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    Subject[PATCH 3.4 031/172] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()
    Date
    From: Roman Gushchin <klamm@yandex-team.ru>

    3.4.108-rc1 review patch. If anyone has any objections, please let me know.

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


    commit 8138a67a5557ffea3a21dfd6f037842d4e748513 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.

    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>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [lizf: Backported to 3.4:
    - adjust context
    - there's no variable reserve]
    Signed-off-by: Zefan Li <lizefan@huawei.com>
    ---
    mm/nommu.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/mm/nommu.c b/mm/nommu.c
    index d3afb47..3f282f9 100644
    --- a/mm/nommu.c
    +++ b/mm/nommu.c
    @@ -1916,7 +1916,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
    */
    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);

    --
    1.9.1


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