[PATCH 0/2] x86/fpu: Allow restoring signal frames with larger xstate_size

[Andrei Vagin <avagin@google.com>]

When a process is checkpointed on one machine and restored on another
(e.g., via CRIU during container/process migration), any signal frame on
its stack carries the xstate layout of the source CPU.

Commit fd14edd82077 ("x86/fpu: Pre-fault only required size of xstate
buffer") enabled restoring signal frames created on CPUs with a smaller
xstate_size than the destination host's default. However,
check_xstate_in_sigframe() still enforces that a signal frame's
xstate_size must not exceed the current task's fpstate->user_size.

This restriction prevents migrating a process from a CPU with more
enabled xstate features to a CPU with fewer features, even when the
process has not actively used any of the unsupported features. Because
the actual set of active features is recorded in the XSAVE header
(XSTATE_BV) and validated during restoration, a larger xstate_size is
safe to restore as long as all active features are present in the
intersection of frame and task features and the buffer is large enough
for them.

Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "Chang S. Bae" <chang.seok.bae@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@kernel.org>

Andrei Vagin (2):
  x86/fpu: Allow restoring signal frames with larger xstate_size
  selftests/x86: Check restoring FPU state with larger xstate_size

 Documentation/arch/x86/xstate.rst                  |  10 +-
 arch/x86/kernel/fpu/signal.c                       |  31 +++++-
 .../selftests/x86/sigframe_fpu_portability.c       | 118 +++++++++++++++++++--
 3 files changed, 143 insertions(+), 16 deletions(-)

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2.55.0.1082.g2b9226bbc0-goog