• Gustavo Romero's avatar
    powerpc/tm: Fix FP/VMX unavailable exceptions inside a transaction · 8205d5d9
    Gustavo Romero authored
    When we take an FP unavailable exception in a transaction we have to
    account for the hardware FP TM checkpointed registers being
    incorrect. In this case for this process we know the current and
    checkpointed FP registers must be the same (since FP wasn't used
    inside the transaction) hence in the thread_struct we copy the current
    FP registers to the checkpointed ones.
    
    This copy is done in tm_reclaim_thread(). We use thread->ckpt_regs.msr
    to determine if FP was on when in userspace. thread->ckpt_regs.msr
    represents the state of the MSR when exiting userspace. This is setup
    by check_if_tm_restore_required().
    
    Unfortunatley there is an optimisation in giveup_all() which returns
    early if tsk->thread.regs->msr (via local variable `usermsr`) has
    FP=VEC=VSX=SPE=0. This optimisation means that
    check_if_tm_restore_required() is not called and hence
    thread->ckpt_regs.msr is not updated and will contain an old value.
    
    This can happen if due to load_fp=255 we start a userspace process
    with MSR FP=1 and then we are context switched out. In this case
    thread->ckpt_regs.msr will contain FP=1. If that same process is then
    context switched in and load_fp overflows, MSR will have FP=0. If that
    process now enters a transaction and does an FP instruction, the FP
    unavailable will not update thread->ckpt_regs.msr (the bug) and MSR
    FP=1 will be retained in thread->ckpt_regs.msr.  tm_reclaim_thread()
    will then not perform the required memcpy and the checkpointed FP regs
    in the thread struct will contain the wrong values.
    
    The code path for this happening is:
    
           Userspace:                      Kernel
                       Start userspace
                        with MSR FP/VEC/VSX/SPE=0 TM=1
                          < -----
           ...
           tbegin
           bne
           fp instruction
                       FP unavailable
                           ---- >
                                            fp_unavailable_tm()
    					  tm_reclaim_current()
    					    tm_reclaim_thread()
    					      giveup_all()
    					        return early since FP/VMX/VSX=0
    						/* ckpt MSR not updated (Incorrect) */
    					      tm_reclaim()
    					        /* thread_struct ckpt FP regs contain junk (OK) */
                                                  /* Sees ckpt MSR FP=1 (Incorrect) */
    					      no memcpy() performed
    					        /* thread_struct ckpt FP regs not fixed (Incorrect) */
    					  tm_recheckpoint()
    					     /* Put junk in hardware checkpoint FP regs */
                                             ....
                          < -----
                       Return to userspace
                         with MSR TM=1 FP=1
                         with junk in the FP TM checkpoint
           TM rollback
           reads FP junk
    
    This is a data integrity problem for the current process as the FP
    registers are corrupted. It's also a security problem as the FP
    registers from one process may be leaked to another.
    
    This patch moves up check_if_tm_restore_required() in giveup_all() to
    ensure thread->ckpt_regs.msr is updated correctly.
    
    A simple testcase to replicate this will be posted to
    tools/testing/selftests/powerpc/tm/tm-poison.c
    
    Similarly for VMX.
    
    This fixes CVE-2019-15030.
    
    Fixes: f48e91e8 ("powerpc/tm: Fix FP and VMX register corruption")
    Cc: stable@vger.kernel.org # 4.12+
    Signed-off-by: default avatarGustavo Romero <gromero@linux.vnet.ibm.com>
    Signed-off-by: default avatarMichael Neuling <mikey@neuling.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20190904045529.23002-1-gromero@linux.vnet.ibm.com
    8205d5d9
process.c 55.3 KB