This includes some things suggested by Ben Hutchings and Kees Cook. Signed-off-by: Jann Horn <jann@xxxxxxxxx> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> --- Documentation/security/ptrace_checks.txt | 229 +++++++++++++++++++++++++++++++ 1 file changed, 229 insertions(+) create mode 100644 Documentation/security/ptrace_checks.txt diff --git a/Documentation/security/ptrace_checks.txt b/Documentation/security/ptrace_checks.txt new file mode 100644 index 0000000..98689d5 --- /dev/null +++ b/Documentation/security/ptrace_checks.txt @@ -0,0 +1,229 @@ + ==================== + PTRACE ACCESS CHECKS + ==================== + +By: Jann Horn <jann@xxxxxxxxx> + +===== +INTRO +===== + +This file describes the rules for ptrace() and ptrace()-like access to another +task (inspecting or modifying the virtual memory of another process, inspecting +another task's register state, inspecting another task's open files and so on). + +This kind of access is particularly security-sensitive because it often allows +the caller to take complete control over the remote process. Therefore, the +kernel must be very careful here. + +There are various kernel APIs that grant debugging access to other processes. + + +=========================== +ORDINARY DEBUGGING SYSCALLS +=========================== + +The following ordinary syscalls grant debugging-like access: + + - ptrace() grants R+W access to registers and virtual memory + - process_vm_readv() / process_vm_writev() grant R/W access to virtual memory + - get_robust_list() reveals an address from the target task, which would be + useful e.g. for an ASLR bypass + - perf_event_open() + - (kcmp() reveals a very small amount of information) + +These syscalls use the caller's *real* UID/GID and his *permitted* capability +set (for historical reasons) to determine the permitted access using +ptrace_may_access(..., PTRACE_MODE_REALCREDS | ...). This function is +responsible for ensuring that the caller can't steal any privileges using the +debugging access; in other words, it has to ensure that the caller's credentials +are (more or less) a superset of the target's credentials. Namespaces aside, +these rules work as follows: + +Introspection, including cross-thread introspection, is always allowed. This +means that it is unsafe to use any of the debugging APIs on behalf of another, +untrusted process unless it is verified somehow that the target pid does not +actually belong to the calling process - dropping privileges is not sufficient. + +root (to be more precise, a process with CAP_SYS_PTRACE effective) can always +use these debugging APIs on any process - unless a LSM denies the access. + +For normal users, there are more checks: + + - The real uid of the caller must be equal to ruid, euid and suid of the target + task, same thing for the gids. All of the uids and gids of the target (apart + from the fs*id) are checked here because a privileged process might stash his + privileged uid into any one of them. (checked in __ptrace_may_access()) + (Note: This doesn't cover the supplementary GIDs.) + - The permitted capabilities of the caller must be a superset of the permitted + capabilities of the target. (checked in cap_ptrace_access_check()) + - The target process must be dumpable. What this means is explained in the next + section. + + +=========== +DUMPABILITY +=========== + +Originally, the dumpability rule didn't exist - as long as a process had ruid, +euid and suid set to the uid of a user (same thing with the gids), that user was +able to gain debugging access to the process. This was problematic: It meant +that if a setuid binary opened some important file, then dropped all privileges, +the user was able to debug the process and steal its file descriptor - which is +also a form of privilege! Other issues are e.g. that when a process drops its +privileges, it might still have secrets in memory that the user isn't meant to +see. + +Therefore, a process now becomes "nondumpable" when it: + + - changes its euid / egid + - changes its fsuid / fsgid + - changes its permitted capability set + +These checks are in commit_creds(). + +When a process is nondumpable, you need to have the CAP_SYS_PTRACE capability to +access it - just having the same uids is not enough anymore. + +The most interesting part of dumpability is what happens on execve(). +(Non-)Dumpability is not inherited; instead, it is recalculated in +setup_new_exec() and commit_creds(): A process will be nondumpable after +execve() if its post-setuid-bit-handling euid and ruid or egid and rgid aren't +the same. Additionally, it will be nondumpable if execve() changes its +credentials in a way that commit_creds() triggers on (as described above). + +In effect, this means that normally, when a setuid/setgid binary executes +another program, the other program is still protected by nondumpability (and +also by secure execution). + +(Additionally, a process will be nondumpable if the euid doesn't have read +access to the executed binary, which can be the case if the binary is +executable, but not readable; however, this restriction is irrelevant from a +security perspective because it isn't enforced consistently, neither through +AT_SECURE nor for bprm->unsafe. In other words, you can e.g. be attached to such +a binary via ptrace while it is executed, or you can LD_PRELOAD a library into +it that dumps its memory. If this was fixed, it would be important to keep +namespacing issues in mind, in particular the interaction with +LSM_UNSAFE_PTRACE_CAP, the privileged kind of ptrace that works across setuid +execve.) + +Dumpability and the secure exec flag can be affected by the following "unsafe +execution" rules. When a process with ruid != euid executes another program and +an unsafe execution is detected, *ITS EUID WILL BE DROPPED TO ITS RUID* by +cap_bprm_set_creds() (unless it has CAP_SETUID effective in the init user ns). +Unsafe execution means one of the following: + + - LSM_UNSAFE_PTRACE: An unprivileged process is currently attached via ptrace. + - LSM_UNSAFE_NO_NEW_PRIVS: The current thread turned on NO_NEW_PRIVS. + - LSM_UNSAFE_SHARE: The fs struct is shared with a non-thread. + + +===================== +FILESYSTEM DEBUG APIS +===================== + +The pid / tgid entries in procfs contain various entries that allow debugging +access to a process. Interesting entries are: + + - auxv permits an ASLR bypass + - cwd can permit bypassing filesystem restrictions in some cases + - environ can leak secret tokens + - fd can permit bypassing filesystem restrictions or leak access to things like + pipes + - maps permits an ASLR bypass + - mem grants R+W access to process memory + - stat permits an ASLR bypass + +Of these, all use both a normal filesystem DAC check (where the file owner is +the process owner for a dumpable process, root for a nondumpable process) and a +ptrace_may_access() check; however, the DAC check may be modified, and the +ptrace_may_access() is performed under PTRACE_FSCREDS, meaning that instead of +the caller's ruid, rgid and permitted capabilities, the fsuid, fsgid and +effective capabilities are used, causing the case where a daemon drops its euid +prior to accessing a file for the user to be treated correctly for this check. + +NEEDS ATTENTION: +Special-case rules that permit introspection become a big problem here. In +particular, if a setuid binary runs with dropped privileges (ruid=euid=user and +suid=0 or so), it is still able to open the following entries under /proc/self: + + - cwd (because normal DAC rules always permit symlink access, proc_pid_get_link + only checks for ptrace access and proc_cwd_link doesn't check anything) + - fd (same as cwd for the entries inside; the directory itself has an override + on the permission handler that makes introspection possible even if the + normal DAC rules would forbid it) + - maps (mode 0444, so DAC doesn't restrict anything) + - stat (mode 0444, and anyone can read from it; however, the interesting + pointers are only printed if the file opener has ptrace access to the + target process) + +In particular the fd directory is dangerous: Imagine a privileged process that +creates some important pipe using pipe() with a dropped EUID and later performs +writes with user-specified data on a file at a user-specified path with the same +dropped EUID. An attacker could let the root-owned process reference the pipe as +/proc/$pid/fd/3 and let the privileged process write to it. + +If necessary, it might make sense to introduce some "enable/disable +introspection" prctl that can be used by userspace to disambiguate whether an +access is intended to be introspection. + + +Note that in all cases in which the DAC check permits access but a ptrace access +check later on (in the VFS open handler or afterwards) restricts access, +faccessat() is broken. However, that's probably not a real problem because +faccessat() is racy anyway. + + +=============== +USER NAMESPACES +=============== + +When ptrace_may_access() checks for CAP_SYS_PTRACE, the capability doesn't need +to be active in the init namespace or in the current namespace; having it in the +namespace of the ptrace target is sufficient. There are currently no further +restrictions on this, meaning that the behavior of a task that creates or enters +a user namespace is somewhat unintuitive: + + - If a nondumpable task enters a user namespace, it will still be + nondumpable - but because the owner of the namespace is capable relative to + it when viewing the namespace from outside, this will often cause a + nondumpable task to effectively become dumpable because of a setns() / + clone() / unshare() call. + + - The root user of a namespace can ptrace any process that enters the namespace + (via setns()) - immediately. This means that a process that wants to enter an + untrusted namespace from outside needs to be *very* careful to drop any + privileges it might have - uids, open file descriptors, secret data that's + still in memory, the controlling TTY and so on. (This also means that a + namespace owner who is unprivileged outside the namespace can't safely enter + the namespace if he doesn't trust the namespace root user: He needs + euid==ns->owner to enter the namespace, but must not have euid==ns->owner + after having entered the namespace. To get around this, it is necessary to + introduce a third namespace between the other two.) + +One way to make it easier for userspace to get this right might be to bind +dumpability to namespaces, as follows: Let a task become nondumpable on +setns() / clone(CLONE_NEWUSER) / unshare(CLONE_NEWUSER). Record the current +namespace on dumpable -> nondumpable transition as "dumpability namespace". +Require the caller to be privileged in the target's dumpability namespace, not +just the target's current namespace. + +============= +FUSE AND CUSE +============= + +Anyone who can open /dev/cuse (root-owned, mode 0600) can arbitrarily read and +write to the memory of various system processes (to be more precise, any process +that opens files in /dev when they appear or so, then calls ioctl() on them; in +particular, acpid does this) without further checks on capabilities or so. + +NEEDS ATTENTION: +FUSE is more accessible than CUSE, but also more strict. ioctl() replies are +restricted in location and size based on the ioctl command number (size) and the +argument (location), but the ability to interactively control the replies to VFS +method calls is still scary. Therefore, FUSE by default rejects filesystem +access by anyone except the filesystem owner. This isn't done using the normal +ptrace check, though; instead, fuse_allow_current_process() is used, which only +looks at the UIDs and GIDs and therefore is a very simplified version of the +normal ptrace access check. In particular, this provides no protection for +setcap binaries. -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
