[PATCH v2] cgroup: Fix memory leak when parsing multiple source parameters

[Qinglang Miao]

A memory leak is found in cgroup1_parse_param() when multiple source
parameters overwrite fc->source in the fs_context struct without free.

unreferenced object 0xffff888100d930e0 (size 16):
  comm "mount", pid 520, jiffies 4303326831 (age 152.783s)
  hex dump (first 16 bytes):
    74 65 73 74 6c 65 61 6b 00 00 00 00 00 00 00 00  testleak........
  backtrace:
    [<000000003e5023ec>] kmemdup_nul+0x2d/0xa0
    [<00000000377dbdaa>] vfs_parse_fs_string+0xc0/0x150
    [<00000000cb2b4882>] generic_parse_monolithic+0x15a/0x1d0
    [<000000000f750198>] path_mount+0xee1/0x1820
    [<0000000004756de2>] do_mount+0xea/0x100
    [<0000000094cafb0a>] __x64_sys_mount+0x14b/0x1f0

Fix this bug by permitting a single source parameter and rejecting with
an error all subsequent ones.

Fixes: 8d2451f4994f ("cgroup1: switch to option-by-option parsing")
Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
Signed-off-by: Qinglang Miao <miaoqinglang@xxxxxxxxxx>
---
 v1->v2: fix compile problems caused by superfluous LF in err message.
 kernel/cgroup/cgroup-v1.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 191c329e4..32596fdbc 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -908,6 +908,8 @@ int cgroup1_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	opt = fs_parse(fc, cgroup1_fs_parameters, param, &result);
 	if (opt == -ENOPARAM) {
 		if (strcmp(param->key, "source") == 0) {
+			if (fc->source)
+				return invalf(fc, "Multiple sources not supported");
 			fc->source = param->string;
 			param->string = NULL;
 			return 0;
-- 
2.23.0