Commit a5af33ce9a16 ("instance: strdup string attributes") has
introduced duplication of various string attributes in order to fix
use-after-free, but missed handling of one `pidfile` attribute case in
instance_config_move() where the new value of `pidfile` is being
copied/assigned. Source of this value is then free()d in following call
to instance_free() and then again for 2nd time during the service stop
command handling, leading to double free crash:

 #0  unmap_chunk at src/malloc/malloc.c:515
 #1  free at src/malloc/malloc.c:526
 #2  instance_free (in=0xd5e300) at instance.c:1100
 #3  instance_delete (in=0xd5e300) at instance.c:559
 #4  instance_stop (in=0xd5e300, halt=true) at instance.c:611

Ref: FS#2723
Cc: Daniel Golle <dan...@makrotopia.org>
Fixes: a5af33ce9a16 ("instance: strdup string attributes")
Signed-off-by: Petr Štetiar <yn...@true.cz>
---
 service/instance.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/service/instance.c b/service/instance.c
index ce5233807dbb..245b9629d99a 100644
--- a/service/instance.c
+++ b/service/instance.c
@@ -1031,7 +1031,6 @@ instance_config_move(struct service_instance *in, struct 
service_instance *in_sr
        blobmsg_list_move(&in->jail.mount, &in_src->jail.mount);
        in->trigger = in_src->trigger;
        in->command = in_src->command;
-       in->pidfile = in_src->pidfile;
        in->respawn = in_src->respawn;
        in->respawn_retry = in_src->respawn_retry;
        in->respawn_threshold = in_src->respawn_threshold;
@@ -1042,6 +1041,10 @@ instance_config_move(struct service_instance *in, struct 
service_instance *in_sr
        in->node.avl.key = in_src->node.avl.key;
        in->syslog_facility = in_src->syslog_facility;
 
+       free(in->pidfile);
+       if (in_src->pidfile)
+               in->pidfile = strdup(in_src->pidfile);
+
        free(in->config);
        in->config = in_src->config;
        in_src->config = NULL;

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