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Diffstat (limited to 'man7/capabilities.7')
| -rw-r--r-- | man7/capabilities.7 | 48 |
1 files changed, 47 insertions, 1 deletions
diff --git a/man7/capabilities.7 b/man7/capabilities.7 index 693c57f025..839b167633 100644 --- a/man7/capabilities.7 +++ b/man7/capabilities.7 @@ -959,7 +959,7 @@ it automatically uses the version 2 scheme. .\" commit 8db6c34f1dbc8e06aa016a9b829b06902c3e1340 Version 3 file capabilities are designed to coexist with version 2 capabilities, but serve a different purpose: -to support namespaced file capabilities. +to support namespaced file capabilities (described below). As with version 2 file capabilities, the capability masks are 64 bits in size. In addition, the namespace root user ID is encoded in the @@ -1411,6 +1411,52 @@ prctl(PR_SET_SECUREBITS, .SS Interaction with user namespaces For a discussion of the interaction of capabilities and user namespaces, see .BR user_namespaces (7). +.\" +.SS Namespaced file capabilities +.\" commit 8db6c34f1dbc8e06aa016a9b829b06902c3e1340 +Traditional (i.e., version 2) file capabilities associate +only a set of capability masks with a binary executable file. +When a process executes a binary with such capabilities, +it gains the associated capabilities (within its user namespace) +as per the rules described above in +"Transformation of capabilities during execve()". +.PP +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 +.BR CAP_SETFCAP +capability in the user namespace where the filesystem was mounted +(normally the initial user namespace). +This limitation renders file capabilities useless for certain use cases. +For example, in user-namespaced containers, +it can be desirable to be able to create a binary that +confers capabilities only to processes executed inside that container, +but not to processes that are executed outside the container. +.PP +Linux 4.14 added so-called namespaced file capabilities +to support such use cases. +Namespaced file capabilities are recorded as version 3 (i.e., +.BR VFS_CAP_REVISION_3 ) +.I security.capability +extended attributes. +Such an attribute is automatically created when a process that resides +in a noninitial user namespace associates +.RB ( setxattr (2)) +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. +.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. .SH CONFORMING TO .PP No standards govern capabilities, but the Linux capability implementation |