coredump: drop caps while we are processing the coredump

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

diff --git a/src/shared/capability.c b/src/shared/capability.c
index b49c5140097542bf8d54899dcef6e3b817d029c8..b39e8e2359d3bc77ea67d52fe92b92370e850f47 100644 (file)
--- a/src/shared/capability.c
+++ b/src/shared/capability.c
@@ -29,12 +29,13 @@
 #include <ctype.h>
 #include <sys/capability.h>
 #include <sys/prctl.h>
+#include "grp.h"
 
 #include "macro.h"
-#include "capability.h"
 #include "util.h"
 #include "log.h"
 #include "fileio.h"
+#include "capability.h"
 
 int have_effective_cap(int value) {
         _cleanup_cap_free_ cap_t cap;
@@ -53,11 +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) {
@@ -84,9 +99,9 @@ unsigned long cap_last_cap(void) {
 }
 
 int capability_bounding_set_drop(uint64_t drop, bool right_now) {
-        unsigned long i;
-        _cleanup_cap_free_ cap_t after_cap = NULL, temp_cap = NULL;
+        _cleanup_cap_free_ cap_t after_cap = NULL;
         cap_flag_value_t fv;
+        unsigned long i;
         int r;
 
         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
@@ -102,6 +117,7 @@ int capability_bounding_set_drop(uint64_t drop, bool right_now) {
                 return -errno;
 
         if (fv != CAP_SET) {
+                _cleanup_cap_free_ cap_t temp_cap = NULL;
                 static const cap_value_t v = CAP_SETPCAP;
 
                 temp_cap = cap_dup(after_cap);
@@ -212,3 +228,80 @@ int capability_bounding_set_drop_usermode(uint64_t drop) {
 
         return r;
 }
+
+int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
+        cap_value_t bits[sizeof(keep_capabilities)*8];
+        _cleanup_cap_free_ cap_t d = NULL;
+        unsigned i, j = 0;
+        int r;
+
+        /* Unfortunately we cannot leave privilege dropping to PID 1
+         * here, since we want to run as user but want to keep some
+         * capabilities. Since file capabilities have been introduced
+         * this cannot be done across exec() anymore, unless our
+         * binary has the capability configured in the file system,
+         * which we want to avoid. */
+
+        if (setresgid(gid, gid, gid) < 0)
+                return log_error_errno(errno, "Failed to change group ID: %m");
+
+        if (setgroups(0, NULL) < 0)
+                return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
+
+        /* Ensure we keep the permitted caps across the setresuid(), if we need them */
+        if (prctl(PR_SET_KEEPCAPS, keep_capabilities != 0) < 0)
+                return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
+
+        r = setresuid(uid, uid, uid);
+        if (r < 0)
+                return log_error_errno(errno, "Failed to change user ID: %m");
+
+        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)
+                return log_error_errno(r, "Failed to drop capabilities: %m");
+
+        if (keep_capabilities == 0) /* All is gone, we can exit early */
+                return 0;
+
+        /* Now upgrade the permitted caps we still kept to effective caps */
+        d = cap_init();
+        if (!d)
+                return log_oom();
+
+        for (i = 0; i < sizeof(keep_capabilities)*8; i++)
+                if (keep_capabilities & (1ULL << i))
+                        bits[j++] = i;
+
+        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_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");
+
+        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;
+}