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jail: Handle a possible race between jail_remove(2) and fork(2)

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jail_remove(2) includes a loop that sends SIGKILL to all processes
in a jail, but skips processes in PRS_NEW state.  Thus it is possible
the a process in mid-fork(2) during jail removal can survive the jail
being removed.

Add a prison flag PR_REMOVE, which is checked before the new process
returns.  If the jail is being removed, the process will then exit.
Also check this flag in jail_attach(2) which has a similar issue.

Reported by:    trasz
Approved by:    kib
MFC after:      3 days
This commit is contained in:
Jamie Gritton 2021-02-16 11:19:13 -08:00
parent 922cf8ac43
commit cc7b730653
3 changed files with 28 additions and 3 deletions
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6
sys/kern/kern_fork.c
View File

@ -1126,6 +1126,12 @@ fork_return(struct thread *td, struct trapframe *frame)
PROC_UNLOCK(p);
}
/*
* If the prison was killed mid-fork, die along with it.
*/
if (!prison_isalive(td->td_ucred->cr_prison))
exit1(td, 0, SIGKILL);
userret(td, frame);
#ifdef KTRACE

24
sys/kern/kern_jail.c
View File

@ -1764,6 +1764,7 @@ kern_jail_set(struct thread *td, struct uio *optuio, int flags)
}
}
pr->pr_flags = (pr->pr_flags & ~ch_flags) | pr_flags;
pr->pr_flags &= ~PR_REMOVE;
mtx_unlock(&pr->pr_mtx);
drflags &= ~PD_LOCKED;
@ -2368,6 +2369,12 @@ prison_remove_one(struct prison *pr)
drflags = PD_DEREF | PD_LOCKED | PD_LIST_XLOCKED;
/*
* Mark the prison as doomed, so it doesn't accidentally come back
* to life. It may still be explicitly brought back by jail_set(2).
*/
pr->pr_flags |= PR_REMOVE;
/* If the prison was persistent, it is not anymore. */
if (pr->pr_flags & PR_PERSIST) {
refcount_release(&pr->pr_ref);
@ -2508,6 +2515,17 @@ do_jail_attach(struct thread *td, struct prison *pr)
#endif
prison_deref(oldcred->cr_prison, PD_DEREF | PD_DEUREF);
crfree(oldcred);
/*
* If the prison was killed while changing credentials, die along
* with it.
*/
if (!prison_isalive(pr)) {
PROC_LOCK(p);
kern_psignal(p, SIGKILL);
PROC_UNLOCK(p);
}
return (0);
e_unlock:
@ -3038,17 +3056,18 @@ prison_ischild(struct prison *pr1, struct prison *pr2)
/*
* Return true if the prison is currently alive. A prison is alive if it is
* valid and it holds user references.
* valid and holds user references, and it isn't being removed.
*/
bool
prison_isalive(struct prison *pr)
{
mtx_assert(&pr->pr_mtx, MA_OWNED);
if (__predict_false(refcount_load(&pr->pr_ref) == 0))
return (false);
if (__predict_false(refcount_load(&pr->pr_uref) == 0))
return (false);
if (__predict_false(pr->pr_flags & PR_REMOVE))
return (false);
return (true);
}
@ -3061,7 +3080,6 @@ bool
prison_isvalid(struct prison *pr)
{
mtx_assert(&pr->pr_mtx, MA_OWNED);
if (__predict_false(refcount_load(&pr->pr_ref) == 0))
return (false);
return (true);

1
sys/sys/jail.h
View File

@ -216,6 +216,7 @@ struct prison_racct {
/* primary jail address. */
/* Internal flag bits */
#define PR_REMOVE 0x01000000 /* In process of being removed */
#define PR_IP4 0x02000000 /* IPv4 restricted or disabled */
/* by this jail or an ancestor */
#define PR_IP6 0x04000000 /* IPv6 restricted or disabled */