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Fix dropping of all capabilities

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From fd.o bug 88898:

systemd-resolved fails to start:
Failed to drop capabilities: Operation not permitted

Broken in f11943c53e.

Drop all capabilities:
1. prctl(PR_SET_KEEPCAPS, keep_capabilities != 0) // 0 when we drop all
capabilities
2. setresuid() // bye bye capabilities
3. Add CAP_SETPCAP // fails because we have no capabilities
4. Reduce capability bounding set
5. Drop capabilities
6. prctl(PR_SET_KEEPCAPS, 0)

Capabilites should always be kept after setresuid() so that the capability
bounding set can be reduced.

Based-on-a-patch-by: mustrumr97@gmail.com

https://bugs.freedesktop.org/show_bug.cgi?id=88898

We must be careful not to leave PR_SET_KEEPCAPS on. We could use the
setresuid() call to drop capabilities, but the rules when capabilities
are dropped are fairly complex, since a transition to non-zero uid must
happen. Let's instead keep the capabilities during setresuid(), and drop
them later.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek
2015-02-01 17:22:32 -05:00
parent 615938651d
commit 51ddf61540
1 changed files with 16 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/shared/capability.c
+16 -16
View File
@@ -230,7 +230,6 @@ int capability_bounding_set_drop_usermode(uint64_t drop) {
}
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;
@@ -248,8 +247,8 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
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)
/* 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);
@@ -264,27 +263,28 @@ int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
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 (keep_capabilities) {
cap_value_t bits[sizeof(keep_capabilities)*8];
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;
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");
}
if (cap_set_proc(d) < 0)
return log_error_errno(errno, "Failed to increase capabilities: %m");
return 0;
}