[PATCH net-next v5 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[PATCH net-next v5 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().

Jason Wang <jasowang@redhat.com> proposed to count XDP_PACKET_HEADROOM
always (e.g. without rcu_access_pointer(tun->xdp_prog)) in
tun_can_build_skb() since there's a window during which XDP program
might be attached between tun_can_build_skb() and 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 (akanner):
    v5:
      - always count XDP_PACKET_HEADROOM in tun_can_build_skb() as there's a
        window between tun_can_build_skb() and tun_build_skb() and XDP
        program might be attached there.
      - rcu_read_lock/unlock() for tun->xdp_prog were completely removed and
        there's no need to use rcu_access_pointer() instead which was noted
        by David Ahern <dsahern@gmail.com>.
    v4: https://lore.kernel.org/all/20230801220710.464-1-andrew.kanner@gmail.com/T/
      - 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.
    v3: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
      - attach the forgotten changelog
    v2: https://lore.kernel.org/all/20230725153941.653-1-andrew.kanner@gmail.com/T/
      - 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>
    v1: https://lore.kernel.org/all/20230724221326.384-1-andrew.kanner@gmail.com/T/

 drivers/net/tun.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 25f0191df00b..100339bc8b04 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1594,7 +1594,7 @@ 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) +
+	if (SKB_DATA_ALIGN(len + TUN_RX_PAD + XDP_PACKET_HEADROOM) +
 	    SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
 		return false;
 
-- 
2.39.3

[PATCH net-next v5 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>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Acked-by: Jason Wang <jasowang@redhat.com>
---

Notes (akanner):
    v5:
      - same as v4, but cc-ed bpf@vger.kernel.org according to v3->v4
        change
    v4: https://lore.kernel.org/all/20230801220710.464-1-andrew.kanner@gmail.com/T/
      - remove bpf_xdp_adjust_tail() check for frame_sz instead.
    v3: https://lore.kernel.org/all/20230725155403.796-1-andrew.kanner@gmail.com/T/
    v2: https://lore.kernel.org/all/20230725153941.653-1-andrew.kanner@gmail.com/T/
    v1: https://lore.kernel.org/all/20230724221326.384-1-andrew.kanner@gmail.com/T/
      - initial attempts to fix drivers/net/tun.c:tun_get_user(),
        e.g. tun_build_skb() or tun_alloc_skb(), to not exceed
        xdp->frame_sz check from net/core/filter.c

 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 net-next v5 1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu,  3 Aug 2023 20:59:48 +0200 you 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().
> 
> Jason Wang <jasowang@redhat.com> proposed to count XDP_PACKET_HEADROOM
> always (e.g. without rcu_access_pointer(tun->xdp_prog)) in
> tun_can_build_skb() since there's a window during which XDP program
> might be attached between tun_can_build_skb() and tun_build_skb().
> 
> [...]

Here is the summary with links:
  - [net-next,v5,1/2] drivers: net: prevent tun_build_skb() to exceed the packet size limit
    https://git.kernel.org/netdev/net/c/59eeb2329405
  - [net-next,v5,2/2] net: core: remove unnecessary frame_sz check in bpf_xdp_adjust_tail()
    https://git.kernel.org/netdev/net/c/d14eea09edf4

You are awesome, thank you!
-- 
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