net: ipa: only reset hashed tables when supported

In the Linux kernel, the following vulnerability has been resolved:

net: ipa: only reset hashed tables when supported

Last year, the code that manages GSI channel transactions switched

from using spinlock-protected linked lists to using indexes into the

ring buffer used for a channel. Recently, Google reported seeing

transaction reference count underflows occasionally during shutdown.

Doug Anderson found a way to reproduce the issue reliably, and

bisected the issue to the commit that eliminated the linked lists

and the lock. The root cause was ultimately determined to be

related to unused transactions being committed as part of the modem

shutdown cleanup activity. Unused transactions are not normally

expected (except in error cases).

The modem uses some ranges of IPA-resident memory, and whenever it

shuts down we zero those ranges. In ipa_filter_reset_table() a

transaction is allocated to zero modem filter table entries. If

hashing is not supported, hashed table memory should not be zeroed.

But currently nothing prevents that, and the result is an unused

transaction. Something similar occurs when we zero routing table

entries for the modem.

By preventing any attempt to clear hashed tables when hashing is not

supported, the reference count underflow is avoided in this case.

Note that there likely remains an issue with properly freeing unused

transactions (if they occur due to errors). This patch addresses

only the underflows that Google originally reported.