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From 1707b9959e67e5e73987e1ff8a72189d24656fa0 Mon Sep 17 00:00:00 2001
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From: Krzysztof Nowicki <krzysztof.a.nowicki+github@gmail.com>
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Date: Wed, 28 Mar 2018 13:36:33 +0200
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Subject: [PATCH] core: dont't remount /sys/fs/cgroup for relabel if not needed
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 (#8595)
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The initial fix for relabelling the cgroup filesystem for
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SELinux delivered in commit 8739f23e3 was based on the assumption that
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the cgroup filesystem is already populated once mount_setup() is
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executed, which was true for my system. What I wasn't aware is that this
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is the case only when another instance of systemd was running before
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this one, which can happen if systemd is used in the initrd (for ex. by
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dracut).
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In case of a clean systemd start-up the cgroup filesystem is actually
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being populated after mount_setup() and does not need relabelling as at
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that moment the SELinux policy is already loaded. Since however the root
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cgroup filesystem was remounted read-only in the meantime this operation
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will now fail.
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To fix this check for the filesystem mount flags before relabelling and
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only remount ro->rw->ro if necessary and leave the filesystem read-write
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otherwise.
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Fixes #7901.
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(cherry picked from commit 6f7729c1767998110c4460c85c94435c5782a613)
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---
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 src/core/mount-setup.c | 35 ++++++++++++++++++++++++++++++-----
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 1 file changed, 30 insertions(+), 5 deletions(-)
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diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
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index 7a2cae4a3..ed493cbe3 100644
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--- a/src/core/mount-setup.c
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+++ b/src/core/mount-setup.c
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@@ -25,6 +25,8 @@
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 #include <stdlib.h>
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 #include <string.h>
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 #include <assert.h>
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+#include <sys/statfs.h>
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+#include <sys/statvfs.h>
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 #include <unistd.h>
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 #include <ftw.h>
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@@ -337,6 +339,31 @@ static int nftw_cb(
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         return FTW_CONTINUE;
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 };
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+
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+static int relabel_cgroup_filesystems(void) {
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+        int r;
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+        struct statfs st;
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+
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+        /* Temporarily remount the root cgroup filesystem to give it a proper label. Do this
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+           only when the filesystem has been already populated by a previous instance of systemd
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+           running from initrd. Otherwise don't remount anything and leave the filesystem read-write
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+           for the cgroup filesystems to be mounted inside. */
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+        r = statfs("/sys/fs/cgroup", &st);
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+        if (r < 0) {
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+                return log_error_errno(errno, "Failed to determine mount flags for /sys/fs/cgroup: %m");
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+        }
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+
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+        if (st.f_flags & ST_RDONLY)
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+                (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT, NULL);
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+
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+        (void) label_fix("/sys/fs/cgroup", false, false);
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+        nftw("/sys/fs/cgroup", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
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+
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+        if (st.f_flags & ST_RDONLY)
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+                (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT|MS_RDONLY, NULL);
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+
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+        return 0;
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+}
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 #endif
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 int mount_setup(bool loaded_policy) {
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@@ -369,11 +396,9 @@ int mount_setup(bool loaded_policy) {
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                 nftw("/dev", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
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                 nftw("/run", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
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-                /* Temporarily remount the root cgroup filesystem to give it a proper label. */
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-                (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT, NULL);
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-                label_fix("/sys/fs/cgroup", false, false);
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-                nftw("/sys/fs/cgroup", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
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-                (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT|MS_RDONLY, NULL);
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+                r = relabel_cgroup_filesystems();
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+                if (r < 0)
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+                        return r;
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                 after_relabel = now(CLOCK_MONOTONIC);
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