• Mel Gorman's avatar
    x86/mm: Check if PUD is large when validating a kernel address · 0ee364eb
    Mel Gorman authored
    A user reported the following oops when a backup process reads
    /proc/kcore:
    
     BUG: unable to handle kernel paging request at ffffbb00ff33b000
     IP: [<ffffffff8103157e>] kern_addr_valid+0xbe/0x110
     [...]
    
     Call Trace:
      [<ffffffff811b8aaa>] read_kcore+0x17a/0x370
      [<ffffffff811ad847>] proc_reg_read+0x77/0xc0
      [<ffffffff81151687>] vfs_read+0xc7/0x130
      [<ffffffff811517f3>] sys_read+0x53/0xa0
      [<ffffffff81449692>] system_call_fastpath+0x16/0x1b
    
    Investigation determined that the bug triggered when reading
    system RAM at the 4G mark. On this system, that was the first
    address using 1G pages for the virt->phys direct mapping so the
    PUD is pointing to a physical address, not a PMD page.
    
    The problem is that the page table walker in kern_addr_valid() is
    not checking pud_large() and treats the physical address as if
    it was a PMD.  If it happens to look like pmd_none then it'll
    silently fail, probably returning zeros instead of real data. If
    the data happens to look like a present PMD though, it will be
    walked resulting in the oops above.
    
    This patch adds the necessary pud_large() check.
    
    Unfortunately the problem was not readily reproducible and now
    they are running the backup program without accessing
    /proc/kcore so the patch has not been validated but I think it
    makes sense.
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Reviewed-by: default avatarRik van Riel <riel@redhat.coM>
    Reviewed-by: default avatarMichal Hocko <mhocko@suse.cz>
    Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Cc: stable@vger.kernel.org
    Cc: linux-mm@kvack.org
    Link: http://lkml.kernel.org/r/20130211145236.GX21389@suse.deSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    0ee364eb
init_64.c 23.9 KB