[PATCH] cifs: fix leak in cifs_smb3_do_mount() ctx

[David Disseldorp <ddiss@suse.de>]

cifs_smb3_do_mount() calls smb3_fs_context_dup() and then
cifs_setup_volume_info(). The latter's subsequent smb3_parse_devname()
call overwrites the cifs_sb->ctx->UNC string already dup'ed by
smb3_fs_context_dup(), resulting in a leak. E.g.

unreferenced object 0xffff888002980420 (size 32):
  comm "mount", pid 160, jiffies 4294892541 (age 30.416s)
  hex dump (first 32 bytes):
    5c 5c 31 39 32 2e 31 36 38 2e 31 37 34 2e 31 30  \\192.168.174.10
    34 5c 72 61 70 69 64 6f 2d 73 68 61 72 65 00 00  4\rapido-share..
  backtrace:
    [<00000000069e12f6>] kstrdup+0x28/0x50
    [<00000000b61f4032>] smb3_fs_context_dup+0x127/0x1d0 [cifs]
    [<00000000c6e3e3bf>] cifs_smb3_do_mount+0x77/0x660 [cifs]
    [<0000000063467a6b>] smb3_get_tree+0xdf/0x220 [cifs]
    [<00000000716f731e>] vfs_get_tree+0x1b/0x90
    [<00000000491d3892>] path_mount+0x62a/0x910
    [<0000000046b2e774>] do_mount+0x50/0x70
    [<00000000ca7b64dd>] __x64_sys_mount+0x81/0xd0
    [<00000000b5122496>] do_syscall_64+0x33/0x40
    [<000000002dd397af>] entry_SYSCALL_64_after_hwframe+0x44/0xae

This change is a bandaid until the cifs_setup_volume_info() TODO and
error handling issues are resolved.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 fs/cifs/cifsfs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 5ddd20b62484..34c125798ad3 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -834,6 +834,12 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 		goto out;
 	}
 
+	/* cifs_setup_volume_info->smb3_parse_devname() redups UNC & prepath */
+	kfree(cifs_sb->ctx->UNC);
+	cifs_sb->ctx->UNC = NULL;
+	kfree(cifs_sb->ctx->prepath);
+	cifs_sb->ctx->prepath = NULL;
+
 	rc = cifs_setup_volume_info(cifs_sb->ctx, NULL, old_ctx->UNC);
 	if (rc) {
 		root = ERR_PTR(rc);
-- 
2.26.2