Messages in this thread | | | Date | Wed, 26 Jul 2023 13:37:40 -0600 | Subject | Re: [PATCH v3] drivers: net: prevent tun_get_user() to exceed xdp size limits | From | David Ahern <> |
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On 7/26/23 3:02 AM, Jesper Dangaard Brouer wrote: > 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())
get_tun_user is passed an iov_iter with a single segment of 65007 total_len. The alloc_skb path is hit with an align size of only 64. That is insufficient for XDP so the netif_receive_generic_xdp hits the pskb_expand_head path. Something is off in the math in netif_receive_generic_xdp resulting in the skb markers being off. That causes bpf_prog_run_generic_xdp to compute the wrong frame_sz.
> >>> 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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