Re: [PATCH net v2 1/2] net: fix SO_DEVMEM_DONTNEED looping too long

[Stanislav Fomichev <stfomichev@gmail.com>]

On 11/07, Mina Almasry wrote:
> Exit early if we're freeing more than 1024 frags, to prevent
> looping too long.
> 
> Also minor code cleanups:
> - Flip checks to reduce indentation.
> - Use sizeof(*tokens) everywhere for consistentcy.
> 
> Cc: Yi Lai <yi1.lai@linux.intel.com>
> Cc: Stanislav Fomichev <sdf@fomichev.me>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
> 
> v2:
> - Retain token check to prevent allocation of too much memory.
> - Exit early instead of pre-checking in a loop so that we don't penalize
>   well behaved applications (sdf)
> 
> ---
>  net/core/sock.c | 42 ++++++++++++++++++++++++------------------
>  1 file changed, 24 insertions(+), 18 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 039be95c40cf..da50df485090 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1052,32 +1052,34 @@ static int sock_reserve_memory(struct sock *sk, int bytes)
>  
>  #ifdef CONFIG_PAGE_POOL
>  
> -/* This is the number of tokens that the user can SO_DEVMEM_DONTNEED in
> - * 1 syscall. The limit exists to limit the amount of memory the kernel
> - * allocates to copy these tokens.
> +/* This is the number of tokens and frags that the user can SO_DEVMEM_DONTNEED
> + * in 1 syscall. The limit exists to limit the amount of memory the kernel
> + * allocates to copy these tokens, and to prevent looping over the frags for
> + * too long.
>   */
>  #define MAX_DONTNEED_TOKENS 128
> +#define MAX_DONTNEED_FRAGS 1024
>  
>  static noinline_for_stack int
>  sock_devmem_dontneed(struct sock *sk, sockptr_t optval, unsigned int optlen)
>  {
>  	unsigned int num_tokens, i, j, k, netmem_num = 0;
>  	struct dmabuf_token *tokens;
> +	int ret = 0, num_frags = 0;
>  	netmem_ref netmems[16];
> -	int ret = 0;
>  
>  	if (!sk_is_tcp(sk))
>  		return -EBADF;
>  
> -	if (optlen % sizeof(struct dmabuf_token) ||
> +	if (optlen % sizeof(*tokens) ||
>  	    optlen > sizeof(*tokens) * MAX_DONTNEED_TOKENS)
>  		return -EINVAL;
>  

[..]

> -	tokens = kvmalloc_array(optlen, sizeof(*tokens), GFP_KERNEL);

Oh, so we currently allocate optlen*8? This is a sneaky fix :-p

> +	num_tokens = optlen / sizeof(*tokens);
> +	tokens = kvmalloc_array(num_tokens, sizeof(*tokens), GFP_KERNEL);
>  	if (!tokens)
>  		return -ENOMEM;
>  
> -	num_tokens = optlen / sizeof(struct dmabuf_token);
>  	if (copy_from_sockptr(tokens, optval, optlen)) {
>  		kvfree(tokens);
>  		return -EFAULT;
> @@ -1086,24 +1088,28 @@ sock_devmem_dontneed(struct sock *sk, sockptr_t optval, unsigned int optlen)
>  	xa_lock_bh(&sk->sk_user_frags);
>  	for (i = 0; i < num_tokens; i++) {
>  		for (j = 0; j < tokens[i].token_count; j++) {

[..]

> +			if (++num_frags > MAX_DONTNEED_FRAGS)
> +				goto frag_limit_reached;
> +

nit: maybe reuse existing ret (and rename it to num_frags) instead of
introducing new num_frags? Both variables now seem to track the same
number.