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Date: Tue, 15 Aug 2023 11:20:36 -0700
From: Stanislav Fomichev <sdf@...gle.com>
To: Tirthendu Sarkar <tirthendu.sarkar@...el.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, bjorn@...nel.org,
magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
jonathan.lemon@...il.com, davem@...emloft.net, kuba@...nel.org,
pabeni@...hat.com, ast@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev,
dan.carpenter@...aro.org
Subject: Re: [PATCH bpf-next v2] xsk: fix xsk_build_skb() error: 'skb'
dereferencing possible ERR_PTR()
On 08/15, Tirthendu Sarkar wrote:
> xsk_build_skb_zerocopy() may return an error other than -EAGAIN and this
> is received as skb and used later in xsk_set_destructor_arg() and
> xsk_drop_skb() which must operate on a valid skb.
>
> Set -EOVERFLOW as error when MAX_SKB_FRAGS are exceeded and packet needs
> to be dropped and use this to distinguish against all other error cases
> where allocation needs to be retried.
>
> Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@...el.com>
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
> Closes: https://lore.kernel.org/r/202307210434.OjgqFcbB-lkp@intel.com/
> Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
>
> Changelog:
> v1 -> v2:
> - Removed err as a parameter to xsk_build_skb_zerocopy()
> [Stanislav Fomichev]
> - use explicit error to distinguish packet drop vs retry
> ---
> net/xdp/xsk.c | 22 +++++++++++++---------
> 1 file changed, 13 insertions(+), 9 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index fcfc8472f73d..55f8b9b0e06d 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -602,7 +602,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>
> for (copied = 0, i = skb_shinfo(skb)->nr_frags; copied < len; i++) {
> if (unlikely(i >= MAX_SKB_FRAGS))
> - return ERR_PTR(-EFAULT);
> + return ERR_PTR(-EOVERFLOW);
>
> page = pool->umem->pgs[addr >> PAGE_SHIFT];
> get_page(page);
> @@ -655,15 +655,17 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> skb_put(skb, len);
>
> err = skb_store_bits(skb, 0, buffer, len);
> - if (unlikely(err))
> + if (unlikely(err)) {
> + kfree_skb(skb);
> goto free_err;
> + }
> } else {
> int nr_frags = skb_shinfo(skb)->nr_frags;
> struct page *page;
> u8 *vaddr;
>
> if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) {
> - err = -EFAULT;
> + err = -EOVERFLOW;
> goto free_err;
> }
>
> @@ -690,12 +692,14 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> return skb;
>
> free_err:
> - if (err == -EAGAIN) {
> - xsk_cq_cancel_locked(xs, 1);
> - } else {
> - xsk_set_destructor_arg(skb);
> - xsk_drop_skb(skb);
> + if (err == -EOVERFLOW) {
Don't think this will work? We have some other error paths in xsk_build_skb
that are not -EOVERFLOW that still need kfree_skb, right?
I feel like we are trying to share some state between xsk_build_skb and
xsk_build_skb_zerocopy which we really shouldn't share. So how about
we try to have a separate cleanup path in xsk_build_skb_zerocopy?
Will something like the following (untested / uncompiled) work instead?
IOW, ideally, xsk_build_skb should look like:
if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
return xsk_build_skb_zerocopy(xs, desc);
} else {
return xsk_build_skb_copy(xs, desc);
/* ^^ current path that should really be a separate func */
}
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 79a96019b7ef..747dd012afdb 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -578,6 +578,19 @@ static void xsk_drop_skb(struct sk_buff *skb)
xsk_consume_skb(skb);
}
+static struct sk_buff *xsk_cleanup_skb(int err, struct sk_buff *skb, struct xdp_sock *xs)
+{
+ if (err == -EAGAIN) {
+ xsk_cq_cancel_locked(xs, 1);
+ } else {
+ xsk_set_destructor_arg(skb);
+ xsk_drop_skb(skb);
+ xskq_cons_release(xs->tx);
+ }
+
+ return ERR_PTR(err);
+}
+
static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
struct xdp_desc *desc)
{
@@ -593,8 +606,10 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
hr = max(NET_SKB_PAD, L1_CACHE_ALIGN(xs->dev->needed_headroom));
skb = sock_alloc_send_skb(&xs->sk, hr, 1, &err);
- if (unlikely(!skb))
- return ERR_PTR(err);
+ if (unlikely(!skb)) {
+ err = ERR_PTR(err);
+ goto free_err;
+ }
skb_reserve(skb, hr);
}
@@ -608,8 +623,10 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
addr = buffer - pool->addrs;
for (copied = 0, i = skb_shinfo(skb)->nr_frags; copied < len; i++) {
- if (unlikely(i >= MAX_SKB_FRAGS))
- return ERR_PTR(-EFAULT);
+ if (unlikely(i >= MAX_SKB_FRAGS)) {
+ err = ERR_PTR(-EFAULT);
+ goto free_err;
+ }
page = pool->umem->pgs[addr >> PAGE_SHIFT];
get_page(page);
@@ -629,6 +646,9 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
refcount_add(ts, &xs->sk.sk_wmem_alloc);
return skb;
+
+free_err:
+ return xsk_cleanup_skb(err, skb, xs);
}
static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
@@ -641,11 +661,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
int err;
if (dev->priv_flags & IFF_TX_SKB_NO_LINEAR) {
- skb = xsk_build_skb_zerocopy(xs, desc);
- if (IS_ERR(skb)) {
- err = PTR_ERR(skb);
- goto free_err;
- }
+ return xsk_build_skb_zerocopy(xs, desc);
} else {
u32 hr, tr, len;
void *buffer;
@@ -729,15 +745,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
return skb;
free_err:
- if (err == -EAGAIN) {
- xsk_cq_cancel_locked(xs, 1);
- } else {
- xsk_set_destructor_arg(skb);
- xsk_drop_skb(skb);
- xskq_cons_release(xs->tx);
- }
-
- return ERR_PTR(err);
+ return xsk_cleanup_skb(err, skb, xs);
}
static int __xsk_generic_xmit(struct sock *sk)
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