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    Subject[PATCH 3.12 094/175] mm/nommu.c: fix arithmetic overflow in __vm_enough_memory()
    Date
    From: Roman Gushchin <klamm@yandex-team.ru>

    3.12-stable 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>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    mm/nommu.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/mm/nommu.c b/mm/nommu.c
    index 1221d2b66e97..97d19be38233 100644
    --- a/mm/nommu.c
    +++ b/mm/nommu.c
    @@ -1904,7 +1904,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
    */
    int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
    {
    - unsigned long free, allowed, reserve;
    + long free, allowed, reserve;

    vm_acct_memory(pages);

    @@ -1969,7 +1969,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
    */
    if (mm) {
    reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
    - allowed -= min(mm->total_vm / 32, reserve);
    + allowed -= min_t(long, mm->total_vm / 32, reserve);
    }

    if (percpu_counter_read_positive(&vm_committed_as) < allowed)
    --
    2.3.0


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