git: f7007a7d0525 - stable/12 - MFC jail: Handle a possible race between jail_remove(2) and fork(2)

[Jamie Gritton]

The branch stable/12 has been updated by jamie:

URL: https://cgit.FreeBSD.org/src/commit/?id=f7007a7d05255a6859dea0982b1f0a6d695e8881

commit f7007a7d05255a6859dea0982b1f0a6d695e8881
Author:     Jamie Gritton <jamie at FreeBSD.org>
AuthorDate: 2021-02-16 19:19:13 +0000
Commit:     Jamie Gritton <jamie at FreeBSD.org>
CommitDate: 2021-02-19 21:44:19 +0000

    MFC jail: Handle a possible race between jail_remove(2) and fork(2)
    
    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
    
    (cherry picked from commit cc7b73065302005ebc4a19503188c8d6d5eb923d)
---
 sys/kern/kern_fork.c |  6 ++++++
 sys/kern/kern_jail.c | 18 ++++++++++++++++++
 sys/sys/jail.h       |  1 +
 3 files changed, 25 insertions(+)

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index e427164cd150..f8e134c30948 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -1138,6 +1138,12 @@ fork_return(struct thread *td, struct trapframe *frame)
 		PROC_UNLOCK(p);
 	}
 
+	/*
+	 * If the prison was killed mid-fork, die along with it.
+	 */
+	if (td->td_ucred->cr_prison->pr_flags & PR_REMOVE)
+		exit1(td, 0, SIGKILL);
+
 	userret(td, frame);
 
 #ifdef KTRACE
diff --git a/sys/kern/kern_jail.c b/sys/kern/kern_jail.c
index 60cbea3c9849..b4a20b4d29e9 100644
--- a/sys/kern/kern_jail.c
+++ b/sys/kern/kern_jail.c
@@ -1768,6 +1768,7 @@ 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);
 
 #ifdef RACCT
@@ -2306,6 +2307,12 @@ prison_remove_one(struct prison *pr)
 	struct proc *p;
 	int deuref;
 
+	/*
+	 * 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. */
 	deuref = 0;
 	if (pr->pr_flags & PR_PERSIST) {
@@ -2450,6 +2457,17 @@ 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 (pr->pr_flags & PR_REMOVE) {
+		PROC_LOCK(p);
+		kern_psignal(p, SIGKILL);
+		PROC_UNLOCK(p);
+	}
+
 	return (0);
 
  e_unlock:
diff --git a/sys/sys/jail.h b/sys/sys/jail.h
index 878b97a064a3..8baf784d6b6d 100644
--- a/sys/sys/jail.h
+++ b/sys/sys/jail.h
@@ -213,6 +213,7 @@ 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 */