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    Subject[PATCH 4.4 053/230] mm: enforce min addr even if capable() in expand_downwards()
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Jann Horn <jannh@google.com>

    commit 0a1d52994d440e21def1c2174932410b4f2a98a1 upstream.

    security_mmap_addr() does a capability check with current_cred(), but
    we can reach this code from contexts like a VFS write handler where
    current_cred() must not be used.

    This can be abused on systems without SMAP to make NULL pointer
    dereferences exploitable again.

    Fixes: 8869477a49c3 ("security: protect from stack expansion into low vm addresses")
    Cc: stable@kernel.org
    Signed-off-by: Jann Horn <jannh@google.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/mmap.c | 7 +++----
    1 file changed, 3 insertions(+), 4 deletions(-)

    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -2294,12 +2294,11 @@ int expand_downwards(struct vm_area_stru
    struct mm_struct *mm = vma->vm_mm;
    struct vm_area_struct *prev;
    unsigned long gap_addr;
    - int error;
    + int error = 0;

    address &= PAGE_MASK;
    - error = security_mmap_addr(address);
    - if (error)
    - return error;
    + if (address < mmap_min_addr)
    + return -EPERM;

    /* Enforce stack_guard_gap */
    gap_addr = address - stack_guard_gap;

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