linux-kernel - Re: [PATCH 3/5] security: secid_to_secctx returns len when data is NULL

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Date:	Tue, 12 Oct 2010 19:04:14 -0400
From:	Paul Moore <paul.moore@...com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
	netfilter@...r.kernel.org, jmorris@...ei.org,
	selinux@...ho.nsa.gov, sds@...ho.nsa.gov, jengelh@...ozas.de,
	linux-security-module@...r.kernel.org, mr.dash.four@...glemail.com,
	pablo@...filter.org
Subject: Re: [PATCH 3/5] security: secid_to_secctx returns len when data is
 NULL

On Tue, 2010-10-12 at 11:40 -0400, Eric Paris wrote:
> With the (long ago) interface change to have the secid_to_secctx functions
> do the string allocation instead of having the caller do the allocation we
> lost the ability to query the security server for the length of the
> upcoming string.  The SECMARK code would like to allocate a netlink skb
> with enough length to hold the string but it is just too unclean to do the
> string allocation twice or to do the allocation the first time and hold
> onto the string and slen.  This patch adds the ability to call
> security_secid_to_secctx() with a NULL data pointer and it will just set
> the slen pointer.

This opens the door for a potential race condition, but only with some
yet to be defined LSM where the both the mapping between secids and
secctxs and secctxs are not like they are with SELinux and Smack.  With
that in mind, it looks good to me I'd just like to see some extra
commentary in security.h warning about the potential pitfalls.

Reviewed-by: Paul Moore <paul.moore@...com>

> Signed-off-by: Eric Paris <eparis@...hat.com>
> ---
> 
>  security/selinux/ss/services.c |   11 +++++++++--
>  security/smack/smack_lsm.c     |    3 ++-
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 73508af..1d4955a 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -998,7 +998,8 @@ static int context_struct_to_string(struct context *context, char **scontext, u3
>  {
>  	char *scontextp;
>  
> -	*scontext = NULL;
> +	if (scontext)
> +		*scontext = NULL;
>  	*scontext_len = 0;
>  
>  	if (context->len) {
> @@ -1015,6 +1016,9 @@ static int context_struct_to_string(struct context *context, char **scontext, u3
>  	*scontext_len += strlen(sym_name(&policydb, SYM_TYPES, context->type - 1)) + 1;
>  	*scontext_len += mls_compute_context_len(context);
>  
> +	if (!scontext)
> +		return 0;
> +
>  	/* Allocate space for the context; caller must free this space. */
>  	scontextp = kmalloc(*scontext_len, GFP_ATOMIC);
>  	if (!scontextp)
> @@ -1054,7 +1058,8 @@ static int security_sid_to_context_core(u32 sid, char **scontext,
>  	struct context *context;
>  	int rc = 0;
>  
> -	*scontext = NULL;
> +	if (scontext)
> +		*scontext = NULL;
>  	*scontext_len  = 0;
>  
>  	if (!ss_initialized) {
> @@ -1062,6 +1067,8 @@ static int security_sid_to_context_core(u32 sid, char **scontext,
>  			char *scontextp;
>  
>  			*scontext_len = strlen(initial_sid_to_string[sid]) + 1;
> +			if (!scontext)
> +				goto out;
>  			scontextp = kmalloc(*scontext_len, GFP_ATOMIC);
>  			if (!scontextp) {
>  				rc = -ENOMEM;
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 174aec4..bc39f40 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3004,7 +3004,8 @@ static int smack_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
>  {
>  	char *sp = smack_from_secid(secid);
>  
> -	*secdata = sp;
> +	if (secdata)
> +		*secdata = sp;
>  	*seclen = strlen(sp);
>  	return 0;
>  }
> 

-- 
paul moore
linux @ hp


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