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    Subject[PATCH 3.4 27/43] powerpc: Fix bad stack check in exception entry
    Date
    3.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Michael Neuling <mikey@neuling.org>

    commit 90ff5d688e61f49f23545ffab6228bd7e87e6dc7 upstream.

    In EXCEPTION_PROLOG_COMMON() we check to see if the stack pointer (r1)
    is valid when coming from the kernel. If it's not valid, we die but
    with a nice oops message.

    Currently we allocate a stack frame (subtract INT_FRAME_SIZE) before we
    check to see if the stack pointer is negative. Unfortunately, this
    won't detect a bad stack where r1 is less than INT_FRAME_SIZE.

    This patch fixes the check to compare the modified r1 with
    -INT_FRAME_SIZE. With this, bad kernel stack pointers (including NULL
    pointers) are correctly detected again.

    Kudos to Paulus for finding this.

    Signed-off-by: Michael Neuling <mikey@neuling.org>
    Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/include/asm/exception-64s.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/arch/powerpc/include/asm/exception-64s.h
    +++ b/arch/powerpc/include/asm/exception-64s.h
    @@ -163,7 +163,7 @@ do_kvm_##n: \
    subi r1,r1,INT_FRAME_SIZE; /* alloc frame on kernel stack */ \
    beq- 1f; \
    ld r1,PACAKSAVE(r13); /* kernel stack to use */ \
    -1: cmpdi cr1,r1,0; /* check if r1 is in userspace */ \
    +1: cmpdi cr1,r1,-INT_FRAME_SIZE; /* check if r1 is in userspace */ \
    blt+ cr1,3f; /* abort if it is */ \
    li r1,(n); /* will be reloaded later */ \
    sth r1,PACA_TRAP_SAVE(r13); \



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