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    SubjectRe: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct
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    On 4/10/18 12:17 PM, Cyrill Gorcunov wrote:
    > On Tue, Apr 10, 2018 at 11:28:13AM -0700, Yang Shi wrote:
    >>> At the first glance, it looks feasible to me. Will look into deeper
    >>> later.
    >> A further look told me this might be *not* feasible.
    >>
    >> It looks the new lock will not break check_data_rlimit since in my patch
    >> both start_brk and brk is protected by mmap_sem. The code flow might look
    >> like below:
    >>
    >> CPU A                             CPU B
    >> --------                       --------
    >> prctl                               sys_brk
    >>                                       down_write
    >> check_data_rlimit           check_data_rlimit (need mm->start_brk)
    >>                                       set brk
    >> down_write                    up_write
    >> set start_brk
    >> set brk
    >> up_write
    >>
    >> If CPU A gets the mmap_sem first, it will set start_brk and brk, then CPU B
    >> will check with the new start_brk. And, prctl doesn't care if sys_brk is run
    >> before it since it gets the new start_brk and brk from parameter.
    >>
    >> If we protect start_brk and brk with the new lock, sys_brk might get old
    >> start_brk, then sys_brk might break rlimit check silently, is that right?
    >>
    >> So, it looks using new lock in prctl and keeping mmap_sem in brk path has
    >> race condition.
    > I fear so. The check_data_rlimit implies that all elements involved into
    > validation (brk, start_brk, start_data, end_data) are not changed unpredicably
    > until written back into mm. In turn if we guard start_brk,brk only (as
    > it is done in the patch) the check_data_rlimit may pass on wrong data
    > I think. And as you mentioned the race above exact the example of such
    > situation. I think for prctl case we can simply left use of mmap_sem
    > as it were before the patch, after all this syscall is really in cold
    > path all the time.

    The race condition is just valid when protecting start_brk, brk,
    start_data and end_data with the new lock, but keep using mmap_sem in
    brk path.

    So, we should just need make a little tweak to have mmap_sem protect
    start_brk, brk, start_data and end_data, then have the new lock protect
    others so that we still can remove mmap_sem in proc as the patch is
    aimed to do.

    Yang

    >
    > Cyrill

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