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Message-ID: <984fe517-44e5-4767-a521-aad4891d5f2b@intel.com>
Date: Thu, 9 Oct 2025 16:50:24 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>
CC: Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski
<maciej.fijalkowski@...el.com>, Stanislav Fomichev <sdf@...ichev.me>, "Alexei
Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
"Jesper Dangaard Brouer" <hawk@...nel.org>, John Fastabend
<john.fastabend@...il.com>, "David S. Miller" <davem@...emloft.net>, Eric
Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
<pabeni@...hat.com>, "Simon Horman" <horms@...nel.org>, Kees Cook
<kees@...nel.org>, <nxne.cnse.osdt.itp.upstreaming@...el.com>,
<bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
<linux-hardening@...r.kernel.org>, <stable@...r.kernel.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf] xsk: harden userspace-supplied &xdp_desc validation
From: Jason Xing <kerneljasonxing@...il.com>
Date: Thu, 9 Oct 2025 22:02:23 +0800
> On Thu, Oct 9, 2025 at 12:59 AM Alexander Lobakin
> <aleksander.lobakin@...el.com> wrote:
>>
>> Turned out certain clearly invalid values passed in &xdp_desc from
>> userspace can pass xp_{,un}aligned_validate_desc() and then lead
>> to UBs or just invalid frames to be queued for xmit.
>>
>> desc->len close to ``U32_MAX`` with a non-zero pool->tx_metadata_len
>> can cause positive integer overflow and wraparound, the same way low
>> enough desc->addr with a non-zero pool->tx_metadata_len can cause
>> negative integer overflow. Both scenarios can then pass the
>> validation successfully.
>> This doesn't happen with valid XSk applications, but can be used
>> to perform attacks.
>>
>> Always promote desc->len to ``u64`` first to exclude positive
>> overflows of it. Use explicit check_{add,sub}_overflow() when
>> validating desc->addr (which is ``u64`` already).
>>
>> bloat-o-meter reports a little growth of the code size:
>>
>> add/remove: 0/0 grow/shrink: 2/1 up/down: 60/-16 (44)
>> Function old new delta
>> xskq_cons_peek_desc 299 330 +31
>> xsk_tx_peek_release_desc_batch 973 1002 +29
>> xsk_generic_xmit 3148 3132 -16
>>
>> but hopefully this doesn't hurt the performance much.
>
> I don't see an evident point that might affect the performance. Since
> you said that, I tested by running './xdpsock -i eth1 -t -S -s 64' and
> didn't spot any degradation.
Thanks for testing!
>
>>
>> Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
>> Cc: stable@...r.kernel.org # 6.8+
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>
> Thanks for the fix!
>
> Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
>
>> ---
>> net/xdp/xsk_queue.h | 45 +++++++++++++++++++++++++++++++++++----------
>> 1 file changed, 35 insertions(+), 10 deletions(-)
>>
>> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
>> index f16f390370dc..1eb8d9f8b104 100644
>> --- a/net/xdp/xsk_queue.h
>> +++ b/net/xdp/xsk_queue.h
>> @@ -143,14 +143,24 @@ static inline bool xp_unused_options_set(u32 options)
>> static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
>> struct xdp_desc *desc)
>> {
>> - u64 addr = desc->addr - pool->tx_metadata_len;
>> - u64 len = desc->len + pool->tx_metadata_len;
>> - u64 offset = addr & (pool->chunk_size - 1);
>> + u64 len = desc->len;
>> + u64 addr, offset;
>>
>> - if (!desc->len)
>> + if (!len)
>> return false;
>>
>> - if (offset + len > pool->chunk_size)
>> + /* Can overflow if desc->addr < pool->tx_metadata_len */
>> + if (check_sub_overflow(desc->addr, pool->tx_metadata_len, &addr))
>> + return false;
>> +
>> + offset = addr & (pool->chunk_size - 1);
>> +
>> + /*
>> + * Can't overflow: @offset is guaranteed to be < ``U32_MAX``
>> + * (pool->chunk_size is ``u32``), @len is guaranteed
>> + * to be <= ``U32_MAX``.
>> + */
>> + if (offset + len + pool->tx_metadata_len > pool->chunk_size)
>> return false;
>>
>> if (addr >= pool->addrs_cnt)
>> @@ -158,27 +168,42 @@ static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
>>
>> if (xp_unused_options_set(desc->options))
>> return false;
>> +
>
> nit?
Yes, probably doesn't fit well for this particular fix. I have no strong
preference and can remove it if the community wishes.
>
>> return true;
>> }
>>
>> static inline bool xp_unaligned_validate_desc(struct xsk_buff_pool *pool,
>> struct xdp_desc *desc)
>> {
>> - u64 addr = xp_unaligned_add_offset_to_addr(desc->addr) - pool->tx_metadata_len;
>> - u64 len = desc->len + pool->tx_metadata_len;
>> + u64 len = desc->len;
>> + u64 addr, end;
>>
>> - if (!desc->len)
>> + if (!len)
>> return false;
>>
>> + /* Can't overflow: @len is guaranteed to be <= ``U32_MAX`` */
>> + len += pool->tx_metadata_len;
>> if (len > pool->chunk_size)
>> return false;
>>
>> - if (addr >= pool->addrs_cnt || addr + len > pool->addrs_cnt ||
>> - xp_desc_crosses_non_contig_pg(pool, addr, len))
>> + /* Can overflow if desc->addr is close to 0 */
>> + if (check_sub_overflow(xp_unaligned_add_offset_to_addr(desc->addr),
>> + pool->tx_metadata_len, &addr))
>> + return false;
>> +
>> + if (addr >= pool->addrs_cnt)
>> + return false;
>> +
>> + /* Can overflow if pool->addrs_cnt is high enough */
>> + if (check_add_overflow(addr, len, &end) || end > pool->addrs_cnt)
>> + return false;
>> +
>> + if (xp_desc_crosses_non_contig_pg(pool, addr, len))
>> return false;
>>
>> if (xp_unused_options_set(desc->options))
>> return false;
>> +
Same here.
>> return true;
>> }
Thanks,
Olek
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