Ktracing kevent(2) calls with unusual arguments might leads to an

overly large allocation requests.

When ktrace-ing io, sys_kevent() allocates memory to copy the
requested changes and reported events.  Allocations are sized by the
incoming syscall lengths arguments, which are user-controlled, and
might cause overflow in calculations or too large allocations.

Since io trace chunks are limited by ktr_geniosize, there is no sense
it even trying to satisfy unbounded allocations.  Export ktr_geniosize
and clamp the buffers sizes in advance.

PR:	217435
Reported by:	Tim Newsham <tim.newsham@nccgroup.trust>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

3 files changed, 17 insertions, 5 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 4473d16230bf..752caa4b76c5 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -887,6 +887,15 @@ kern_kqueue(struct thread *td, int flags, struct filecaps *fcaps)
 	return (0);
 }
 
+static size_t
+kev_iovlen(int n, u_int kgio)
+{
+
+	if (n < 0 || n >= kgio / sizeof(struct kevent))
+		return (kgio);
+	return (n * sizeof(struct kevent));
+}
+
 #ifndef _SYS_SYSPROTO_H_
 struct kevent_args {
 	int	fd;
@@ -910,6 +919,7 @@ sys_kevent(struct thread *td, struct kevent_args *uap)
 	struct iovec ktriov;
 	struct uio *ktruioin = NULL;
 	struct uio *ktruioout = NULL;
+	u_int kgio;
 #endif
 
 	if (uap->timeout != NULL) {
@@ -922,13 +932,15 @@ sys_kevent(struct thread *td, struct kevent_args *uap)
 
 #ifdef KTRACE
 	if (KTRPOINT(td, KTR_GENIO)) {
+		kgio = ktr_geniosize;
 		ktriov.iov_base = uap->changelist;
-		ktriov.iov_len = uap->nchanges * sizeof(struct kevent);
+		ktriov.iov_len = kev_iovlen(uap->nchanges, kgio);
 		ktruio = (struct uio){ .uio_iov = &ktriov, .uio_iovcnt = 1,
 		    .uio_segflg = UIO_USERSPACE, .uio_rw = UIO_READ,
 		    .uio_td = td };
 		ktruioin = cloneuio(&ktruio);
 		ktriov.iov_base = uap->eventlist;
+		ktriov.iov_len = kev_iovlen(uap->nevents, kgio);
 		ktriov.iov_len = uap->nevents * sizeof(struct kevent);
 		ktruioout = cloneuio(&ktruio);
 	}
@@ -939,9 +951,9 @@ sys_kevent(struct thread *td, struct kevent_args *uap)
 
 #ifdef KTRACE
 	if (ktruioin != NULL) {
-		ktruioin->uio_resid = uap->nchanges * sizeof(struct kevent);
+		ktruioin->uio_resid = kev_iovlen(uap->nchanges, kgio);
 		ktrgenio(uap->fd, UIO_WRITE, ktruioin, 0);
-		ktruioout->uio_resid = td->td_retval[0] * sizeof(struct kevent);
+		ktruioout->uio_resid = kev_iovlen(td->td_retval[0], kgio);
 		ktrgenio(uap->fd, UIO_READ, ktruioout, error);
 	}
 #endif
diff --git a/sys/kern/kern_ktrace.c b/sys/kern/kern_ktrace.c
index 472017cdbbe6..2b1b9944ad08 100644
--- a/sys/kern/kern_ktrace.c
+++ b/sys/kern/kern_ktrace.c
@@ -132,7 +132,7 @@ static SYSCTL_NODE(_kern, OID_AUTO, ktrace, CTLFLAG_RD, 0, "KTRACE options");
 static u_int ktr_requestpool = KTRACE_REQUEST_POOL;
 TUNABLE_INT("kern.ktrace.request_pool", &ktr_requestpool);
 
-static u_int ktr_geniosize = PAGE_SIZE;
+u_int ktr_geniosize = PAGE_SIZE;
 SYSCTL_UINT(_kern_ktrace, OID_AUTO, genio_size, CTLFLAG_RWTUN, &ktr_geniosize,
     0, "Maximum size of genio event payload");
 
diff --git a/sys/sys/ktrace.h b/sys/sys/ktrace.h
index dce00756c6b2..03cd5cc17010 100644
--- a/sys/sys/ktrace.h
+++ b/sys/sys/ktrace.h
@@ -276,7 +276,7 @@ void	ktrcapfail(enum ktr_cap_fail_type, const cap_rights_t *,
 	ktrstruct("sockaddr", (s), ((struct sockaddr *)(s))->sa_len)
 #define ktrstat(s) \
 	ktrstruct("stat", (s), sizeof(struct stat))
-
+extern u_int ktr_geniosize;
 #else
 
 #include <sys/cdefs.h>