Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits
From: Jesper Dangaard Brouer
Date: Wed Jul 26 2023 - 05:07:34 EST
Cc. John and Ahern
On 26/07/2023 04.09, Jason Wang wrote:
On Tue, Jul 25, 2023 at 11:54 PM Andrew Kanner <andrew.kanner@xxxxxxxxx> wrote:
Syzkaller reported the following issue:
=======================================
Too BIG xdp->frame_sz = 131072
Is this a contiguous physical memory allocation?
131072 bytes equal order 5 page.
Looking at tun.c code I cannot find a code path that could create
order-5 skb->data, but only SKB with order-0 fragments. But I guess it
is the netif_receive_generic_xdp() what will realloc to make this linear
(via skb_linearize())
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
tun_get_user() still provides an execution path with do_xdp_generic()
and exceed XDP limits for packet size.
I added this check and maybe it is too strict. XDP can work on higher
order pages, as long as this is contiguous physical memory (e.g. a
page). And
An order 5 page (131072 bytes) seems excessive, but maybe TUN have a
use-case for having such large packets? (Question to Ahern?)
I'm considering we should change the size-limit to order-2 (16384) or
order-3 (32768).
Order-3 because netstack have:
#define SKB_FRAG_PAGE_ORDER get_order(32768)
And order-2 because netstack have: SKB_MAX_ALLOC (16KiB)
- See discussion in commit 6306c1189e77 ("bpf: Remove MTU check in
__bpf_skb_max_len").
- https://git.kernel.org/torvalds/c/6306c1189e77
Using the syzkaller repro with reduced packet size it was also
discovered that XDP_PACKET_HEADROOM is not checked in
tun_can_build_skb(), although pad may be incremented in
tun_build_skb().
If we move the limit check from tun_can_build_skb() to tun_build_skb()
we will make xdp to be used only in tun_build_skb(), without falling
in tun_alloc_skb(), etc. And moreover we will drop the packet which
can't be processed in tun_build_skb().
Looking at tun_build_skb() is uses the page_frag system, and can thus
create up-to SKB_FRAG_PAGE_ORDER (size 32768 / order-3).
Reported-and-tested-by: syzbot+f817490f5bd20541b90a@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@xxxxxxxxxx/T/
Link: https://syzkaller.appspot.com/bug?id=5335c7c62bfff89bbb1c8f14cdabebe91909060f
Fixes: 7df13219d757 ("tun: reserve extra headroom only when XDP is set")
Signed-off-by: Andrew Kanner <andrew.kanner@xxxxxxxxx>
---
Notes:
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@xxxxxxxxxx>
* 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@xxxxxxxxxx>
Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
Thanks
drivers/net/tun.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..7c2b05ce0421 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1594,10 +1594,6 @@ 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) +
- SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) > PAGE_SIZE)
- return false;
-
return true;
}
@@ -1673,6 +1669,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
buflen += SKB_DATA_ALIGN(len + pad);
rcu_read_unlock();
+ if (buflen > PAGE_SIZE)
+ return ERR_PTR(-EFAULT);
Concretely I'm saying maybe use SKB_FRAG_PAGE_ORDER "size" here?
e.g. create SKB_FRAG_PAGE_SIZE define as below.
if (buflen > SKB_FRAG_PAGE_SIZE)
diff --git a/include/net/sock.h b/include/net/sock.h
index 656ea89f60ff..4c4b3c257b52 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2886,7 +2886,8 @@ extern int sysctl_optmem_max;
extern __u32 sysctl_wmem_default;
extern __u32 sysctl_rmem_default;
-#define SKB_FRAG_PAGE_ORDER get_order(32768)
+#define SKB_FRAG_PAGE_SIZE 32768
+#define SKB_FRAG_PAGE_ORDER get_order(SKB_FRAG_PAGE_SIZE)
DECLARE_STATIC_KEY_FALSE(net_high_order_alloc_disable_key);
+
alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
if (unlikely(!skb_page_frag_refill(buflen, alloc_frag, GFP_KERNEL)))
return ERR_PTR(-ENOMEM);
--Jesper