• Sebastian Andrzej Siewior's avatar
    x86/fpu: Disable bottom halves while loading FPU registers · 68239654
    Sebastian Andrzej Siewior authored
    The sequence
    
      fpu->initialized = 1;		/* step A */
      preempt_disable();		/* step B */
      fpu__restore(fpu);
      preempt_enable();
    
    in __fpu__restore_sig() is racy in regard to a context switch.
    
    For 32bit frames, __fpu__restore_sig() prepares the FPU state within
    fpu->state. To ensure that a context switch (switch_fpu_prepare() in
    particular) does not modify fpu->state it uses fpu__drop() which sets
    fpu->initialized to 0.
    
    After fpu->initialized is cleared, the CPU's FPU state is not saved
    to fpu->state during a context switch. The new state is loaded via
    fpu__restore(). It gets loaded into fpu->state from userland and
    ensured it is sane. fpu->initialized is then set to 1 in order to avoid
    fpu__initialize() doing anything (overwrite the new state) which is part
    of fpu__restore().
    
    A context switch between step A and B above would save CPU's current FPU
    registers to fpu->state and overwrite the newly prepared state. This
    looks like a tiny race window but the Kernel Test Robot reported this
    back in 2016 while we had lazy FPU support. Borislav Petkov made the
    link between that report and another patch that has been posted. Since
    the removal of the lazy FPU support, this race goes unnoticed because
    the warning has been removed.
    
    Disable bottom halves around the restore sequence to avoid the race. BH
    need to be disabled because BH is allowed to run (even with preemption
    disabled) and might invoke kernel_fpu_begin() by doing IPsec.
    
     [ bp: massage commit message a bit. ]
    Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
    Acked-by: Ingo Molnar's avatarIngo Molnar <mingo@kernel.org>
    Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
    Cc: kvm ML <kvm@vger.kernel.org>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Radim Krčmář <rkrcmar@redhat.com>
    Cc: Rik van Riel <riel@surriel.com>
    Cc: stable@vger.kernel.org
    Cc: x86-ml <x86@kernel.org>
    Link: http://lkml.kernel.org/r/20181120102635.ddv3fvavxajjlfqk@linutronix.de
    Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic
    68239654
signal.c 11.8 KB