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    Subject[PATCH 3.2 037/221] mm/mmap.c: fix arithmetic overflow in __vm_enough_memory()
    3.2.69-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Roman Gushchin <klamm@yandex-team.ru>

    commit 5703b087dc8eaf47bfb399d6cf512d471beff405 upstream.

    I noticed, that "allowed" can easily overflow by falling below 0,
    because (total_vm / 32) can be larger than "allowed". The problem
    occurs in OVERCOMMIT_NONE mode.

    In this case, a huge allocation can success and overcommit the system
    (despite OVERCOMMIT_NONE mode). All subsequent allocations will fall
    (system-wide), so system become unusable.

    The problem was masked out by commit c9b1d0981fcc
    ("mm: limit growth of 3% hardcoded other user reserve"),
    but it's easy to reproduce it on older kernels:
    1) set overcommit_memory sysctl to 2
    2) mmap() large file multiple times (with VM_SHARED flag)
    3) try to malloc() large amount of memory

    It also can be reproduced on newer kernels, but miss-configured
    sysctl_user_reserve_kbytes is required.

    Fix this issue by switching to signed arithmetic here.

    [akpm@linux-foundation.org: use min_t]
    Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
    Cc: Andrew Shewmaker <agshew@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Reviewed-by: Michal Hocko <mhocko@suse.cz>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.2: there is no 'reserved' variable]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -111,7 +111,7 @@ struct percpu_counter vm_committed_as __
    */
    int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
    {
    - unsigned long free, allowed;
    + long free, allowed;

    vm_acct_memory(pages);



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