• Suresh Siddha's avatar
    crypto: padlock - fix VIA PadLock instruction usage with irq_ts_save/restore() · e4914012
    Suresh Siddha authored
    Wolfgang Walter reported this oops on his via C3 using padlock for
    AES-encryption:
    
    ##################################################################
    
    BUG: unable to handle kernel NULL pointer dereference at 000001f0
    IP: [<c01028c5>] __switch_to+0x30/0x117
    *pde = 00000000
    Oops: 0002 [#1] PREEMPT
    Modules linked in:
    
    Pid: 2071, comm: sleep Not tainted (2.6.26 #11)
    EIP: 0060:[<c01028c5>] EFLAGS: 00010002 CPU: 0
    EIP is at __switch_to+0x30/0x117
    EAX: 00000000 EBX: c0493300 ECX: dc48dd00 EDX: c0493300
    ESI: dc48dd00 EDI: c0493530 EBP: c04cff8c ESP: c04cff7c
     DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 0068
    Process sleep (pid: 2071, ti=c04ce000 task=dc48dd00 task.ti=d2fe6000)
    Stack: dc48df30 c0493300 00000000 00000000 d2fe7f44 c03b5b43 c04cffc8 00000046
           c0131856 0000005a dc472d3c c0493300 c0493470 d983ae00 00002696 00000000
           c0239f54 00000000 c04c4000 c04cffd8 c01025fe c04f3740 00049800 c04cffe0
    Call Trace:
     [<c03b5b43>] ? schedule+0x285/0x2ff
     [<c0131856>] ? pm_qos_requirement+0x3c/0x53
     [<c0239f54>] ? acpi_processor_idle+0x0/0x434
     [<c01025fe>] ? cpu_idle+0x73/0x7f
     [<c03a4dcd>] ? rest_init+0x61/0x63
     =======================
    
    Wolfgang also found out that adding kernel_fpu_begin() and kernel_fpu_end()
    around the padlock instructions fix the oops.
    
    Suresh wrote:
    
    These padlock instructions though don't use/touch SSE registers, but it behaves
    similar to other SSE instructions. For example, it might cause DNA faults
    when cr0.ts is set. While this is a spurious DNA trap, it might cause
    oops with the recent fpu code changes.
    
    This is the code sequence  that is probably causing this problem:
    
    a) new app is getting exec'd and it is somewhere in between
       start_thread() and flush_old_exec() in the load_xyz_binary()
    
    b) At pont "a", task's fpu state (like TS_USEDFPU, used_math() etc) is
       cleared.
    
    c) Now we get an interrupt/softirq which starts using these encrypt/decrypt
       routines in the network stack. This generates a math fault (as
       cr0.ts is '1') which sets TS_USEDFPU and restores the math that is
       in the task's xstate.
    
    d) Return to exec code path, which does start_thread() which does
       free_thread_xstate() and sets xstate pointer to NULL while
       the TS_USEDFPU is still set.
    
    e) At the next context switch from the new exec'd task to another task,
       we have a scenarios where TS_USEDFPU is set but xstate pointer is null.
       This can cause an oops during unlazy_fpu() in __switch_to()
    
    Now:
    
    1) This should happen with or with out pre-emption. Viro also encountered
       similar problem with out CONFIG_PREEMPT.
    
    2) kernel_fpu_begin() and kernel_fpu_end() will fix this problem, because
       kernel_fpu_begin() will manually do a clts() and won't run in to the
       situation of setting TS_USEDFPU in step "c" above.
    
    3) This was working before the fpu changes, because its a spurious
       math fault  which doesn't corrupt any fpu/sse registers and the task's
       math state was always in an allocated state.
    
    With out the recent lazy fpu allocation changes, while we don't see oops,
    there is a possible race still present in older kernels(for example,
    while kernel is using kernel_fpu_begin() in some optimized clear/copy
    page and an interrupt/softirq happens which uses these padlock
    instructions generating DNA fault).
    
    This is the failing scenario that existed even before the lazy fpu allocation
    changes:
    
    0. CPU's TS flag is set
    
    1. kernel using FPU in some optimized copy  routine and while doing
    kernel_fpu_begin() takes an interrupt just before doing clts()
    
    2. Takes an interrupt and ipsec uses padlock instruction. And we
    take a DNA fault as TS flag is still set.
    
    3. We handle the DNA fault and set TS_USEDFPU and clear cr0.ts
    
    4. We complete the padlock routine
    
    5. Go back to step-1, which resumes clts() in kernel_fpu_begin(), finishes
    the optimized copy routine and does kernel_fpu_end(). At this point,
    we have cr0.ts again set to '1' but the task's TS_USEFPU is stilll
    set and not cleared.
    
    6. Now kernel resumes its user operation. And at the next context
    switch, kernel sees it has do a FP save as TS_USEDFPU is still set
    and then will do a unlazy_fpu() in __switch_to(). unlazy_fpu()
    will take a DNA fault, as cr0.ts is '1' and now, because we are
    in __switch_to(), math_state_restore() will get confused and will
    restore the next task's FP state and will save it in prev tasks's FP state.
    Remember, in __switch_to() we are already on the stack of the next task
    but take a DNA fault for the prev task.
    
    This causes the fpu leakage.
    
    Fix the padlock instruction usage by calling them inside the
    context of new routines irq_ts_save/restore(), which clear/restore cr0.ts
    manually in the interrupt context. This will not generate spurious DNA
    in the  context of the interrupt which will fix the oops encountered and
    the possible FPU leakage issue.
    Reported-and-bisected-by: default avatarWolfgang Walter <wolfgang.walter@stwm.de>
    Signed-off-by: default avatarSuresh Siddha <suresh.b.siddha@intel.com>
    Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    e4914012
via-rng.c 5.34 KB