diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/prctl/seccomp_filter.txt | 163 | ||||
| -rw-r--r-- | Documentation/security/Smack.txt | 204 | ||||
| -rw-r--r-- | Documentation/security/Yama.txt | 10 | ||||
| -rw-r--r-- | Documentation/security/keys.txt | 17 |
4 files changed, 353 insertions, 41 deletions
diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt new file mode 100644 index 00000000000..597c3c58137 --- /dev/null +++ b/Documentation/prctl/seccomp_filter.txt @@ -0,0 +1,163 @@ + SECure COMPuting with filters + ============================= + +Introduction +------------ + +A large number of system calls are exposed to every userland process +with many of them going unused for the entire lifetime of the process. +As system calls change and mature, bugs are found and eradicated. A +certain subset of userland applications benefit by having a reduced set +of available system calls. The resulting set reduces the total kernel +surface exposed to the application. System call filtering is meant for +use with those applications. + +Seccomp filtering provides a means for a process to specify a filter for +incoming system calls. The filter is expressed as a Berkeley Packet +Filter (BPF) program, as with socket filters, except that the data +operated on is related to the system call being made: system call +number and the system call arguments. This allows for expressive +filtering of system calls using a filter program language with a long +history of being exposed to userland and a straightforward data set. + +Additionally, BPF makes it impossible for users of seccomp to fall prey +to time-of-check-time-of-use (TOCTOU) attacks that are common in system +call interposition frameworks. BPF programs may not dereference +pointers which constrains all filters to solely evaluating the system +call arguments directly. + +What it isn't +------------- + +System call filtering isn't a sandbox. It provides a clearly defined +mechanism for minimizing the exposed kernel surface. It is meant to be +a tool for sandbox developers to use. Beyond that, policy for logical +behavior and information flow should be managed with a combination of +other system hardening techniques and, potentially, an LSM of your +choosing. Expressive, dynamic filters provide further options down this +path (avoiding pathological sizes or selecting which of the multiplexed +system calls in socketcall() is allowed, for instance) which could be +construed, incorrectly, as a more complete sandboxing solution. + +Usage +----- + +An additional seccomp mode is added and is enabled using the same +prctl(2) call as the strict seccomp. If the architecture has +CONFIG_HAVE_ARCH_SECCOMP_FILTER, then filters may be added as below: + +PR_SET_SECCOMP: + Now takes an additional argument which specifies a new filter + using a BPF program. + The BPF program will be executed over struct seccomp_data + reflecting the system call number, arguments, and other + metadata. The BPF program must then return one of the + acceptable values to inform the kernel which action should be + taken. + + Usage: + prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, prog); + + The 'prog' argument is a pointer to a struct sock_fprog which + will contain the filter program. If the program is invalid, the + call will return -1 and set errno to EINVAL. + + If fork/clone and execve are allowed by @prog, any child + processes will be constrained to the same filters and system + call ABI as the parent. + + Prior to use, the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) or + run with CAP_SYS_ADMIN privileges in its namespace. If these are not + true, -EACCES will be returned. This requirement ensures that filter + programs cannot be applied to child processes with greater privileges + than the task that installed them. + + Additionally, if prctl(2) is allowed by the attached filter, + additional filters may be layered on which will increase evaluation + time, but allow for further decreasing the attack surface during + execution of a process. + +The above call returns 0 on success and non-zero on error. + +Return values +------------- +A seccomp filter may return any of the following values. If multiple +filters exist, the return value for the evaluation of a given system +call will always use the highest precedent value. (For example, +SECCOMP_RET_KILL will always take precedence.) + +In precedence order, they are: + +SECCOMP_RET_KILL: + Results in the task exiting immediately without executing the + system call. The exit status of the task (status & 0x7f) will + be SIGSYS, not SIGKILL. + +SECCOMP_RET_TRAP: + Results in the kernel sending a SIGSYS signal to the triggering + task without executing the system call. The kernel will + rollback the register state to just before the system call + entry such that a signal handler in the task will be able to + inspect the ucontext_t->uc_mcontext registers and emulate + system call success or failure upon return from the signal + handler. + + The SECCOMP_RET_DATA portion of the return value will be passed + as si_errno. + + SIGSYS triggered by seccomp will have a si_code of SYS_SECCOMP. + +SECCOMP_RET_ERRNO: + Results in the lower 16-bits of the return value being passed + to userland as the errno without executing the system call. + +SECCOMP_RET_TRACE: + When returned, this value will cause the kernel to attempt to + notify a ptrace()-based tracer prior to executing the system + call. If there is no tracer present, -ENOSYS is returned to + userland and the system call is not executed. + + A tracer will be notified if it requests PTRACE_O_TRACESECCOMP + using ptrace(PTRACE_SETOPTIONS). The tracer will be notified + of a PTRACE_EVENT_SECCOMP and the SECCOMP_RET_DATA portion of + the BPF program return value will be available to the tracer + via PTRACE_GETEVENTMSG. + +SECCOMP_RET_ALLOW: + Results in the system call being executed. + +If multiple filters exist, the return value for the evaluation of a +given system call will always use the highest precedent value. + +Precedence is only determined using the SECCOMP_RET_ACTION mask. When +multiple filters return values of the same precedence, only the +SECCOMP_RET_DATA from the most recently installed filter will be +returned. + +Pitfalls +-------- + +The biggest pitfall to avoid during use is filtering on system call +number without checking the architecture value. Why? On any +architecture that supports multiple system call invocation conventions, +the system call numbers may vary based on the specific invocation. If +the numbers in the different calling conventions overlap, then checks in +the filters may be abused. Always check the arch value! + +Example +------- + +The samples/seccomp/ directory contains both an x86-specific example +and a more generic example of a higher level macro interface for BPF +program generation. + + + +Adding architecture support +----------------------- + +See arch/Kconfig for the authoritative requirements. In general, if an +architecture supports both ptrace_event and seccomp, it will be able to +support seccomp filter with minor fixup: SIGSYS support and seccomp return +value checking. Then it must just add CONFIG_HAVE_ARCH_SECCOMP_FILTER +to its arch-specific Kconfig. diff --git a/Documentation/security/Smack.txt b/Documentation/security/Smack.txt index d2f72ae6643..a416479b8a1 100644 --- a/Documentation/security/Smack.txt +++ b/Documentation/security/Smack.txt @@ -15,7 +15,7 @@ at hand. Smack consists of three major components: - The kernel - - A start-up script and a few modified applications + - Basic utilities, which are helpful but not required - Configuration data The kernel component of Smack is implemented as a Linux @@ -23,37 +23,28 @@ Security Modules (LSM) module. It requires netlabel and works best with file systems that support extended attributes, although xattr support is not strictly required. It is safe to run a Smack kernel under a "vanilla" distribution. + Smack kernels use the CIPSO IP option. Some network configurations are intolerant of IP options and can impede access to systems that use them as Smack does. -The startup script etc-init.d-smack should be installed -in /etc/init.d/smack and should be invoked early in the -start-up process. On Fedora rc5.d/S02smack is recommended. -This script ensures that certain devices have the correct -Smack attributes and loads the Smack configuration if -any is defined. This script invokes two programs that -ensure configuration data is properly formatted. These -programs are /usr/sbin/smackload and /usr/sin/smackcipso. -The system will run just fine without these programs, -but it will be difficult to set access rules properly. - -A version of "ls" that provides a "-M" option to display -Smack labels on long listing is available. +The current git repositories for Smack user space are: -A hacked version of sshd that allows network logins by users -with specific Smack labels is available. This version does -not work for scp. You must set the /etc/ssh/sshd_config -line: - UsePrivilegeSeparation no + git@gitorious.org:meego-platform-security/smackutil.git + git@gitorious.org:meego-platform-security/libsmack.git -The format of /etc/smack/usr is: +These should make and install on most modern distributions. +There are three commands included in smackutil: - username smack +smackload - properly formats data for writing to /smack/load +smackcipso - properly formats data for writing to /smack/cipso +chsmack - display or set Smack extended attribute values In keeping with the intent of Smack, configuration data is minimal and not strictly required. The most important configuration step is mounting the smackfs pseudo filesystem. +If smackutil is installed the startup script will take care +of this, but it can be manually as well. Add this line to /etc/fstab: @@ -61,19 +52,148 @@ Add this line to /etc/fstab: and create the /smack directory for mounting. -Smack uses extended attributes (xattrs) to store file labels. -The command to set a Smack label on a file is: +Smack uses extended attributes (xattrs) to store labels on filesystem +objects. The attributes are stored in the extended attribute security +name space. A process must have CAP_MAC_ADMIN to change any of these +attributes. + +The extended attributes that Smack uses are: + +SMACK64 + Used to make access control decisions. In almost all cases + the label given to a new filesystem object will be the label + of the process that created it. +SMACK64EXEC + The Smack label of a process that execs a program file with + this attribute set will run with this attribute's value. +SMACK64MMAP + Don't allow the file to be mmapped by a process whose Smack + label does not allow all of the access permitted to a process + with the label contained in this attribute. This is a very + specific use case for shared libraries. +SMACK64TRANSMUTE + Can only have the value "TRUE". If this attribute is present + on a directory when an object is created in the directory and + the Smack rule (more below) that permitted the write access + to the directory includes the transmute ("t") mode the object + gets the label of the directory instead of the label of the + creating process. If the object being created is a directory + the SMACK64TRANSMUTE attribute is set as well. +SMACK64IPIN + This attribute is only available on file descriptors for sockets. + Use the Smack label in this attribute for access control + decisions on packets being delivered to this socket. +SMACK64IPOUT + This attribute is only available on file descriptors for sockets. + Use the Smack label in this attribute for access control + decisions on packets coming from this socket. + +There are multiple ways to set a Smack label on a file: # attr -S -s SMACK64 -V "value" path + # chsmack -a value path -NOTE: Smack labels are limited to 23 characters. The attr command - does not enforce this restriction and can be used to set - invalid Smack labels on files. - -If you don't do anything special all users will get the floor ("_") -label when they log in. If you do want to log in via the hacked ssh -at other labels use the attr command to set the smack value on the -home directory and its contents. +A process can see the smack label it is running with by +reading /proc/self/attr/current. A process with CAP_MAC_ADMIN +can set the process smack by writing there. + +Most Smack configuration is accomplished by writing to files +in the smackfs filesystem. This pseudo-filesystem is usually +mounted on /smack. + +access + This interface reports whether a subject with the specified + Smack label has a particular access to an object with a + specified Smack label. Write a fixed format access rule to + this file. The next read will indicate whether the access + would be permitted. The text will be either "1" indicating + access, or "0" indicating denial. +access2 + This interface reports whether a subject with the specified + Smack label has a particular access to an object with a + specified Smack label. Write a long format access rule to + this file. The next read will indicate whether the access + would be permitted. The text will be either "1" indicating + access, or "0" indicating denial. +ambient + This contains the Smack label applied to unlabeled network + packets. +cipso + This interface allows a specific CIPSO header to be assigned + to a Smack label. The format accepted on write is: + "%24s%4d%4d"["%4d"]... + The first string is a fixed Smack label. The first number is + the level to use. The second number is the number of categories. + The following numbers are the categories. + "level-3-cats-5-19 3 2 5 19" +cipso2 + This interface allows a specific CIPSO header to be assigned + to a Smack label. The format accepted on write is: + "%s%4d%4d"["%4d"]... + The first string is a long Smack label. The first number is + the level to use. The second number is the number of categories. + The following numbers are the categories. + "level-3-cats-5-19 3 2 5 19" +direct + This contains the CIPSO level used for Smack direct label + representation in network packets. +doi + This contains the CIPSO domain of interpretation used in + network packets. +load + This interface allows access control rules in addition to + the system defined rules to be specified. The format accepted + on write is: + "%24s%24s%5s" + where the first string is the subject label, the second the + object label, and the third the requested access. The access + string may contain only the characters "rwxat-", and specifies + which sort of access is allowed. The "-" is a placeholder for + permissions that are not allowed. The string "r-x--" would + specify read and execute access. Labels are limited to 23 + characters in length. +load2 + This interface allows access control rules in addition to + the system defined rules to be specified. The format accepted + on write is: + "%s %s %s" + where the first string is the subject label, the second the + object label, and the third the requested access. The access + string may contain only the characters "rwxat-", and specifies + which sort of access is allowed. The "-" is a placeholder for + permissions that are not allowed. The string "r-x--" would + specify read and execute access. +load-self + This interface allows process specific access rules to be + defined. These rules are only consulted if access would + otherwise be permitted, and are intended to provide additional + restrictions on the process. The format is the same as for + the load interface. +load-self2 + This interface allows process specific access rules to be + defined. These rules are only consulted if access would + otherwise be permitted, and are intended to provide additional + restrictions on the process. The format is the same as for + the load2 interface. +logging + This contains the Smack logging state. +mapped + This contains the CIPSO level used for Smack mapped label + representation in network packets. +netlabel + This interface allows specific internet addresses to be + treated as single label hosts. Packets are sent to single + label hosts without CIPSO headers, but only from processes + that have Smack write access to the host label. All packets + received from single label hosts are given the specified + label. The format accepted on write is: + "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label". +onlycap + This contains the label processes must have for CAP_MAC_ADMIN + and CAP_MAC_OVERRIDE to be effective. If this file is empty + these capabilities are effective at for processes with any + label. The value is set by writing the desired label to the + file or cleared by writing "-" to the file. You can add access rules in /etc/smack/accesses. They take the form: @@ -83,10 +203,6 @@ access is a combination of the letters rwxa which specify the kind of access permitted a subject with subjectlabel on an object with objectlabel. If there is no rule no access is allowed. -A process can see the smack label it is running with by -reading /proc/self/attr/current. A privileged process can -set the process smack by writing there. - Look for additional programs on http://schaufler-ca.com From the Smack Whitepaper: @@ -186,7 +302,7 @@ team. Smack labels are unstructured, case sensitive, and the only operation ever performed on them is comparison for equality. Smack labels cannot contain unprintable characters, the "/" (slash), the "\" (backslash), the "'" (quote) and '"' (double-quote) characters. -Smack labels cannot begin with a '-', which is reserved for special options. +Smack labels cannot begin with a '-'. This is reserved for special options. There are some predefined labels: @@ -194,7 +310,7 @@ There are some predefined labels: ^ Pronounced "hat", a single circumflex character. * Pronounced "star", a single asterisk character. ? Pronounced "huh", a single question mark character. - @ Pronounced "Internet", a single at sign character. + @ Pronounced "web", a single at sign character. Every task on a Smack system is assigned a label. System tasks, such as init(8) and systems daemons, are run with the floor ("_") label. User tasks @@ -246,13 +362,14 @@ The format of an access rule is: Where subject-label is the Smack label of the task, object-label is the Smack label of the thing being accessed, and access is a string specifying the sort -of access allowed. The Smack labels are limited to 23 characters. The access -specification is searched for letters that describe access modes: +of access allowed. The access specification is searched for letters that +describe access modes: a: indicates that append access should be granted. r: indicates that read access should be granted. w: indicates that write access should be granted. x: indicates that execute access should be granted. + t: indicates that the rule requests transmutation. Uppercase values for the specification letters are allowed as well. Access mode specifications can be in any order. Examples of acceptable rules @@ -273,7 +390,7 @@ Examples of unacceptable rules are: Spaces are not allowed in labels. Since a subject always has access to files with the same label specifying a rule for that case is pointless. Only -valid letters (rwxaRWXA) and the dash ('-') character are allowed in +valid letters (rwxatRWXAT) and the dash ('-') character are allowed in access specifications. The dash is a placeholder, so "a-r" is the same as "ar". A lone dash is used to specify that no access should be allowed. @@ -297,6 +414,13 @@ but not any of its attributes by the circumstance of having read access to the containing directory but not to the differently labeled file. This is an artifact of the file name being data in the directory, not a part of the file. +If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the +access rule that allows a process to create an object in that directory +includes 't' access the label assigned to the new object will be that +of the directory, not the creating process. This makes it much easier +for two processes with different labels to share data without granting +access to all of their files. + IPC objects, message queues, semaphore sets, and memory segments exist in flat namespaces and access requests are only required to match the object in question. diff --git a/Documentation/security/Yama.txt b/Documentation/security/Yama.txt index a9511f17906..e369de2d48c 100644 --- a/Documentation/security/Yama.txt +++ b/Documentation/security/Yama.txt @@ -34,7 +34,7 @@ parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still work), or with CAP_SYS_PTRACE (i.e. "gdb --pid=PID", and "strace -p PID" still work as root). -For software that has defined application-specific relationships +In mode 1, software that has defined application-specific relationships between a debugging process and its inferior (crash handlers, etc), prctl(PR_SET_PTRACER, pid, ...) can be used. An inferior can declare which other process (and its descendents) are allowed to call PTRACE_ATTACH @@ -46,6 +46,8 @@ restrictions, it can call prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...) so that any otherwise allowed process (even those in external pid namespaces) may attach. +These restrictions do not change how ptrace via PTRACE_TRACEME operates. + The sysctl settings are: 0 - classic ptrace permissions: a process can PTRACE_ATTACH to any other @@ -60,6 +62,12 @@ The sysctl settings are: inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare an allowed debugger PID to call PTRACE_ATTACH on the inferior. +2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace + with PTRACE_ATTACH. + +3 - no attach: no processes may use ptrace with PTRACE_ATTACH. Once set, + this sysctl cannot be changed to a lower value. + The original children-only logic was based on the restrictions in grsecurity. ============================================================== diff --git a/Documentation/security/keys.txt b/Documentation/security/keys.txt index d389acd31e1..aa0dbd74b71 100644 --- a/Documentation/security/keys.txt +++ b/Documentation/security/keys.txt @@ -805,6 +805,23 @@ The keyctl syscall functions are: kernel and resumes executing userspace. + (*) Invalidate a key. + + long keyctl(KEYCTL_INVALIDATE, key_serial_t key); + + This function marks a key as being invalidated and then wakes up the + garbage collector. The garbage collector immediately removes invalidated + keys from all keyrings and deletes the key when its reference count + reaches zero. + + Keys that are marked invalidated become invisible to normal key operations + immediately, though they are still visible in /proc/keys until deleted + (they're marked with an 'i' flag). + + A process must have search permission on the key for this function to be + successful. + + =============== KERNEL SERVICES =============== |