[PATCH] kernfs: preserve security xattrs without allocating iattrs

[hengyul]

From: Hengyu Liang <hengyul@xxxxxxxxxx>

Commit d5e81a5650b5 ("kernfs: avoid iattr allocation in listxattr")
made kernfs_iop_listxattr() return an empty list when the kernfs node
has no allocated kernfs_iattrs.

However, this also skips security xattr names provided by
simple_xattr_list(). As of now, applications can retrieve the SELinux
label of a sysfs file with getxattr(), but cannot do it through
listxattr().

A similar issue happened before in commit b09e0fa4b4ea ("tmpfs: implement generic xattr support").
It was fixed by commit 8b0ba61df5a1c ("fs/xattr.c: fix simple_xattr_list to always include security.* xattrs").
Perhaps this recent commit needs a fix as well.

The issue can be reproduced with a simple python program:

    python3 - <<'PY'
    import os

    path = "/sys/kernel/warn_count"

    print("getxattr:", os.getxattr(path, "security.selinux"))
    print("listxattr:", os.listxattr(path))
    PY

Before commit d5e81a5650b5 ("kernfs: avoid iattr allocation in listxattr"),
the result is:

    getxattr: b'system_u:object_r:sysfs_t:s0\x00'
    listxattr: ['security.selinux']

After that commit, the result is:

    getxattr: b'system_u:object_r:sysfs_t:s0\x00'
    listxattr: []

This patch will keep listxattr() consistent with getxattr() when security
xattrs are available.

Fixes: d5e81a5650b5 ("kernfs: avoid iattr allocation in listxattr")

Signed-off-by: Hengyu Liang <hengyul@xxxxxxxxxx>
---
 fs/kernfs/inode.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index 237dcdd73fc2..bc3c6b939b0e 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -142,10 +142,9 @@ ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size)
 	struct kernfs_iattrs *attrs;
 
 	attrs = kernfs_iattrs_noalloc(kn);
-	if (!attrs)
-		return 0;
 
-	return simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size);
+	return simple_xattr_list(d_inode(dentry),
+				 attrs ? &attrs->xattrs : NULL, buf, size);
 }
 
 static inline void set_default_inode_attr(struct inode *inode, umode_t mode)
-- 
2.55.0