Re: [PATCH -next 3/5] ceph: fix possible null-ptr-deref when parsing param

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On 24/10/2022 00:39, Hawkins Jiawei wrote:
According to commit "vfs: parse: deal with zero length string value",
kernel will set the param->string to null pointer in vfs_parse_fs_string()
if fs string has zero length.

Yet the problem is that, ceph_parse_mount_param() will dereferences the
param->string, without checking whether it is a null pointer, which may
trigger a null-ptr-deref bug.

This patch solves it by adding sanity check on param->string
in ceph_parse_mount_param().

Signed-off-by: Hawkins Jiawei <yin31149@xxxxxxxxx>
---
  fs/ceph/super.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 3fc48b43cab0..341e23fe29eb 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -417,6 +417,9 @@ static int ceph_parse_mount_param(struct fs_context *fc,
  		param->string = NULL;
  		break;
  	case Opt_mds_namespace:
+		if (!param->string)
+			return invalfc(fc, "Bad value '%s' for mount option '%s'\n",
+				       param->string, param->key);
  		if (!namespace_equals(fsopt, param->string, strlen(param->string)))
  			return invalfc(fc, "Mismatching mds_namespace");
  		kfree(fsopt->mds_namespace);

BTW, did you hit any crash issue when testing this ?

$ ./bin/mount.ceph :/ /mnt/kcephfs -o mds_namespace=

<5>[  375.535442] ceph: module verification failed: signature and/or required key missing - tainting kernel
<6>[  375.698145] ceph: loaded (mds proto 32)
<3>[  375.801621] ceph: Bad value for 'mds_namespace'

From my test, the 'fsparam_string()' has already make sure it won't trigger the null-ptr-deref bug.

But it will always make sense to fix it in ceph code with your patch.

- Xiubo






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