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bpf: Fix the write filter for detached descriptors

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A BPF descriptor only has an associated interface descriptor once it is
attached to an interface, e.g., with BIOCSETIF.  Avoid dereferencing a
NULL pointer in filt_bpfwrite() if the BPF descriptor is not attached.

Reviewed by:	ae
Reported by:	syzbot+ae45d5166afe15a5a21d@syzkaller.appspotmail.com
Fixes:	ded77e0237a8 ("Allow the BPF to be select for write.")
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32561
This commit is contained in:
Mark Johnston 2021-10-26 09:57:27 -04:00
parent c0cf36bc02
commit 426682b05a
2 changed files with 14 additions and 5 deletions
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4
lib/libc/sys/kqueue.2
View File

@ -390,8 +390,8 @@ For eventfds,
will contain the maximum value that can be added to the counter
without blocking.
.Pp
For BPF devices, the filter always indicates that it is possible to
write and
For BPF devices, when the descriptor is attached to an interface the filter
always indicates that it is possible to write and
.Va data
will contain the MTU size of the underlying interface.
.It Dv EVFILT_EMPTY

15
sys/net/bpf.c
View File

@ -763,6 +763,10 @@ bpf_attachd(struct bpf_d *d, struct bpf_if *bp)
CK_LIST_INSERT_HEAD(&bp->bif_dlist, d, bd_next);
reset_d(d);
/* Trigger EVFILT_WRITE events. */
bpf_wakeup(d);
BPFD_UNLOCK(d);
bpf_bpfd_cnt++;
@ -2229,11 +2233,16 @@ static int
filt_bpfwrite(struct knote *kn, long hint)
{
struct bpf_d *d = (struct bpf_d *)kn->kn_hook;
BPFD_LOCK_ASSERT(d);
kn->kn_data = d->bd_bif->bif_ifp->if_mtu;
return (1);
if (d->bd_bif == NULL) {
kn->kn_data = 0;
return (0);
} else {
kn->kn_data = d->bd_bif->bif_ifp->if_mtu;
return (1);
}
}
#define BPF_TSTAMP_NONE 0