• Davidlohr Bueso's avatar
    ipc/shm: Fix shmat mmap nil-page protection · 95e91b83
    Davidlohr Bueso authored
    The issue is described here, with a nice testcase:
    
        https://bugzilla.kernel.org/show_bug.cgi?id=192931
    
    The problem is that shmat() calls do_mmap_pgoff() with MAP_FIXED, and
    the address rounded down to 0.  For the regular mmap case, the
    protection mentioned above is that the kernel gets to generate the
    address -- arch_get_unmapped_area() will always check for MAP_FIXED and
    return that address.  So by the time we do security_mmap_addr(0) things
    get funky for shmat().
    
    The testcase itself shows that while a regular user crashes, root will
    not have a problem attaching a nil-page.  There are two possible fixes
    to this.  The first, and which this patch does, is to simply allow root
    to crash as well -- this is also regular mmap behavior, ie when hacking
    up the testcase and adding mmap(...  |MAP_FIXED).  While this approach
    is the safer option, the second alternative is to ignore SHM_RND if the
    rounded address is 0, thus only having MAP_SHARED flags.  This makes the
    behavior of shmat() identical to the mmap() case.  The downside of this
    is obviously user visible, but does make sense in that it maintains
    semantics after the round-down wrt 0 address and mmap.
    
    Passes shm related ltp tests.
    
    Link: http://lkml.kernel.org/r/1486050195-18629-1-git-send-email-dave@stgolabs.netSigned-off-by: default avatarDavidlohr Bueso <dbueso@suse.de>
    Reported-by: default avatarGareth Evans <gareth.evans@contextis.co.uk>
    Cc: Manfred Spraul <manfred@colorfullife.com>
    Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    95e91b83
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