linux-kernel - Re: [PATCH] apparmor: enforce nullbyte at end of tag string

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Date:   Tue, 28 May 2019 12:05:41 -0700
From:   John Johansen <john.johansen@...onical.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] apparmor: enforce nullbyte at end of tag string

On 5/28/19 8:32 AM, Jann Horn wrote:
> A packed AppArmor policy contains null-terminated tag strings that are read
> by unpack_nameX(). However, unpack_nameX() uses string functions on them
> without ensuring that they are actually null-terminated, potentially
> leading to out-of-bounds accesses.
> 
> Make sure that the tag string is null-terminated before passing it to
> strcmp().
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>

gah! yes!

Acked-by: John Johansen <john.johansen@...onical.com>


> ---
> Warning: The existence of this bug has not been verified at runtime, and
> the patch is compile-tested only. I noticed this while browsing through
> the code, but didn't want to spend the time necessary to figure out how
> to actually test this at runtime.
> 
> 
>  security/apparmor/policy_unpack.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/apparmor/policy_unpack.c b/security/apparmor/policy_unpack.c
> index f6c2bcb2ab14..33041c4fb69f 100644
> --- a/security/apparmor/policy_unpack.c
> +++ b/security/apparmor/policy_unpack.c
> @@ -276,7 +276,7 @@ static bool unpack_nameX(struct aa_ext *e, enum aa_code code, const char *name)
>  		char *tag = NULL;
>  		size_t size = unpack_u16_chunk(e, &tag);
>  		/* if a name is specified it must match. otherwise skip tag */
> -		if (name && (!size || strcmp(name, tag)))
> +		if (name && (!size || tag[size-1] != '\0' || strcmp(name, tag)))
>  			goto fail;
>  	} else if (name) {
>  		/* if a name is specified and there is no name tag fail */
>