[PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Andrew Kanner]

Using the syzkaller repro with reduced packet size it was discovered
that XDP_PACKET_HEADROOM is not checked in tun_can_build_skb(),
although pad may be incremented in tun_build_skb(). This may end up
with exceeding the PAGE_SIZE limit in tun_build_skb().

Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
---

Notes:
    v3 -> v4:
    * fall back to v1, fixing only missing XDP_PACKET_HEADROOM in pad and
      removing bpf_xdp_adjust_tail() check for frame_sz.
    * added rcu read lock, noted by Jason Wang <jasowang@redhat.com> in v1
    * I decided to leave the packet length check in tun_can_build_skb()
      instead of moving to tun_build_skb() suggested by Jason Wang
      <jasowang@redhat.com>. Otherwise extra packets will be dropped
      without falling back to tun_alloc_skb(). And in the discussion of v3
      Jesper Dangaard Brouer <jbrouer@redhat.com> noticed that XDP is ok
      with a higher order pages if it's a contiguous physical memory
      allocation, so falling to tun_alloc_skb() -> do_xdp_generic() should
      be ok.
    
    v2 -> v3:
    * attach the forgotten changelog
    
    v1 -> v2:
    * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
      syzkaller repro and missing XDP_PACKET_HEADROOM in pad
    * changed the title and description of the execution path, suggested
      by Jason Wang <jasowang@redhat.com>
    * move the limit check from tun_can_build_skb() to tun_build_skb() to
      remove duplication and locking issue, and also drop the packet in
      case of a failed check - noted by Jason Wang <jasowang@redhat.com>

 drivers/net/tun.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..a1d04bc9485f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1582,6 +1582,9 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
 static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
 			      int len, int noblock, bool zerocopy)
 {
+	struct bpf_prog *xdp_prog;
+	int pad = TUN_RX_PAD;
+
 	if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
 		return false;
 
@@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
 	if (zerocopy)
 		return false;
 
-	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
+	rcu_read_lock();
+	xdp_prog = rcu_dereference(tun->xdp_prog);
+	if (xdp_prog)
+		pad += XDP_PACKET_HEADROOM;
+	rcu_read_unlock();
+
+	if (SKB_DATA_ALIGN(len + pad) +
 	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
 		return false;
 
-- 
2.39.3

[PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()

[Andrew Kanner]

Syzkaller reported the following issue:
=======================================
Too BIG xdp->frame_sz = 131072
WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
  ____bpf_xdp_adjust_tail net/core/filter.c:4121 [inline]
WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
  bpf_xdp_adjust_tail+0x466/0xa10 net/core/filter.c:4103
...
Call Trace:
 <TASK>
 bpf_prog_4add87e5301a4105+0x1a/0x1c
 __bpf_prog_run include/linux/filter.h:600 [inline]
 bpf_prog_run_xdp include/linux/filter.h:775 [inline]
 bpf_prog_run_generic_xdp+0x57e/0x11e0 net/core/dev.c:4721
 netif_receive_generic_xdp net/core/dev.c:4807 [inline]
 do_xdp_generic+0x35c/0x770 net/core/dev.c:4866
 tun_get_user+0x2340/0x3ca0 drivers/net/tun.c:1919
 tun_chr_write_iter+0xe8/0x210 drivers/net/tun.c:2043
 call_write_iter include/linux/fs.h:1871 [inline]
 new_sync_write fs/read_write.c:491 [inline]
 vfs_write+0x650/0xe40 fs/read_write.c:584
 ksys_write+0x12f/0x250 fs/read_write.c:637
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

xdp->frame_sz > PAGE_SIZE check was introduced in commit c8741e2bfe87
("xdp: Allow bpf_xdp_adjust_tail() to grow packet size"). But Jesper
Dangaard Brouer <jbrouer@redhat.com> noted that after introducing the
xdp_init_buff() which all XDP driver use - it's safe to remove this
check. The original intend was to catch cases where XDP drivers have
not been updated to use xdp.frame_sz, but that is not longer a concern
(since xdp_init_buff).

Running the initial syzkaller repro it was discovered that the
contiguous physical memory allocation is used for both xdp paths in
tun_get_user(), e.g. tun_build_skb() and tun_alloc_skb(). It was also
stated by Jesper Dangaard Brouer <jbrouer@redhat.com> that XDP can
work on higher order pages, as long as this is contiguous physical
memory (e.g. a page).

Reported-and-tested-by: syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/T/
Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
Link: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
Fixes: 43b5169d8355 ("net, xdp: Introduce xdp_init_buff utility routine")
Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
---
 net/core/filter.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 06ba0e56e369..28a59596987a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4116,12 +4116,6 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
 	if (unlikely(data_end > data_hard_end))
 		return -EINVAL;
 
-	/* ALL drivers MUST init xdp->frame_sz, chicken check below */
-	if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
-		WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
-		return -EINVAL;
-	}
-
 	if (unlikely(data_end < xdp->data + ETH_HLEN))
 		return -EINVAL;
 
-- 
2.39.3

Re: [PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()

[Jesper Dangaard Brouer]

On 02/08/2023 00.07, Andrew Kanner wrote:
> Syzkaller reported the following issue:
> =======================================
> Too BIG xdp->frame_sz = 131072
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>    ____bpf_xdp_adjust_tail net/core/filter.c:4121 [inline]
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>    bpf_xdp_adjust_tail+0x466/0xa10 net/core/filter.c:4103
> ...
> Call Trace:
>   <TASK>
>   bpf_prog_4add87e5301a4105+0x1a/0x1c
>   __bpf_prog_run include/linux/filter.h:600 [inline]
>   bpf_prog_run_xdp include/linux/filter.h:775 [inline]
>   bpf_prog_run_generic_xdp+0x57e/0x11e0 net/core/dev.c:4721
>   netif_receive_generic_xdp net/core/dev.c:4807 [inline]
>   do_xdp_generic+0x35c/0x770 net/core/dev.c:4866
>   tun_get_user+0x2340/0x3ca0 drivers/net/tun.c:1919
>   tun_chr_write_iter+0xe8/0x210 drivers/net/tun.c:2043
>   call_write_iter include/linux/fs.h:1871 [inline]
>   new_sync_write fs/read_write.c:491 [inline]
>   vfs_write+0x650/0xe40 fs/read_write.c:584
>   ksys_write+0x12f/0x250 fs/read_write.c:637
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> xdp->frame_sz > PAGE_SIZE check was introduced in commit c8741e2bfe87
> ("xdp: Allow bpf_xdp_adjust_tail() to grow packet size"). But Jesper
> Dangaard Brouer <jbrouer@redhat.com> noted that after introducing the
> xdp_init_buff() which all XDP driver use - it's safe to remove this
> check. The original intend was to catch cases where XDP drivers have
> not been updated to use xdp.frame_sz, but that is not longer a concern
> (since xdp_init_buff).
> 
> Running the initial syzkaller repro it was discovered that the
> contiguous physical memory allocation is used for both xdp paths in
> tun_get_user(), e.g. tun_build_skb() and tun_alloc_skb(). It was also
> stated by Jesper Dangaard Brouer <jbrouer@redhat.com> that XDP can
> work on higher order pages, as long as this is contiguous physical
> memory (e.g. a page).
> 
> Reported-and-tested-by: syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/T/
> Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> Link: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
> Fixes: 43b5169d8355 ("net, xdp: Introduce xdp_init_buff utility routine")
> Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
> ---

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

You can include that ACK in V5.

>   net/core/filter.c | 6 ------
>   1 file changed, 6 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 06ba0e56e369..28a59596987a 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4116,12 +4116,6 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>   	if (unlikely(data_end > data_hard_end))
>   		return -EINVAL;
>   
> -	/* ALL drivers MUST init xdp->frame_sz, chicken check below */
> -	if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
> -		WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
> -		return -EINVAL;
> -	}
> -
>   	if (unlikely(data_end < xdp->data + ETH_HLEN))
>   		return -EINVAL;
>   

Re: [PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()

[Jason Wang]

On Wed, Aug 2, 2023 at 6:09 AM Andrew Kanner <andrew.kanner@gmail.com> wrote:
>
> Syzkaller reported the following issue:
> =======================================
> Too BIG xdp->frame_sz = 131072
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>   ____bpf_xdp_adjust_tail net/core/filter.c:4121 [inline]
> WARNING: CPU: 0 PID: 5020 at net/core/filter.c:4121
>   bpf_xdp_adjust_tail+0x466/0xa10 net/core/filter.c:4103
> ...
> Call Trace:
>  <TASK>
>  bpf_prog_4add87e5301a4105+0x1a/0x1c
>  __bpf_prog_run include/linux/filter.h:600 [inline]
>  bpf_prog_run_xdp include/linux/filter.h:775 [inline]
>  bpf_prog_run_generic_xdp+0x57e/0x11e0 net/core/dev.c:4721
>  netif_receive_generic_xdp net/core/dev.c:4807 [inline]
>  do_xdp_generic+0x35c/0x770 net/core/dev.c:4866
>  tun_get_user+0x2340/0x3ca0 drivers/net/tun.c:1919
>  tun_chr_write_iter+0xe8/0x210 drivers/net/tun.c:2043
>  call_write_iter include/linux/fs.h:1871 [inline]
>  new_sync_write fs/read_write.c:491 [inline]
>  vfs_write+0x650/0xe40 fs/read_write.c:584
>  ksys_write+0x12f/0x250 fs/read_write.c:637
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> xdp->frame_sz > PAGE_SIZE check was introduced in commit c8741e2bfe87
> ("xdp: Allow bpf_xdp_adjust_tail() to grow packet size"). But Jesper
> Dangaard Brouer <jbrouer@redhat.com> noted that after introducing the
> xdp_init_buff() which all XDP driver use - it's safe to remove this
> check. The original intend was to catch cases where XDP drivers have
> not been updated to use xdp.frame_sz, but that is not longer a concern
> (since xdp_init_buff).
>
> Running the initial syzkaller repro it was discovered that the
> contiguous physical memory allocation is used for both xdp paths in
> tun_get_user(), e.g. tun_build_skb() and tun_alloc_skb(). It was also
> stated by Jesper Dangaard Brouer <jbrouer@redhat.com> that XDP can
> work on higher order pages, as long as this is contiguous physical
> memory (e.g. a page).
>
> Reported-and-tested-by: syzbot+f817490f5bd20541b90a@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/T/
> Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> Link: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
> Fixes: 43b5169d8355 ("net, xdp: Introduce xdp_init_buff utility routine")
> Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

> ---
>  net/core/filter.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 06ba0e56e369..28a59596987a 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4116,12 +4116,6 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>         if (unlikely(data_end > data_hard_end))
>                 return -EINVAL;
>
> -       /* ALL drivers MUST init xdp->frame_sz, chicken check below */
> -       if (unlikely(xdp->frame_sz > PAGE_SIZE)) {
> -               WARN_ONCE(1, "Too BIG xdp->frame_sz = %d\n", xdp->frame_sz);
> -               return -EINVAL;
> -       }
> -
>         if (unlikely(data_end < xdp->data + ETH_HLEN))
>                 return -EINVAL;
>
> --
> 2.39.3
>

Re: [PATCH v4 2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()

[Andrew Kanner]

On Thu, Aug 03, 2023 at 11:20:20AM +0800, Jason Wang wrote:
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 
> Thanks
> 

Thanks, Jason.
I will use this tag in v5, if you don't mind.
PATCH 2/2 will be the same but with more recipients.

-- 
Andrew Kanner

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[David Ahern]

On 8/1/23 4:07 PM, Andrew Kanner wrote:
> @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>  	if (zerocopy)
>  		return false;
>  
> -	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> +	rcu_read_lock();
> +	xdp_prog = rcu_dereference(tun->xdp_prog);
> +	if (xdp_prog)
> +		pad += XDP_PACKET_HEADROOM;
> +	rcu_read_unlock();


since you do not care about the actual xdp_prog (only that one is set) I
believe you can use rcu_access_pointer here.

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Andrew Kanner]

On Tue, Aug 01, 2023 at 07:07:39PM -0600, David Ahern wrote:
> On 8/1/23 4:07 PM, Andrew Kanner wrote:
> > @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> >  	if (zerocopy)
> >  		return false;
> >  
> > -	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > +	rcu_read_lock();
> > +	xdp_prog = rcu_dereference(tun->xdp_prog);
> > +	if (xdp_prog)
> > +		pad += XDP_PACKET_HEADROOM;
> > +	rcu_read_unlock();
> 
> 
> since you do not care about the actual xdp_prog (only that one is set) I
> believe you can use rcu_access_pointer here.

Good point. Thanks, David.

I'll resend both as v5.

The correct cc-list for PATCH 2/2 is also needed. It fixes
net/core/filter.c instead of drivers/net/tun.c now.

pw-bot: changes-requested

-- 
Andrew Kanner

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Jesper Dangaard Brouer]

On 02/08/2023 00.07, Andrew Kanner wrote:
> Using the syzkaller repro with reduced packet size it was discovered
> that XDP_PACKET_HEADROOM is not checked in tun_can_build_skb(),
> although pad may be incremented in tun_build_skb(). This may end up
> with exceeding the PAGE_SIZE limit in tun_build_skb().
> 
> Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
> ---
> 
> Notes:
>      v3 -> v4:
>      * fall back to v1, fixing only missing XDP_PACKET_HEADROOM in pad and
>        removing bpf_xdp_adjust_tail() check for frame_sz.
>      * added rcu read lock, noted by Jason Wang <jasowang@redhat.com> in v1
>      * I decided to leave the packet length check in tun_can_build_skb()
>        instead of moving to tun_build_skb() suggested by Jason Wang
>        <jasowang@redhat.com>. Otherwise extra packets will be dropped
>        without falling back to tun_alloc_skb(). And in the discussion of v3
>        Jesper Dangaard Brouer <jbrouer@redhat.com> noticed that XDP is ok
>        with a higher order pages if it's a contiguous physical memory
>        allocation, so falling to tun_alloc_skb() -> do_xdp_generic() should
>        be ok.
>      
>      v2 -> v3:
>      * attach the forgotten changelog
>      
>      v1 -> v2:
>      * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
>        syzkaller repro and missing XDP_PACKET_HEADROOM in pad
>      * changed the title and description of the execution path, suggested
>        by Jason Wang <jasowang@redhat.com>
>      * move the limit check from tun_can_build_skb() to tun_build_skb() to
>        remove duplication and locking issue, and also drop the packet in
>        case of a failed check - noted by Jason Wang <jasowang@redhat.com>
> 
>   drivers/net/tun.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index d75456adc62a..a1d04bc9485f 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1582,6 +1582,9 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
>   static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>   			      int len, int noblock, bool zerocopy)
>   {
> +	struct bpf_prog *xdp_prog;
> +	int pad = TUN_RX_PAD;
> +
>   	if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
>   		return false;
>   
> @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
>   	if (zerocopy)
>   		return false;
>   
> -	if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> +	rcu_read_lock();
> +	xdp_prog = rcu_dereference(tun->xdp_prog);
> +	if (xdp_prog)
> +		pad += XDP_PACKET_HEADROOM;
> +	rcu_read_unlock();
> +

Isolated seen, I guess, this is a correct fix to 7df13219d757.

> +	if (SKB_DATA_ALIGN(len + pad) +
>   	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
>   		return false;
>   

Question to Jason Wang:
Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?

AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
from it's page_frag.  Is there a reason for this limitation?

(To Andrew, I assume a change in this area is another patch).

--Jesper

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Jason Wang]

On Wed, Aug 2, 2023 at 10:14 PM Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>
>
>
> On 02/08/2023 00.07, Andrew Kanner wrote:
> > Using the syzkaller repro with reduced packet size it was discovered
> > that XDP_PACKET_HEADROOM is not checked in tun_can_build_skb(),
> > although pad may be incremented in tun_build_skb(). This may end up
> > with exceeding the PAGE_SIZE limit in tun_build_skb().
> >
> > Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
> > Link: https://syzkaller.appspot.com/bug?extid=f817490f5bd20541b90a
> > Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
> > ---
> >
> > Notes:
> >      v3 -> v4:
> >      * fall back to v1, fixing only missing XDP_PACKET_HEADROOM in pad and
> >        removing bpf_xdp_adjust_tail() check for frame_sz.
> >      * added rcu read lock, noted by Jason Wang <jasowang@redhat.com> in v1
> >      * I decided to leave the packet length check in tun_can_build_skb()
> >        instead of moving to tun_build_skb() suggested by Jason Wang
> >        <jasowang@redhat.com>. Otherwise extra packets will be dropped
> >        without falling back to tun_alloc_skb(). And in the discussion of v3
> >        Jesper Dangaard Brouer <jbrouer@redhat.com> noticed that XDP is ok
> >        with a higher order pages if it's a contiguous physical memory
> >        allocation, so falling to tun_alloc_skb() -> do_xdp_generic() should
> >        be ok.
> >
> >      v2 -> v3:
> >      * attach the forgotten changelog
> >
> >      v1 -> v2:
> >      * merged 2 patches in 1, fixing both issues: WARN_ON_ONCE with
> >        syzkaller repro and missing XDP_PACKET_HEADROOM in pad
> >      * changed the title and description of the execution path, suggested
> >        by Jason Wang <jasowang@redhat.com>
> >      * move the limit check from tun_can_build_skb() to tun_build_skb() to
> >        remove duplication and locking issue, and also drop the packet in
> >        case of a failed check - noted by Jason Wang <jasowang@redhat.com>
> >
> >   drivers/net/tun.c | 11 ++++++++++-
> >   1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index d75456adc62a..a1d04bc9485f 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1582,6 +1582,9 @@ static void tun_rx_batched(struct tun_struct *tun, struct tun_file *tfile,
> >   static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> >                             int len, int noblock, bool zerocopy)
> >   {
> > +     struct bpf_prog *xdp_prog;
> > +     int pad = TUN_RX_PAD;
> > +
> >       if ((tun->flags & TUN_TYPE_MASK) != IFF_TAP)
> >               return false;
> >
> > @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> >       if (zerocopy)
> >               return false;
> >
> > -     if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > +     rcu_read_lock();
> > +     xdp_prog = rcu_dereference(tun->xdp_prog);
> > +     if (xdp_prog)
> > +             pad += XDP_PACKET_HEADROOM;
> > +     rcu_read_unlock();
> > +
>
> Isolated seen, I guess, this is a correct fix to 7df13219d757.

I think so.

Actually, I think we can probably always count XDP_PACKET_HEADROOM
here. Since there's a window that XDP program might be attached in the
middle of tun_can_build_skb() and tun_build_skb().

>
> > +     if (SKB_DATA_ALIGN(len + pad) +
> >           SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
> >               return false;
> >
>
> Question to Jason Wang:
> Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?
>
> AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
> from it's page_frag.  Is there a reason for this limitation?

I couldn't recall but I think we can relax.

Thanks

>
> (To Andrew, I assume a change in this area is another patch).
>
> --Jesper
>
>
>

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Andrew Kanner]

On Thu, Aug 03, 2023 at 11:19:47AM +0800, Jason Wang wrote:
> > > @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> > >       if (zerocopy)
> > >               return false;
> > >
> > > -     if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > > +     rcu_read_lock();
> > > +     xdp_prog = rcu_dereference(tun->xdp_prog);
> > > +     if (xdp_prog)
> > > +             pad += XDP_PACKET_HEADROOM;
> > > +     rcu_read_unlock();
> > > +
> >
> > Isolated seen, I guess, this is a correct fix to 7df13219d757.
> 
> I think so.
> 
> Actually, I think we can probably always count XDP_PACKET_HEADROOM
> here. Since there's a window that XDP program might be attached in the
> middle of tun_can_build_skb() and tun_build_skb().


Thanks, that makes sense.
I will do it in v5.


> > Question to Jason Wang:
> > Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?
> >
> > AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
> > from it's page_frag.  Is there a reason for this limitation?
> 
> I couldn't recall but I think we can relax.

Jesper already sent enough info for this idea in v2, I will use it for
the next patch/series.

Jesper, I will add this tag for this next patch/series if you don't
mind:
Suggested-by: Jesper Dangaard Brouer <hawk@kernel.org>

-- 
Andrew Kanner

Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[Jesper Dangaard Brouer]

On 03/08/2023 19.53, Andrew Kanner wrote:
>>> Question to Jason Wang:
>>> Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?
>>>
>>> AFAIK tun_build_skb()*can*  create get larger packets than PAGE_SIZE
>>> from it's page_frag.  Is there a reason for this limitation?
>> >> I couldn't recall but I think we can relax.
> > Jesper already sent enough info for this idea in v2, I will use it for
> the next patch/series.
> 

I have some more input and considerations when selecting the new
constant that replace PAGE_SIZE.

Lets see if Eric Dumazet or Alex Duyck disagree?
("inventors" of page_frag scheme)

The function tun_alloc_skb() uses a page_frag scheme for allocation.
The maximim size is 32768 bytes (Order-3), but using something that is
close to this max alloc size can cause memory waste and fragmentation.

My suggestion would be to use the constant SKB_MAX_ALLOC (16KiB).

Maybe Eric or Alex would recommend using something smaller? (e.g. 8192)

page_frag limit comes from:
  #define SKB_FRAG_PAGE_ORDER get_order(32768)


> Jesper, I will add this tag for this next patch/series if you don't
> mind:
> Suggested-by: Jesper Dangaard Brouer <hawk@kernel.org>

ACK