Linus Torvalds 698b368275
fbcon: add lifetime refcount to opened frame buffers ...
This just adds the refcount and the new registration lock logic. It
does not (for example) actually change the read/write/ioctl routines to
actually use the frame buffer that was opened: those function still end
up alway susing whatever the current frame buffer is at the time of the
call.
Without this, if something holds the frame buffer open over a
framebuffer switch, the close() operation after the switch will access a
fb_info that has been free'd by the unregistering of the old frame
buffer.
(The read/write/ioctl operations will normally not cause problems,
because they will - illogically - pick up the new fbcon instead. But a
switch that happens just as one of those is going on might see problems
too, the window is just much smaller: one individual op rather than the
whole open-close sequence.)
This use-after-free is apparently fairly easily triggered by the Ubuntu
11.04 boot sequence.
Acked-by: Tim Gardner <tim.gardner@canonical.com >
Tested-by: Daniel J Blueman <daniel.blueman@gmail.com >
Tested-by: Anca Emanuel <anca.emanuel@gmail.com >
Cc: Bruno Prémont <bonbons@linux-vserver.org >
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk >
Cc: Paul Mundt <lethal@linux-sh.org >
Cc: Dave Airlie <airlied@redhat.com >
Cc: Andy Whitcroft <andy.whitcroft@canonical.com >
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org >
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