Re: [PATCH] wifi: iwlwifi: prevent NAPI processing after firmware error

[Cole Leavitt <cole@unwrap.rs>]

Ben,

Good catch on both fronts.

On the build_tfd dangling pointer -- you're right. The failure path at
line 775 leaves entries[idx].skb/cmd pointing at caller-owned objects
(set at lines 763-764). The caller gets -1 and presumably frees the
skb, so entries[idx].skb becomes a dangling pointer. While write_ptr
not advancing means current unmap paths won't iterate to that index,
it's a latent UAF waiting for a flush path change or future code to
touch it. Two NULL stores inside a held spinlock cost nothing. I think
this should go upstream as its own patch.

On the TOCTOU question -- this is the part I spent the most time on.
The window you're asking about is: firmware starts producing corrupt
completion data *before* STATUS_FW_ERROR gets set. Our NAPI/TX handler
checks can't help there because the flag isn't set yet.

The primary guard in that window is iwl_txq_used() in
iwl_pcie_reclaim(). It validates that the firmware's SSN falls within
[read_ptr, write_ptr). This catches wild values -- out-of-range SSNs,
wraparound corruption, etc.

What it can't catch is an in-range corrupt SSN -- e.g., firmware says
reclaim up to index 15 when legitimate is 8, but write_ptr is 20.
That passes bounds checking and the reclaim loop frees skbs for
entries still in-flight (active DMA). The NULL skb WARN_ONCE in the
loop catches double-reclaim but not first-time over-reclaim.

The complete fix for this would be a per-entry generation counter --
tag each entry on submit, validate on reclaim. But that adds per-entry
overhead on the TX hot path to protect against a condition (firmware
producing corrupt completions) that is already terminal. I think the
right trade-off is:

  1. Your build_tfd NULL fix (eliminates one dangling pointer class)
  2. STATUS_FW_ERROR checks in NAPI poll + TX handlers (this series --
     shrinks the detection window to near-zero)
  3. The existing iwl_txq_used() bounds check (catches most corrupt
     SSNs)

Together these make the damage window small enough that a per-entry
generation scheme isn't justified -- by the time firmware is sending
corrupt SSNs, we're in dump-and-reset territory anyway.

That said, if you're seeing corruption patterns in your customer
testing where a valid-looking-but-wrong SSN gets through before
FW_ERROR fires, I'd be very interested in the traces. That would
change the cost/benefit on the generation counter approach.

Thanks,
Cole