all arches, signal: move restart_block to struct task_struct

If an attacker can cause a controlled kernel stack overflow, overwriting
the restart block is a very juicy exploit target.  This is because the
restart_block is held in the same memory allocation as the kernel stack.

Moving the restart block to struct task_struct prevents this exploit by
making the restart_block harder to locate.

Note that there are other fields in thread_info that are also easy
targets, at least on some architectures.

It's also a decent simplification, since the restart code is more or less
identical on all architectures.

[[email protected]: metag: align thread_info::supervisor_stack]
Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Al Viro <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: David Miller <[email protected]>
Acked-by: Richard Weinberger <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Russell King <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Haavard Skinnemoen <[email protected]>
Cc: Hans-Christian Egtvedt <[email protected]>
Cc: Steven Miao <[email protected]>
Cc: Mark Salter <[email protected]>
Cc: Aurelien Jacquiot <[email protected]>
Cc: Mikael Starvik <[email protected]>
Cc: Jesper Nilsson <[email protected]>
Cc: David Howells <[email protected]>
Cc: Richard Kuo <[email protected]>
Cc: "Luck, Tony" <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Jonas Bonn <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Acked-by: Michael Ellerman <[email protected]> (powerpc)
Tested-by: Michael Ellerman <[email protected]> (powerpc)
Cc: Martin Schwidefsky <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Chen Liqin <[email protected]>
Cc: Lennox Wu <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Guan Xuetao <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Guenter Roeck <[email protected]>
Signed-off-by: James Hogan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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