Fix core corruption caused by race in note_procstat_vmmap

This fix is spiritually similar to r287442 and was discovered thanks to
the KASSERT added in that revision.

NT_PROCSTAT_VMMAP output length, when packing kinfo structs, is tied to
the length of filenames corresponding to vnodes in the process' vm map
via vn_fullpath.  As vnodes may move during coredump, this is racy.

We do not remove the race, only prevent it from causing coredump
corruption.

- Add a sysctl, kern.coredump_pack_vmmapinfo, to allow users to disable
  kinfo packing for PROCSTAT_VMMAP notes.  This avoids VMMAP corruption
  and truncation, even if names change, at the cost of up to PATH_MAX
  bytes per mapped object.  The new sysctl is documented in core.5.

- Fix note_procstat_vmmap to self-limit in the second pass.  This
  addresses corruption, at the cost of sometimes producing a truncated
  result.

- Fix PROCSTAT_VMMAP consumers libutil (and libprocstat, via copy-paste)
  to grok the new zero padding.

Reported by:	pho (https://people.freebsd.org/~pho/stress/log/datamove4-2.txt)
Relnotes:	yes
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D3824

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/libprocstat/libprocstat.c b/lib/libprocstat/libprocstat.c
index 932b5a0..3de64aa 100644
--- a/lib/libprocstat/libprocstat.c
+++ b/lib/libprocstat/libprocstat.c
@@ -1867,6 +1867,8 @@ kinfo_getvmmap_core(struct procstat_core *core, int *cntp)
 	eb = buf + len;
 	while (bp < eb) {
 		kv = (struct kinfo_vmentry *)(uintptr_t)bp;
+		if (kv->kve_structsize == 0)
+			break;
 		bp += kv->kve_structsize;
 		cnt++;
 	}
@@ -1882,6 +1884,8 @@ kinfo_getvmmap_core(struct procstat_core *core, int *cntp)
 	/* Pass 2: unpack */
 	while (bp < eb) {
 		kv = (struct kinfo_vmentry *)(uintptr_t)bp;
+		if (kv->kve_structsize == 0)
+			break;
 		/* Copy/expand into pre-zeroed buffer */
 		memcpy(kp, kv, kv->kve_structsize);
 		/* Advance to next packed record */