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mpr, mps: Fix a stack buffer overflow in the user passthru ioctl

Authored by markj on Jan 4 2021, 11:37 PM.



Previously we copied in the request into a stack-allocated structure
that could be smaller than the request size. Furthermore, we checked
the request size only after doing the copyin.

Fix this by allocating a buffer to hold the request, then copying the
buffer's contents into a command descriptor. This is a bit heavy-handed
but I expect the overhead will not be noticeable. The approach of
coping the header in first is susceptible to TOCTOU problems.

Test Plan

I don't have any hardware driven by mpr or mps. Would anyone be willing
to test?

Diff Detail

R10 FreeBSD src repository
Automatic diff as part of commit; lint not applicable.
Automatic diff as part of commit; unit tests not applicable.