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>

1 files changed, 3 insertions, 1 deletions

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 9a99f3c485cb..a5d86ffbf9a0 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -960,18 +960,20 @@ 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