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| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2018-04-13 21:04:29 +0200 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2018-04-13 21:23:28 +0200 |
| commit | bcaa30c985fc5366496abd80765c313ddd95fcce (patch) | |
| tree | a61608935a5489cef1b104baeda0f25598f7531f /man7 | |
| parent | 6442c03b6815eda1202def03cc1e4eb9a57830f1 (diff) | |
| download | man-pages-bcaa30c985fc5366496abd80765c313ddd95fcce.tar.gz | |
capabilities.7: Rework file capability versioning and namespaced file caps text
There was some confused missing of concepts between the
two subsections, and some other details that needed fixing up.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Diffstat (limited to 'man7')
| -rw-r--r-- | man7/capabilities.7 | 85 |
1 files changed, 54 insertions, 31 deletions
diff --git a/man7/capabilities.7 b/man7/capabilities.7 index 697bb079a2..8f54a4c68c 100644 --- a/man7/capabilities.7 +++ b/man7/capabilities.7 @@ -932,7 +932,7 @@ then the effective flag must also be specified as enabled for all other capabilities for which the corresponding permitted or inheritable flags is enabled. .\" -.SS File capability version numbering +.SS File capability mask versioning To allow extensibility, the kernel supports a scheme to encode a version number inside the .I security.capability @@ -953,34 +953,44 @@ The kernel transparently continues to support the execution of files that have 32-bit version 1 capability masks, but when adding capabilities to files that did not previously have capabilities, or modifying the capabilities of existing files, -it automatically uses the version 2 scheme. +it automatically uses the version 2 scheme +(or possibly the version 3 scheme, as described below). .TP .BR VFS_CAP_REVISION_3 " (since Linux 4.14)" .\" commit 8db6c34f1dbc8e06aa016a9b829b06902c3e1340 -Version 3 file capabilities are designed to coexist -with version 2 capabilities, but serve a different purpose: +Version 3 file capabilities are provided to support namespaced file capabilities (described below). +.IP As with version 2 file capabilities, -the capability masks are 64 bits in size. -In addition, the root user ID of namespace is encoded in the +version 3 capability masks are 64 bits in size. +But in addition, the root user ID of namespace is encoded in the .I security.capability extended attribute. (A namespace's root user ID is the value that user ID 0 inside that namespace maps to in the initial user namespace.) .IP -A binary with -.BR VFS_CAP_REVISION_3 -file capabilities confers capabilities only when executed by a -process that resides in a user namespace whose -UID 0 maps to the root user ID that is saved in the extended attribute, -or when executed by a process that resides in descendant of such a namespace. -.IP +Version 3 file capabilities are designed to coexist +with version 2 capabilities; +that is, on a modern Linux system, +there may be some files with version 2 capabilities +while others have version 3 capabilities. +.PP +Before Linux 4.14, +the only kind of capability mask that could be attached to a file was a +.B VFS_CAP_REVISION_2 +mask. +Since Linux 4.14, +the version of the capability mask that is attached to a file +depends on the circumstances in which the +.I security.capability +extended attribute was created. +.PP Starting with Linux 4.14, a -.BR VFS_CAP_REVISION_3 .I security.capability extended attribute is automatically created as (or converted to) -a version 3 attribute if both of the following are true: -.RS +a version 3 +.RB ( VFS_CAP_REVISION_3 ) +attribute if both of the following are true: .IP (1) 4 The thread writing the attribute resides in a noninitial namespace. (More precisely: the thread resides in a user namespace other @@ -994,15 +1004,14 @@ meaning that (a) the thread has the capability in its own user namespace; and (b) the UID and GID of the file inode have mappings in the writer's user namespace. -.RE -.IP +.PP When a .BR VFS_CAP_REVISION_3 .I security.capability extended attribute is created, the root user ID of the creating thread's user namespace is saved in the extended attribute. -.IP -Creating a +.PP +By contrast, creating a .I security.capability extended attribute from a privileged .RB ( CAP_SETFCAP ) @@ -1010,8 +1019,17 @@ thread that resides in the namespace where the the underlying filesystem was mounted (this normally means the initial user namespace) automatically results in a version 2 -.RB ( VFS_CAP_REVISION_3 ) +.RB ( VFS_CAP_REVISION_2 ) attribute. +.PP +Note that a file can have either a version 2 or a version 3 +.I security.capability +extended attribute associated with it, but not both: +creation or modification of the +.I security.capability +extended attribute will automatically modify the version +according to the circumstances in which the extended attribute is +created or modified. .\" .SS Transformation of capabilities during execve() .PP @@ -1471,7 +1489,7 @@ as per the rules described above in Because version 2 file capabilities confer capabilities to the executing process regardless of which user namespace it resides in, only privileged processes are permitted to associate capabilities with a file. -Here, privileged means a process that has the +Here, "privileged" means a process that has the .BR CAP_SETFCAP capability in the user namespace where the filesystem was mounted (normally the initial user namespace). @@ -1494,16 +1512,21 @@ file capabilities with a file whose user ID matches the user ID of the creator of the namespace. In this case, the kernel records not just the capability masks in the extended attribute, -but also the user ID of the namespace creator. +but also the namespace root user ID. +For further details, see +.IR "File capability mask versioning" , +above. .PP -Note that a file can have either a version 2 or a version 3 -.I security.capability -extended attribute associated with it, but not both: -creation or modification of the -.I security.capability -extended attribute will automatically modify the version -according to the circumstances in which the extended attribute is -created or modified. +As with a binary that has +.BR VFS_CAP_REVISION_2 +file capabilities, a binary with +.BR VFS_CAP_REVISION_3 +file capabilities confers capabilities to a process during +.BR execve (). +However, capabilities are conferred only if the binary is executed by +a process that resides in a user namespace whose +UID 0 maps to the root user ID that is saved in the extended attribute, +or when executed by a process that resides in descendant of such a namespace. .SH CONFORMING TO .PP No standards govern capabilities, but the Linux capability implementation |