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    powerpc/mm: Add support for runtime configuration of ASLR limits · 9fea59bd
    Michael Ellerman authored
    
    
    Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two
    sysctls that allow a user to configure the number of bits of randomness used for
    ASLR.
    
    Because of the way the Kconfig for ARCH_MMAP_RND_BITS is defined, we have to
    construct at least the MIN value in Kconfig, vs in a header which would be more
    natural. Given that we just go ahead and do it all in Kconfig.
    
    At least according to the code (the documentation makes no mention of it), the
    value is defined as the number of bits of randomisation *of the page*, not the
    address. This makes some sense, with larger page sizes more of the low bits are
    forced to zero, which would reduce the randomisation if we didn't take the
    PAGE_SIZE into account. However it does mean the min/max values have to change
    depending on the PAGE_SIZE in order to actually limit the amount of address
    space consumed by the randomisation.
    
    The result of that is that we have to define the default values based on both
    32-bit vs 64-bit, but also the configured PAGE_SIZE. Furthermore now that we
    have 128TB address space support on Book3S, we also have to take that into
    account.
    
    Finally we can wire up the value in arch_mmap_rnd().
    
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: default avatarBhupesh Sharma <bhsharma@redhat.com>
    Tested-by: default avatarBhupesh Sharma <bhsharma@redhat.com>
    Reviewed-by: default avatarKees Cook <keescook@chromium.org>
    Reviewed-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    9fea59bd