Messages in this thread Patch in this message | | | From | Jesper Dangaard Brouer <> | Date | Wed, 26 Jul 2023 11:02:26 +0200 | Subject | Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits |
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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@gmail.com> 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@syzkaller.appspotmail.com >> Closes: https://lore.kernel.org/all/000000000000774b9205f1d8a80d@google.com/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@gmail.com> >> --- >> >> 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@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> > > Acked-by: Jason Wang <jasowang@redhat.com> > > 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
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