drm/radeon: r100/r200 ums: block ability for userspace app to trash 0 page and beyond

radeon's have a special ability to passthrough writes in their internal
memory space directly to PCI, this ability means that if some of the internal
surfaces like the depth buffer point at 0x0, any writes to these will
go directly to RAM at 0x0 via PCI busmastering.

Now mesa used to always emit clears after emitting state, since the
radeon mesa driver was refactored a year or more ago, it was found it
could generate a clear request without ever sending any setup state to the
card. So the clear would attempt to clear the depth buffer at 0x0, which
would overwrite main memory at this point. fs corruption ensues.

Also once one app did this correctly, it would never get set back to 0
making this messy to reproduce.

The kernel should block this from happening as mesa runs without privs,
though it does require the user be connected to the current running X session.

This patch implements a check to make sure the depth offset has been set
before a depth clear occurs and if it finds one it prints a warning and
ignores the depth clear request. There is also a mesa fix to avoid sending
the badness going into mesa.

This only affects r100/r200 GPUs in user modesetting mode.

Signed-off-by: Dave Airlie <airlied@redhat.com>

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