BACKPORT: SMACK: Fix handling value==NULL in post setxattr

The function `smack_inode_post_setxattr` is called each
time that a setxattr is done, for any value of name.
The kernel allow to put value==NULL when size==0
to set an empty attribute value. The systematic
call to smk_import_entry was causing the dereference
of a NULL pointer hence a KERNEL PANIC!

The problem can be produced easily by issuing the
command `setfattr -n user.data file` under bash prompt
when SMACK is active.

Moving the call to smk_import_entry as proposed by this
patch is correcting the behaviour because the function
smack_inode_post_setxattr is called for the SMACK's
attributes only if the function smack_inode_setxattr validated
the value and its size (what will not be the case when size==0).

It also has a benefical effect to not fill the smack hash
with garbage values coming from any extended attribute
write.

Change-Id: Iaf0039c2be9bccb6cee11c24a3b44d209101fe47
Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
(cherry-picked from upstream 9598f4c9e7069aee8639be1e04e8af26b5a77fa2)

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 7eac0d1f2a349df25d2cb0936f920655c7d2711e..ab46b3dc543c998b7bf3b8709072081b1c295292 100644 (file)
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -963,18 +963,20 @@ static void smack_inode_post_setxattr(struct dentry *dentry, const char *name,
                return;
        }
 
-       skp = smk_import_entry(value, size);
        if (strcmp(name, XATTR_NAME_SMACK) == 0) {
+               skp = smk_import_entry(value, size);
                if (skp != NULL)
                        isp->smk_inode = skp->smk_known;
                else
                        isp->smk_inode = smack_known_invalid.smk_known;
        } else if (strcmp(name, XATTR_NAME_SMACKEXEC) == 0) {
+               skp = smk_import_entry(value, size);
                if (skp != NULL)
                        isp->smk_task = skp;
                else
                        isp->smk_task = &smack_known_invalid;
        } else if (strcmp(name, XATTR_NAME_SMACKMMAP) == 0) {
+               skp = smk_import_entry(value, size);
                if (skp != NULL)
                        isp->smk_mmap = skp;
                else