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Tue, 12 May 2026 15:34:45 -0700 (PDT) Date: Tue, 12 May 2026 15:34:44 -0700 From: Stanislav Fomichev To: Jason Xing Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, horms@kernel.org, andrew+netdev@lunn.ch, bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: Re: [PATCH net 1/4] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata() Message-ID: References: <20260510012310.88570-1-kerneljasonxing@gmail.com> <20260510012310.88570-2-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 05/12, Jason Xing wrote: > On Mon, May 11, 2026 at 11:03 PM Stanislav Fomichev > wrote: > > > > On 05/10, Jason Xing wrote: > > > From: Jason Xing > > > > > > The TX metadata area resides in the UMEM buffer which is memory-mapped > > > and concurrently writable by userspace. In xsk_skb_metadata(), > > > csum_start and csum_offset are read from shared memory for bounds > > > validation, then read again for skb assignment. A malicious userspace > > > application can race to overwrite these values between the two reads, > > > bypassing the bounds check and causing out-of-bounds memory access > > > during checksum computation in the transmit path. > > > > > > Fix this by reading csum_start and csum_offset into local variables > > > once, then using the local copies for both validation and assignment. > > > > > > Note that other metadata fields (flags, launch_time) and the cached > > > csum fields may be mutually inconsistent due to concurrent userspace > > > writes, but this is benign: the only security-critical invariant is > > > that each field's validated value is the same one used, which local > > > caching guarantees. > > > > > > Closes: https://lore.kernel.org/all/20260503200927.73EA1C2BCB4@smtp.kernel.org/ > > > Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support") > > > Signed-off-by: Jason Xing > > > --- > > > net/xdp/xsk.c | 11 +++++++---- > > > 1 file changed, 7 insertions(+), 4 deletions(-) > > > > > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c > > > index 6bcd77068e52..cd039e397018 100644 > > > --- a/net/xdp/xsk.c > > > +++ b/net/xdp/xsk.c > > > @@ -722,6 +722,7 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, > > > u32 hr) > > > { > > > struct xsk_tx_metadata *meta = NULL; > > > + u16 csum_start, csum_offset; > > > > > > if (unlikely(pool->tx_metadata_len == 0)) > > > return -EINVAL; > > > @@ -731,13 +732,15 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, > > > return -EINVAL; > > > > > > if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) { > > > - if (unlikely(meta->request.csum_start + > > > - meta->request.csum_offset + > > > + csum_start = meta->request.csum_start; > > > + csum_offset = meta->request.csum_offset; > > > > Wondering if it's better to READ_ONCE(x) these? > > I still chose not to use it after reading the suggestion from local > AI. The reason is there is no WRITE_ONCE pair to make sure everything > is no data-race. I also checked some existing implementations around > the shared buffer (between userspace and kernel) and didn't manage to > see the usage of XXXX_ONCE(). Does it make any sense to you :) ? Without READ_ONCE your patch relies on the compiler honoring exactly the loads and stores as written. Which I don't think it does (hence that whole WRITE_ONCE/READ_ONCE mess). IOW, it can pretty much generate the same code (and read csum_start twice) even with your patch applied. Happy to argue with your local AI if you give me the output :-D I grepped io_uring/ and I see similar pattern for reading user supplied entries (via READ_ONCE).