net/mlx5: Fix error path in multi-packet WQE transmit

[ Upstream commit 2bcae12c795f32ddfbf8c80d1b5f1d3286341c32 ]

Remove the erroneous unmap in case no DMA mapping was established

The multi-packet WQE transmit code attempts to obtain a DMA mapping for
the skb. This could fail, e.g. under memory pressure, when the IOMMU
driver just can't allocate more memory for page tables. While the code
tries to handle this in the path below the err_unmap label it erroneously
unmaps one entry from the sq's FIFO list of active mappings. Since the
current map attempt failed this unmap is removing some random DMA mapping
that might still be required. If the PCI function now presents that IOVA,
the IOMMU may assumes a rogue DMA access and e.g. on s390 puts the PCI
function in error state.

The erroneous behavior was seen in a stress-test environment that created
memory pressure.

BUG=b/377000541
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2024-50001 in the Linux kernel.

cos-patch: security-moderate
Fixes: 5af75c747e2a ("net/mlx5e: Enhanced TX MPWQE for SKBs")
Change-Id: I25de3762eee6b976dac88c0c97518e72b7243713
Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Acked-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kernel CVE Triage Automation <cloud-image-kernel-cve-triage-automation@prod.google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/85132
Reviewed-by: Arnav Kansal <rnv@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Kevin Berry <kpberry@google.com>