it
+ * for a root user just to cause least surprise to an admin.
+ */
+ if (effective && new->uid != 0 && new->euid == 0) {
+ warn_setuid_and_fcaps_mixed(bprm->filename);
+ goto skip;
+ }
+ /*
* To support inheritance of root-permissions and suid-root
* executables under compatibility mode, we override the
* capability sets for the file.
@@ -478,6 +509,7 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
if (new->euid == 0)
effective = true;
}
+skip:
/* Don't let someone trace a set[ug]id/setpcap binary with the revised
* credentials unless they have the appropriate permit
--
1.5.4.3
On Thu, 2 Apr 2009, Serge E. Hallyn wrote:
> Changelog:
> Apr 2 2009: Print a message once when such a binary is loaded,
> as per James Morris' suggestion.
> Apr 2 2009: Fix the condition to only catch uid!=0 && euid==0.
>
> Signed-off-by: Serge E. Hallyn <[email protected]>
> Acked-by: Casey Schaufler <[email protected]>
Applied to
git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/security-testing-2.6#next
--
James Morris
<[email protected]>
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Serge E. Hallyn wrote:
> Distributions face a backward compatibility problem with starting to use
> file capabilities. For instance, removing setuid root from ping and
> doing setcap cap_net_raw=pe means that booting with an older kernel
> or one compiled without file capabilities means ping won't work for
> non-root users.
>
> In order to replace the setuid root bit on a capability-unaware
> program, one has to set the effective, or legacy, file capability,
> which makes the capability effective immediately. This patch
> uses the legacy bit as a queue to not automatically add full
s/queue/cue/
> privilege to a setuid-root program.
>
> So, with this patch, an ordinary setuid-root program will run with
> privilege. But if /bin/ping has both setuid-root and cap_net_raw in
> fP and fE, then ping (when run by non-root user) will not run
> with only cap_net_raw.
Acked-by: Andrew G. Morgan <[email protected]>
Cheers
Andrew
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> Serge E. Hallyn wrote:
> > Distributions face a backward compatibility problem with starting to use
> > file capabilities. For instance, removing setuid root from ping and
> > doing setcap cap_net_raw=pe means that booting with an older kernel
> > or one compiled without file capabilities means ping won't work for
> > non-root users.
> >
> > In order to replace the setuid root bit on a capability-unaware
> > program, one has to set the effective, or legacy, file capability,
> > which makes the capability effective immediately. This patch
> > uses the legacy bit as a queue to not automatically add full
>
> s/queue/cue/
Oops.
(I wonder whether this is worth resending?)
> > privilege to a setuid-root program.
> >
> > So, with this patch, an ordinary setuid-root program will run with
> > privilege. But if /bin/ping has both setuid-root and cap_net_raw in
> > fP and fE, then ping (when run by non-root user) will not run
> > with only cap_net_raw.
>
> Acked-by: Andrew G. Morgan <[email protected]>
thanks,
-serge