features/shard: Remove ctx from LRU in shard_forget

Problem:
There is a race when the following two commands are executed on the mount in
parallel from two different terminals on a sharded volume,
which leads to use-after-free.

Terminal-1:
while true; do dd if=/dev/zero of=file1 bs=1M count=4; done

Terminal-2:
while true; do cat file1 > /dev/null; done

In the normal case this is the life-cycle of a shard-inode
1) Shard is added to LRU when it is first looked-up
2) For every operation on the shard it is moved up in LRU
3) When "unlink of the shard"/"LRU limit is hit" happens it is removed from LRU

But we are seeing a race where the inode stays in Shard LRU even after it is
forgotten which leads to Use-after-free and then some memory-corruptions.

These are the steps:
1) Shard is added to LRU when it is first looked-up
2) For every operation on the shard it is moved up in LRU

Reader-handler                                    Truncate-handler
1) Reader handler needs shard-x to be read.     1) Truncate has just deleted shard-x
2) In shard_common_resolve_shards(), it does
   inode_resolve() and that leads to
   a hit in LRU, so it is going to call
__shard_update_shards_inode_list() to move the
   inode to top of LRU
						2) shard-x gets unlinked from the itable
						   and inode_forget(inode, 0) is called
						   to make sure the inode can be purged
						   upon last unref
3) when __shard_update_shards_inode_list() is
   called it finds that the inode is not in LRU
   so it adds it back to the LRU-list

Both these operations complete and call inode_unref(shard-x) which leads to the inode
getting freed and forgotten, even when it is in Shard LRU list. When more inodes are
added to LRU, use-after-free will happen and it leads to undefined behaviors.

Fix:
I see that the inode can be removed from LRU even by the protocol layers like gfapi/gNFS
when LRU limit is reached. So it is better to add a check in shard_forget() to remove itself
from LRU list if it exists.

BUG: 1466037
Change-Id: Ia79c0c5c9d5febc56c41ddb12b5daf03e5281638
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: https://review.gluster.org/17644
Smoke: Gluster Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/xlators/features/shard/src/shard.c b/xlators/features/shard/src/shard.c
index d7526339591..f892fb69efa 100644
--- a/xlators/features/shard/src/shard.c
+++ b/xlators/features/shard/src/shard.c
@@ -5011,13 +5011,26 @@ shard_forget (xlator_t *this, inode_t *inode)
 {
         uint64_t            ctx_uint = 0;
         shard_inode_ctx_t  *ctx      = NULL;
+        shard_priv_t       *priv     = NULL;
 
+        priv = this->private;
         inode_ctx_del (inode, this, &ctx_uint);
         if (!ctx_uint)
                 return 0;
 
         ctx = (shard_inode_ctx_t *)ctx_uint;
 
+        /* When LRU limit reaches inode will be forcefully removed from the
+         * table, inode needs to be removed from LRU of shard as well.
+         */
+        if (!list_empty (&ctx->ilist)) {
+                LOCK(&priv->lock);
+                {
+                        list_del_init (&ctx->ilist);
+                        priv->inode_count--;
+                }
+                UNLOCK(&priv->lock);
+        }
         GF_FREE (ctx);
 
         return 0;