Revert "x86/fpu: Refine and simplify the magic number check during signal return"

Revert "x86/fpu: Refine and simplify the magic number check during signal return"

This reverts

  dc8aa31a7ac2 ("x86/fpu: Refine and simplify the magic number check during signal return").

The aforementioned commit broke applications that construct signal frames in
userspace (such as CRIU and gVisor) if the frame's xstate size is smaller than
the kernel's fpstate->user_size.

Furthermore, this introduces a critical issue for checkpoint/restore tools
like CRIU. If a process is checkpointed while inside a signal handler, its
stack contains a signal frame formatted according to the source host's xstate
capabilities.

If that process is later restored on a destination host with larger xstate
capabilities (e.g., a newer CPU with more features enabled, resulting in
a larger fpstate->user_size), the kernel will look for FP_XSTATE_MAGIC2 at the
destination host's larger user_size offset instead of the offset encoded in
the frame's fx_sw->xstate_size.

This causes the magic2 check to fail, forcing sigreturn to silently fall back
to "FX-only" mode. Upon return from the signal handler, the process's extended
state is reset to initial values instead of being restored, leading to silent
data corruption.

The aforementioned commit cited

  d877550eaf2d ("x86/fpu: Stop relying on userspace for info to fault in xsave buffer")

as justification to stop relying on userspace for the magic number check.

However, these two changes are fundamentally different. The last one only
changed how much memory the kernel ensures is paged-in before running XRSTOR
to prevent an infinite loop. It did not change the signal frame format or how
the layout is validated.

Reverting this change restores the use of fx_sw->xstate_size for
locating magic2 and restores the necessary sanity checks, ensuring that
the signal frame remains self-describing and portable.

  [ bp: Massage commit message. ]

Fixes: dc8aa31a7ac2 ("x86/fpu: Refine and simplify the magic number check during signal return")
Signed-off-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20260429000623.3356606-1-avagin@google.com