linux-kernel - [PATCH] cifs: retry lookup and readdir when EAGAIN is returned.

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Message-Id: <20210615164256.173715-1-trbecker@gmail.com>
Date:   Tue, 15 Jun 2021 13:42:56 -0300
From:   Thiago Rafael Becker <trbecker@...il.com>
To:     linux-cifs@...r.kernel.org
Cc:     Steve French <sfrench@...ba.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thiago Rafael Becker <trbecker@...il.com>
Subject: [PATCH] cifs: retry lookup and readdir when EAGAIN is returned.

According to the investigation performed by Jacob Shivers at Red Hat,
cifs_lookup and cifs_readdir leak EAGAIN when the user session is
deleted on the server. Fix this issue by implementing a retry with
limits, as is implemented in cifs_revalidate_dentry_attr.

Reproducer based on the work by Jacob Shivers:

  ~~~
  $ cat readdir-cifs-test.sh
  #!/bin/bash

  # Install and configure powershell and sshd on the windows
  #  server as descibed in
  # https://docs.microsoft.com/en-us/windows-server/administration/openssh/openssh_overview
  # This script uses expect(1)

  USER=dude
  SERVER=192.168.0.2
  RPATH=root
  PASS='password'

  function debug_funcs {
  	for line in $@ ; do
  		echo "func $line +p" > /sys/kernel/debug/dynamic_debug/control
  	done
  }

  function setup {
  	echo 1 > /proc/fs/cifs/cifsFYI
  	debug_funcs wait_for_compound_request \
                smb2_query_dir_first cifs_readdir \
                compound_send_recv cifs_reconnect_tcon \
                generic_ip_connect cifs_reconnect \
                smb2_reconnect_server smb2_reconnect \
                cifs_readv_from_socket cifs_readv_receive
  	tcpdump -i eth0 -w cifs.pcap host 192.168.2.182 & sleep 5
  	dmesg -C
  }

  function test_call {
  	if [[ $1 == 1 ]] ; then
  		tracer="strace -tt -f -s 4096 -o trace-$(date -Iseconds).txt"
  	fi
        # Change the command here to anything apropriate
  	$tracer ls $2 > /dev/null
  	res=$?
  	if [[ $1 == 1 ]] ; then
  		if [[ $res == 0 ]] ; then
  			1>&2 echo success
  		else
  			1>&2 echo "failure ($res)"
  		fi
  	fi
  }

  mountpoint /mnt > /dev/null || mount -t cifs -o username=$USER,pass=$PASS //$SERVER/$RPATH /mnt

  test_call 0 /mnt/

  /usr/bin/expect << EOF
  	set timeout 60

  	spawn ssh $USER@...RVER

  	expect "yes/no" {
  		send "yes\r"
  		expect "*?assword" { send "$PASS\r" }
  	} "*?assword" { send "$PASS\r" }

  	expect ">" { send "powershell close-smbsession -force\r" }
  	expect ">" { send "exit\r" }
  	expect eof
  EOF

  sysctl -w vm.drop_caches=2 > /dev/null
  sysctl -w vm.drop_caches=2 > /dev/null

  setup

  test_call 1 /mnt/
  ~~~

Signed-off-by: Thiago Rafael Becker <trbecker@...il.com>
---
 fs/cifs/dir.c     | 4 ++++
 fs/cifs/smb2ops.c | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/fs/cifs/dir.c b/fs/cifs/dir.c
index 6bcd3e8f7cda..7c641f9a3dac 100644
--- a/fs/cifs/dir.c
+++ b/fs/cifs/dir.c
@@ -630,6 +630,7 @@ cifs_lookup(struct inode *parent_dir_inode, struct dentry *direntry,
 	struct inode *newInode = NULL;
 	const char *full_path;
 	void *page;
+	int retry_count = 0;
 
 	xid = get_xid();
 
@@ -673,6 +674,7 @@ cifs_lookup(struct inode *parent_dir_inode, struct dentry *direntry,
 	cifs_dbg(FYI, "Full path: %s inode = 0x%p\n",
 		 full_path, d_inode(direntry));
 
+again:
 	if (pTcon->posix_extensions)
 		rc = smb311_posix_get_inode_info(&newInode, full_path, parent_dir_inode->i_sb, xid);
 	else if (pTcon->unix_ext) {
@@ -687,6 +689,8 @@ cifs_lookup(struct inode *parent_dir_inode, struct dentry *direntry,
 		/* since paths are not looked up by component - the parent
 		   directories are presumed to be good here */
 		renew_parental_timestamps(direntry);
+	} else if (rc == -EAGAIN && retry_count++ < 10) {
+		goto again;
 	} else if (rc == -ENOENT) {
 		cifs_set_time(direntry, jiffies);
 		newInode = NULL;
diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index 21ef51d338e0..d241e6af8fe4 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -2325,6 +2325,7 @@ smb2_query_dir_first(const unsigned int xid, struct cifs_tcon *tcon,
 	struct smb2_query_directory_rsp *qd_rsp = NULL;
 	struct smb2_create_rsp *op_rsp = NULL;
 	struct TCP_Server_Info *server = cifs_pick_channel(tcon->ses);
+	int retry_count = 0;
 
 	utf16_path = cifs_convert_path_to_utf16(path, cifs_sb);
 	if (!utf16_path)
@@ -2372,10 +2373,14 @@ smb2_query_dir_first(const unsigned int xid, struct cifs_tcon *tcon,
 
 	smb2_set_related(&rqst[1]);
 
+again:
 	rc = compound_send_recv(xid, tcon->ses, server,
 				flags, 2, rqst,
 				resp_buftype, rsp_iov);
 
+	if (rc == -EAGAIN && retry_count++ < 10)
+		goto again;
+
 	/* If the open failed there is nothing to do */
 	op_rsp = (struct smb2_create_rsp *)rsp_iov[0].iov_base;
 	if (op_rsp == NULL || op_rsp->sync_hdr.Status != STATUS_SUCCESS) {
-- 
2.31.1