man7/cgroup_namespaces.7

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  25 .\"
  26 .TH CGROUP_NAMESPACES 7 2017-09-15 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 cgroup_namespaces \- overview of Linux cgroup namespaces
  29 .SH DESCRIPTION
  30 For an overview of namespaces, see
  31 .BR namespaces (7).
  32 .PP
  33 Cgroup namespaces virtualize the view of a process's cgroups (see
  34 .BR cgroups (7))
  35 as seen via
  36 .IR /proc/[pid]/cgroup
  37 and
  38 .IR /proc/[pid]/mountinfo .
  39 .PP
  40 Each cgroup namespace has its own set of cgroup root directories.
  41 These root directories are the base points for the relative
  42 locations displayed in the corresponding records in the
  43 .IR /proc/[pid]/cgroup
  44 file.
  45 When a process creates a new cgroup namespace using
  46 .BR clone (2)
  47 or
  48 .BR unshare (2)
  49 with the
  50 .BR CLONE_NEWCGROUP
  51 flag, it enters a new cgroup namespace in which its current
  52 cgroups directories become the cgroup root directories
  53 of the new namespace.
  54 (This applies both for the cgroups version 1 hierarchies
  55 and the cgroups version 2 unified hierarchy.)
  56 .PP
  57 When viewing
  58 .IR /proc/[pid]/cgroup ,
  59 the pathname shown in the third field of each record will be
  60 relative to the reading process's root directory
  61 for the corresponding cgroup hierarchy.
  62 If the cgroup directory of the target process lies outside
  63 the root directory of the reading process's cgroup namespace,
  64 then the pathname will show
  65 .I ../
  66 entries for each ancestor level in the cgroup hierarchy.
  67 .PP
  68 The following shell session demonstrates the effect of creating
  69 a new cgroup namespace.
  70 .PP
  71 First, (as superuser) we create a child cgroup in the
  72 .I freezer
  73 hierarchy, and place a process in that cgroup that we will
  74 use as part of the demonstration below:
  75 .PP
  76 .in +4n
  77 .EX
  78 # \fBmkdir \-p /sys/fs/cgroup/freezer/sub2\fP
  79 # \fBsleep 10000 &\fP     # Create a process that lives for a while
  80 [1] 20124
  81 # \fBecho 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs\fP
  82 .EE
  83 .in
  84 .PP
  85 We then create another child cgroup in the
  86 .I freezer
  87 hierarchy and put the shell into that cgroup:
  88 .PP
  89 .in +4n
  90 .EX
  91 # \fBmkdir \-p /sys/fs/cgroup/freezer/sub\fP
  92 # \fBecho $$\fP                      # Show PID of this shell
  93 30655
  94 # \fBecho 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs\fP
  95 # \fBcat /proc/self/cgroup | grep freezer\fP
  96 7:freezer:/sub
  97 .EE
  98 .in
  99 .PP
 100 Next, we use
 101 .BR unshare (1)
 102 to create a process running a new shell in new cgroup and mount namespaces:
 103 .PP
 104 .EX
 105 .in +4n
 106 # \fBunshare \-Cm bash\fP
 107 .in
 108 .EE
 109 .PP
 110 We then inspect the
 111 .IR /proc/[pid]/cgroup
 112 files of, respectively, the new shell process started by the
 113 .BR unshare (1)
 114 command, a process that is in the original cgroup namespace
 115 .RI ( init ,
 116 with PID 1), and the process in the sibling cgroup
 117 .RI ( sub2 ):
 118 .PP
 119 .EX
 120 .in +4n
 121 $ \fBcat /proc/self/cgroup | grep freezer\fP
 122 7:freezer:/
 123 $ \fBcat /proc/1/cgroup | grep freezer\fP
 124 7:freezer:/..
 125 $ \fBcat /proc/20124/cgroup | grep freezer\fP
 126 7:freezer:/../sub2
 127 .in
 128 .EE
 129 .PP
 130 From the output of the first command,
 131 we see that the freezer cgroup membership of the new shell
 132 (which is in the same cgroup as the initial shell)
 133 is shown defined relative to the freezer cgroup root directory
 134 that was established when the new cgroup namespace was created.
 135 (In absolute terms,
 136 the new shell is in the
 137 .I /sub
 138 freezer cgroup,
 139 and the root directory of the freezer cgroup hierarchy
 140 in the new cgroup namespace is also
 141 .IR /sub .
 142 Thus, the new shell's cgroup membership is displayed as \(aq/\(aq.)
 143 .PP
 144 However, when we look in
 145 .IR /proc/self/mountinfo
 146 we see the following anomaly:
 147 .PP
 148 .EX
 149 .in +4n
 150 # \fBcat /proc/self/mountinfo | grep freezer\fP
 151 155 145 0:32 /.. /sys/fs/cgroup/freezer ...
 152 .in
 153 .EE
 154 .PP
 155 The fourth field of this line
 156 .RI ( /.. )
 157 should show the
 158 directory in the cgroup filesystem which forms the root of this mount.
 159 Since by the definition of cgroup namespaces, the process's current
 160 freezer cgroup directory became its root freezer cgroup directory,
 161 we should see \(aq/\(aq in this field.
 162 The problem here is that we are seeing a mount entry for the cgroup
 163 filesystem corresponding to our initial shell process's cgroup namespace
 164 (whose cgroup filesystem is indeed rooted in the parent directory of
 165 .IR sub ).
 166 We need to remount the freezer cgroup filesystem
 167 inside this cgroup namespace, after which we see the expected results:
 168 .PP
 169 .EX
 170 .in +4n
 171 # \fBmount \-\-make\-rslave /\fP     # Don't propagate mount events
 172                             # to other namespaces
 173 # \fBumount /sys/fs/cgroup/freezer\fP
 174 # \fBmount \-t cgroup \-o freezer freezer /sys/fs/cgroup/freezer\fP
 175 # \fBcat /proc/self/mountinfo | grep freezer\fP
 176 155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...
 177 .in
 178 .EE
 179 .\"
 180 .SH CONFORMING TO
 181 Namespaces are a Linux-specific feature.
 182 .SH NOTES
 183 Use of cgroup namespaces requires a kernel that is configured with the
 184 .B CONFIG_CGROUPS
 185 option.
 186 .PP
 187 The virtualization provided by cgroup namespaces serves a number of purposes:
 188 .IP * 2
 189 It prevents information leaks whereby cgroup directory paths outside of
 190 a container would otherwise be visible to processes in the container.
 191 Such leakages could, for example,
 192 reveal information about the container framework
 193 to containerized applications.
 194 .IP *
 195 It eases tasks such as container migration.
 196 The virtualization provided by cgroup namespaces
 197 allows containers to be isolated from knowledge of
 198 the pathnames of ancestor cgroups.
 199 Without such isolation, the full cgroup pathnames (displayed in
 200 .IR /proc/self/cgroups )
 201 would need to be replicated on the target system when migrating a container;
 202 those pathnames would also need to be unique,
 203 so that they don't conflict with other pathnames on the target system.
 204 .IP *
 205 It allows better confinement of containerized processes,
 206 because it is possible to mount the container's cgroup filesystems such that
 207 the container processes can't gain access to ancestor cgroup directories.
 208 Consider, for example, the following scenario:
 209 .RS 4
 210 .IP \(bu 2
 211 We have a cgroup directory,
 212 .IR /cg/1 ,
 213 that is owned by user ID 9000.
 214 .IP \(bu
 215 We have a process,
 216 .IR X ,
 217 also owned by user ID 9000,
 218 that is namespaced under the cgroup
 219 .IR /cg/1/2
 220 (i.e.,
 221 .I X
 222 was placed in a new cgroup namespace via
 223 .BR clone (2)
 224 or
 225 .BR unshare (2)
 226 with the
 227 .BR CLONE_NEWCGROUP
 228 flag).
 229 .RE
 230 .IP
 231 In the absence of cgroup namespacing, because the cgroup directory
 232 .IR /cg/1
 233 is owned (and writable) by UID 9000 and process
 234 .I X
 235 is also owned by user ID 9000, then process
 236 .I X
 237 would be able to modify the contents of cgroups files
 238 (i.e., change cgroup settings) not only in
 239 .IR /cg/1/2
 240 but also in the ancestor cgroup directory
 241 .IR /cg/1 .
 242 Namespacing process
 243 .IR X
 244 under the cgroup directory
 245 .IR /cg/1/2 ,
 246 in combination with suitable mount operations
 247 for the cgroup filesystem (as shown above),
 248 prevents it modifying files in
 249 .IR /cg/1 ,
 250 since it cannot even see the contents of that directory
 251 (or of further removed cgroup ancestor directories).
 252 Combined with correct enforcement of hierarchical limits,
 253 this prevents process
 254 .I X
 255 from escaping the limits imposed by ancestor cgroups.
 256 .SH SEE ALSO
 257 .BR unshare (1),
 258 .BR clone (2),
 259 .BR setns (2),
 260 .BR unshare (2),
 261 .BR proc (5),
 262 .BR cgroups (7),
 263 .BR credentials (7),
 264 .BR namespaces (7),
 265 .BR user_namespaces (7)