cxl/features: Reject feature offset that overflows 16-bit field

cxl_get_feature() and cxl_set_feature() build each mailbox command's
offset from the starting offset plus the amount of data already
transferred, then store it in a 16-bit field. A user-controlled fwctl
offset and transfer size can exceed the feature extent, allowing a later
offset to be truncated by cpu_to_le16() and target the wrong feature
data.

Reject requests whose transfer size exceeds the remaining 16-bit feature
range. Express the check as "size > U16_MAX - offset" so the validation
itself cannot wrap on 32-bit systems.

Change cxl_get_feature() to return ssize_t so invalid input and mailbox
failures are reported as negative errno rather than being conflated with
a zero-byte result. Update the EDAC callers to handle negative results.
Keep fwctl behavior unchanged by translating helper failures to the same
header-only RPC response carrying the CXL mailbox return code.

Fixes: 5e5ac21f629d ("cxl/mbox: Add GET_FEATURE mailbox command")
Fixes: 14d502cc2718 ("cxl/mbox: Add SET_FEATURE mailbox command")
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Richard Cheng <icheng@nvidia.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20260902053839.25595-2-icheng@nvidia.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

0 files changed, 0 insertions, 0 deletions