Re: [PATCH] 9p/cache.c: Fix memory leak in v9fs_cache_session_get_cookie

[Dominique Martinet <asmadeus@codewreck.org>]

Bharath Vedartham wrote on Thu, May 23, 2019:
> v9fs_cache_session_get_cookie assigns a random cachetag to
> v9ses->cachetag, if the cachetag is not assigned previously.
> 
> v9fs_random_cachetag allocates memory to v9ses->cachetag with kmalloc
> and uses scnprintf to fill it up with a cachetag.
> 
> But if scnprintf fails, v9ses->cachetag is not freed in the current code causing a memory leak.
> 
> Fix this by freeing v9ses->cachetag it v9fs_random_cachetag fails.
> 
> This was reported by syzbot, the link to the report is below:
> https://syzkaller.appspot.com/bug?id=f012bdf297a7a4c860c38a88b44fbee43fd9bbf3
> 
> Reported-by: syzbot+3a030a73b6c1e9833815@syzkaller.appspotmail.com 
> Signed-off-by: Bharath Vedartham <linux.bhar@gmail.com>
> ---
>  fs/9p/cache.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/9p/cache.c b/fs/9p/cache.c
> index 9eb3470..4463b91 100644
> --- a/fs/9p/cache.c
> +++ b/fs/9p/cache.c
> @@ -66,6 +66,7 @@ void v9fs_cache_session_get_cookie(struct v9fs_session_info *v9ses)
>  	if (!v9ses->cachetag) {
>  		if (v9fs_random_cachetag(v9ses) < 0) {
>  			v9ses->fscache = NULL;
> +			kfree(v9ses->cachetag);

I would also reset v9ses->cachetag to NULL just in case,
v9fs_cache_session_get_cookie will use v9ses->cachetag as it is if it is
not null and you were leaving an invalid pointer there

I do not see any reason it could be called multiple times but
v9fs_cache_session_get_cookie does not return any error (void function)
so something later on could try to use that cachetag incorrectly later
on

Thanks,
-- 
Dominique