I was interested in modifying haveged to drop privileges after opening
/dev/random but discovered this was not possible because it uses the
ioctl RNDADDENTROPY which requires CAP_SYS_ADMIN.
Retaining CAP_SYS_ADMIN after dropping GID/UID would defeat the point
of doing so, so this program must always run with UID 0 and/or
CAP_SYS_ADMIN, which is undesirable.
I attach a patch to add a new capability CAP_RND_ADD, which allows the
use of ioctls RNDADDENTROPY and RNDADDTOENTCNT. It further modifies
drivers/char/random.c to also check for this capability before returning
-EPERM.
==================
--- a/drivers/char/random.c 2012-07-14 02:52:10.781202854 +0100
+++ b/drivers/char/random.c 2012-07-14 02:52:55.369201089 +0100
@@ -1154,14 +1154,14 @@
return -EFAULT;
return 0;
case RNDADDTOENTCNT:
- if (!capable(CAP_SYS_ADMIN))
+ if (!capable(CAP_SYS_ADMIN) && !capable(CAP_RND_ADD))
return -EPERM;
if (get_user(ent_count, p))
return -EFAULT;
credit_entropy_bits(&input_pool, ent_count);
return 0;
case RNDADDENTROPY:
- if (!capable(CAP_SYS_ADMIN))
+ if (!capable(CAP_SYS_ADMIN) && !capable(CAP_RND_ADD))
return -EPERM;
if (get_user(ent_count, p++))
return -EFAULT;
--- a/include/linux/capability.h 2012-07-14 03:15:52.378624902 +0100
+++ b/include/linux/capability.h 2012-07-14 03:16:47.508624928 +0100
@@ -364,7 +364,18 @@
#define CAP_EPOLLWAKEUP 36
-#define CAP_LAST_CAP CAP_EPOLLWAKEUP
+/* Allow adding of random entropy and updating entropy estimate,
+ but not clearing the entropy pool (see drivers/char/random.c)
+ Introduced so that software like haveged can drop gid/uid
+ on startup and drop all capabilities except this one.
+ Otherwise it would require CAP_SYS_ADMIN, which would
+ defeat the point of dropping gid/uid. */
+
+#define CAP_RND_ADD 37
+
+
+
+#define CAP_LAST_CAP CAP_RND_ADD
#define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
--
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