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Avoid memory accesses reordering which can result in fget_unlocked()

seeing a stale fd_ofiles table once fd_nfiles is already updated,
resulting in OOB accesses.

Approved by:	re (kib)
Sponsored by:	EMC / Isilon storage division
Reported and tested by:	pho
Reviewed by:	benno
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1 parent 244d5cf commit 29d161240ea3c6786da8eaac6f6e04b60a3b2316 attilio committed Sep 25, 2013
Showing with 14 additions and 2 deletions.
  1. +14 −2 sys/kern/kern_descrip.c
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@@ -1512,12 +1512,20 @@ fdgrowtable(struct filedesc *fdp, int nfd)
memcpy(nmap, omap, NDSLOTS(onfiles) * sizeof(*omap));
/* update the pointers and counters */
- fdp->fd_nfiles = nnfiles;
memcpy(ntable, otable, onfiles * sizeof(ntable[0]));
fdp->fd_ofiles = ntable;
fdp->fd_map = nmap;
/*
+ * In order to have a valid pattern for fget_unlocked()
+ * fdp->fd_nfiles might be the last member to be updated, otherwise
+ * fget_unlocked() consumers may reference a new, higher value for
+ * fdp->fd_nfiles before to access the fdp->fd_ofiles array,
+ * resulting in OOB accesses.
+ */
+ atomic_store_rel_int(&fdp->fd_nfiles, nnfiles);
+
+ /*
* Do not free the old file table, as some threads may still
* reference entries within it. Instead, place it on a freelist
* which will be processed when the struct filedesc is released.
@@ -2308,7 +2316,11 @@ fget_unlocked(struct filedesc *fdp, int fd, cap_rights_t *needrightsp,
int error;
#endif
- if (fd < 0 || fd >= fdp->fd_nfiles)
+ /*
+ * Avoid reads reordering and then a first access to the
+ * fdp->fd_ofiles table which could result in OOB operation.
+ */
+ if (fd < 0 || fd >= atomic_load_acq_int(&fdp->fd_nfiles))
return (EBADF);
/*
* Fetch the descriptor locklessly. We avoid fdrop() races by

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