Hi, Jann:
Regarding the security considerations, it seems simpler and more secure to
just clear the "pre-coredump signal" cross execve(2), and let the new program
decide for itself. What do you think?
---
Changes to prctl(2):
DESCRIPTION
PR_SET_PREDUMP_SIG (since Linux 4.20.x)
This allows the calling process to receive a signal (arg2,
if nonzero) from a child process prior to the coredump of
the child process. arg2 must be SIGUSR1, or SIGUSR2, or
SIGCHLD, or 0 (for clear).
When SIGCHLD is specified, the signal code is set to
CLD_PREDUMP in such an SIGCHLD signal.
The value of the pre-coredump signal is cleared across
execve(2), or for the child of a fork(2).
PR_GET_PREDUMP_SIG (since Linux 4.20.x)
Return the current value of the pre-coredump signal for the
calling process, in the location pointed to by (int *) arg2.
---
Thanks. -- Enke
On 10/15/18 11:54 AM, Jann Horn wrote:
> On Mon, Oct 15, 2018 at 8:36 PM Enke Chen <enkechen@xxxxxxxxx> wrote:
>> On 10/13/18 11:27 AM, Jann Horn wrote:
>>> On Sat, Oct 13, 2018 at 2:33 AM Enke Chen <enkechen@xxxxxxxxx> wrote:
>>>> For simplicity and consistency, this patch provides an implementation
>>>> for signal-based fault notification prior to the coredump of a child
>>>> process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can
>>>> be used by an application to express its interest and to specify the
>>>> signal (SIGCHLD or SIGUSR1 or SIGUSR2) for such a notification. A new
>>>> signal code (si_code), CLD_PREDUMP, is also defined for SIGCHLD.
>>>
>>> Your suggested API looks vaguely similar to PR_SET_PDEATHSIG, but with
>>> some important differences:
>>>
>>> - You don't reset the signal on setuid execution.
> [...]
>>>
>>> For both of these: Are these differences actually necessary, and if
>>> so, can you provide a specific rationale? From a security perspective,
>>> I would very much prefer it if this API had semantics closer to
>>> PR_SET_PDEATHSIG.
>>
> [...]
>>
>> Regarding the impact of "setuid", this property "PR_SET_PREDUMP_SIG" has to
>> do with the application/process whether the signal handler is set for receiving
>> such a notification. If it is set, the "uid" should not matter.
>
> If an attacker's process first calls PR_SET_PREDUMP_SIG, then forks
> off a child, then calls execve() on a setuid binary, the setuid binary
> calls setuid(0), and the attacker-controlled child then crashes, the
> privileged process will receive an unexpected signal that the attacker
> wouldn't have been allowed to send otherwise. For similar reasons, the
> parent death signal is reset when a setuid binary is executed:
>
> void setup_new_exec(struct linux_binprm * bprm)
> {
> /*
> * Once here, prepare_binrpm() will not be called any more, so
> * the final state of setuid/setgid/fscaps can be merged into the
> * secureexec flag.
> */
> bprm->secureexec |= bprm->cap_elevated;
>
> if (bprm->secureexec) {
> /* Make sure parent cannot signal privileged process. */
> current->pdeath_signal = 0;
> [...]
> }
> [...]
> }
>
> int commit_creds(struct cred *new)
> {
> [...]
> /* dumpability changes */
> if (!uid_eq(old->euid, new->euid) ||
> !gid_eq(old->egid, new->egid) ||
> !uid_eq(old->fsuid, new->fsuid) ||
> !gid_eq(old->fsgid, new->fsgid) ||
> !cred_cap_issubset(old, new)) {
> if (task->mm)
> set_dumpable(task->mm, suid_dumpable);
> task->pdeath_signal = 0;
> smp_wmb();
> }
> [...]
> }
>
> AppArmor and SELinux also do related changes:
>
> static void apparmor_bprm_committing_creds(struct linux_binprm *bprm)
> {
> [...]
> /* bail out if unconfined or not changing profile */
> if ((new_label->proxy == label->proxy) ||
> (unconfined(new_label)))
> return;
>
> aa_inherit_files(bprm->cred, current->files);
>
> current->pdeath_signal = 0;
> [...]
> }
>
> static void selinux_bprm_committing_creds(struct linux_binprm *bprm)
> {
> [...]
> new_tsec = bprm->cred->security;
> if (new_tsec->sid == new_tsec->osid)
> return;
>
> /* Close files for which the new task SID is not authorized. */
> flush_unauthorized_files(bprm->cred, current->files);
>
> /* Always clear parent death signal on SID transitions. */
> current->pdeath_signal = 0;
> [...]
> }
>
> You should probably reset the coredump signal in the same places - or
> even better, add a new helper for resetting the parent death signal,
> and then add code for resetting the coredump signal in there.
>
