[PATCH 1/1] SMACK: Fix handling value==NULL in post setxattr

[PATCH 1/1] SMACK: Fix handling value==NULL in post setxattr

[José Bollo]

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>
---
 security/smack/smack_lsm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 3f01cf5..28d482c 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -893,18 +893,20 @@ 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
-- 
1.8.1.4

Re: [PATCH 1/1] SMACK: Fix handling value==NULL in post setxattr

[Casey Schaufler]

On 4/3/2014 4:48 AM, José Bollo wrote:
> 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>

Acked-by: Casey Schaufler <casey@schaufler-ca.com>

Applied to git://git.gitorious.org/smack-next/kernel.git smack-for-3.16

> ---
>  security/smack/smack_lsm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 3f01cf5..28d482c 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -893,18 +893,20 @@ 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