[RFC] make reading /proc/sys/kernel/cap-bould not require CAP_SYS_MODULE

[RFC] make reading /proc/sys/kernel/cap-bould not require CAP_SYS_MODULE

[Eric Paris]

Reading /proc/sys/kernel/cap-bound requires CAP_SYS_MODULE.  (see
proc_dointvec_bset in kernel/sysctl.c)

sysctl appears to drive all over proc reading everything it can get it's
hands on and is complaining when it is being denied access to read
cap-bound.  Clearly writing to cap-bound should be a sensitive operation
but requiring CAP_SYS_MODULE to read cap-bound seems a bit to strong.  I
believe the information could with reasonable certainty be obtained by
looking at a bunch of the output of /proc/pid/status which has very low
security protection, so at best we are just getting a little obfuscation
of information.

Currently SELinux policy has to 'dontaudit' capability checks for
CAP_SYS_MODULE for things like sysctl which just want to read cap-bound.
In doing so we also as a by product have to hide warnings of potential
exploits such as if at some time that sysctl actually tried to load a
module.  I wondered if anyone would have a problem opening cap-bound up
to read from anyone?  Possibly with something like the patch below?

-Eric

 kernel/sysctl.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-2.6-upstream/kernel/sysctl.c.cap.sys.module
+++ linux-2.6-upstream/kernel/sysctl.c
@@ -2020,7 +2020,7 @@ int proc_dointvec_bset(ctl_table *table,
 {
 	int op;
 
-	if (!capable(CAP_SYS_MODULE)) {
+	if (write && !capable(CAP_SYS_MODULE)) {
 		return -EPERM;
 	}