shared/capability: don't be too frugal on space for caps

[elogind.git] / src / shared / capability.c

diff --git a/src/shared/capability.c b/src/shared/capability.c
index f7513379a2c9f2c18cb43a1b194d18cca56f9740..57f392a4da887228c616577f63c8b37ca8ad03b5 100644 (file)
--- a/src/shared/capability.c
+++ b/src/shared/capability.c
@@ -54,11 +54,25 @@ int have_effective_cap(int value) {
 unsigned long cap_last_cap(void) {
         static thread_local unsigned long saved;
         static thread_local bool valid = false;
+        _cleanup_free_ char *content = NULL;
         unsigned long p;
+        int r;
 
         if (valid)
                 return saved;
 
+        /* available since linux-3.2 */
+        r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
+        if (r >= 0) {
+                r = safe_atolu(content, &p);
+                if (r >= 0) {
+                        saved = p;
+                        valid = true;
+                        return p;
+                }
+        }
+
+        /* fall back to syscall-probing for pre linux-3.2 */
         p = (unsigned long) CAP_LAST_CAP;
 
         if (prctl(PR_CAPBSET_READ, p) < 0) {
@@ -216,8 +230,8 @@ int capability_bounding_set_drop_usermode(uint64_t drop) {
 }
 
 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
-
         _cleanup_cap_free_ cap_t d = NULL;
+        unsigned i, j = 0;
         int r;
 
         /* Unfortunately we cannot leave privilege dropping to PID 1
@@ -227,61 +241,72 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
          * binary has the capability configured in the file system,
          * which we want to avoid. */
 
-        if (setresgid(gid, gid, gid) < 0) {
-                log_error("Failed to change group ID: %m");
-                return -errno;
-        }
+        if (setresgid(gid, gid, gid) < 0)
+                return log_error_errno(errno, "Failed to change group ID: %m");
 
-        if (setgroups(0, NULL) < 0) {
-                log_error("Failed to drop auxiliary groups list: %m");
-                return -errno;
-        }
+        if (setgroups(0, NULL) < 0)
+                return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 
-        if (prctl(PR_SET_KEEPCAPS, 1) < 0) {
-                log_error("Failed to enable keep capabilities flag: %m");
-                return -errno;
-        }
+        /* Ensure we keep the permitted caps across the setresuid() */
+        if (prctl(PR_SET_KEEPCAPS, 1) < 0)
+                return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 
         r = setresuid(uid, uid, uid);
-        if (r < 0) {
-                log_error("Failed to change user ID: %m");
-                return -errno;
-        }
+        if (r < 0)
+                return log_error_errno(errno, "Failed to change user ID: %m");
 
-        if (prctl(PR_SET_KEEPCAPS, 0) < 0) {
-                log_error("Failed to disable keep capabilities flag: %m");
-                return -errno;
-        }
+        if (prctl(PR_SET_KEEPCAPS, 0) < 0)
+                return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 
+        /* Drop all caps from the bounding set, except the ones we want */
         r = capability_bounding_set_drop(~keep_capabilities, true);
-        if (r < 0) {
-                log_error_errno(-r, "Failed to drop capabilities: %m");
-                return r;
-        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to drop capabilities: %m");
 
+        /* Now upgrade the permitted caps we still kept to effective caps */
         d = cap_init();
         if (!d)
                 return log_oom();
 
         if (keep_capabilities) {
-                cap_value_t bits[sizeof(keep_capabilities)*8];
-                unsigned i, j = 0;
+                cap_value_t bits[u64log2(keep_capabilities) + 1];
 
-                for (i = 0; i < sizeof(keep_capabilities)*8; i++)
+                for (i = 0; i < ELEMENTSOF(bits); i++)
                         if (keep_capabilities & (1ULL << i))
                                 bits[j++] = i;
 
+                /* don't keep too many bits */
+                assert((keep_capabilities & (~1ULL << i)) == 0);
+                /* don't throw away too many bits */
+                assert(((keep_capabilities >> i) & (~1ULL >> i)) == 0);
+
                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
-                        log_error("Failed to enable capabilities bits: %m");
+                        log_error_errno(errno, "Failed to enable capabilities bits: %m");
                         return -errno;
                 }
+
+                if (cap_set_proc(d) < 0)
+                        return log_error_errno(errno, "Failed to increase capabilities: %m");
         }
 
-        if (cap_set_proc(d) < 0) {
-                log_error("Failed to increase capabilities: %m");
+        return 0;
+}
+
+int drop_capability(cap_value_t cv) {
+        _cleanup_cap_free_ cap_t tmp_cap = NULL;
+
+        tmp_cap = cap_get_proc();
+        if (!tmp_cap)
+                return -errno;
+
+        if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
+            (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
+            (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
+                return -errno;
+
+        if (cap_set_proc(tmp_cap) < 0)
                 return -errno;
-        }
 
         return 0;
 }