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

[Casey Schaufler]

On 4/3/2014 4:48 AM, Josà 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: Josà Bollo <jose.bollo@xxxxxxxxxxxxxxxxxxx>

Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>

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

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