x86/coredump: Always use user_regs_struct for compat_elf_gregset_t

x86/coredump: Always use user_regs_struct for compat_elf_gregset_t

Commit:

  90954e7b9407 ("x86/coredump: Use pr_reg size, rather that TIF_IA32 flag")

changed the coredumping code to construct the elf coredump file according
to register set size - and that's good: if binary crashes with 32-bit code
selector, generate 32-bit ELF core, otherwise - 64-bit core.

That was made for restoring 32-bit applications on x86_64: we want
32-bit application after restore to generate 32-bit ELF dump on crash.

All was quite good and recently I started reworking 32-bit applications
dumping part of CRIU: now it has two parasites (32 and 64) for seizing
compat/native tasks, after rework it'll have one parasite, working in
64-bit mode, to which 32-bit prologue long-jumps during infection.

And while it has worked for my work machine, in VM with
!CONFIG_X86_X32_ABI during reworking I faced that segfault in 32-bit
binary, that has long-jumped to 64-bit mode results in dereference
of garbage:

 32-victim[19266]: segfault at f775ef65 ip 00000000f775ef65 sp 00000000f776aa50 error 14
 BUG: unable to handle kernel paging request at ffffffffffffffff
 IP: [<ffffffff81332ce0>] strlen+0x0/0x20
 [...]
 Call Trace:
  [] elf_core_dump+0x11a9/0x1480
  [] do_coredump+0xa6b/0xe60
  [] get_signal+0x1a8/0x5c0
  [] do_signal+0x23/0x660
  [] exit_to_usermode_loop+0x34/0x65
  [] prepare_exit_to_usermode+0x2f/0x40
  [] retint_user+0x8/0x10

That's because we have 64-bit registers set (with according total size)
and we're writing it to elf_thread_core_info which has smaller size
on !CONFIG_X86_X32_ABI. That lead to overwriting ELF notes part.

Tested on 32-, 64-bit ELF crashes and on 32-bit binaries that have
jumped with 64-bit code selector - all is readable with gdb.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Fixes: 90954e7b9407 ("x86/coredump: Use pr_reg size, rather that TIF_IA32 flag")
Signed-off-by: Ingo Molnar <mingo@kernel.org>