x86/ptrace: Clean up PTRACE_GETREGS/PTRACE_PUTREGS regset selection

task_user_regset_view() has nonsensical semantics, but those semantics
appear to be relied on by existing users of PTRACE_GETREGSET and
PTRACE_SETREGSET.  (See added comments below for details.)

It shouldn't be used for PTRACE_GETREGS or PTRACE_SETREGS, though. A
native 64-bit ptrace() call and an x32 ptrace() call using GETREGS
or SETREGS wants the 64-bit regset views, and a 32-bit ptrace() call
(native or compat) should use the 32-bit regset.

task_user_regset_view() almost does this except that it will
malfunction if a ptracer is itself ptraced and the outer ptracer
modifies CS on entry to a ptrace() syscall.  Hopefully that has never
happened.  (The compat ptrace() code already hardcoded the 32-bit
regset, so this change has no effect on that path.)

Improve the situation and deobfuscate the code by hardcoding the
64-bit view in the x32 ptrace() and selecting the view based on the
kernel config in the native ptrace().

I tried to figure out the history behind this API. I naïvely assumed
that PTRAGE_GETREGSET and PTRACE_SETREGSET were ancient APIs that
predated compat, but no. They were introduced by

  2225a122

 ("ptrace: Add support for generic PTRACE_GETREGSET/PTRACE_SETREGSET")

in 2010, and they are simply a poor design.  ELF core dumps have the
ELF e_machine field and a bunch of register sets in ELF notes, and the
pair (e_machine, NT_XXX) indicates the format of the regset blob.  But
the new PTRACE_GET/SETREGSET API coopted the NT_XXX numbering without
any way to specify which e_machine was in effect.  This is especially
bad on x86, where a process can freely switch between 32-bit and
64-bit mode, and, in fact, the PTRAGE_SETREGSET call itself can cause
this switch to happen.  Oops.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/9daa791d0c7eaebd59c5bc2b2af1b0e7bebe707d.1612375698.git.luto@kernel.org