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    Subject[PATCH] install_special_mapping skips security_file_mmap check.
    The install_special_mapping routine (used, for example, to setup the vdso)
    skips the security check before insert_vm_struct, allowing a local attacker to
    bypass the mmap_min_addr security restriction by limiting the available pages
    for special mappings. bprm_mm_init() also skips the check, although I don't
    think this can be used to bypass any restrictions, I don't see any reason not
    to have the security check.

    $ uname -m
    x86_64
    $ cat /proc/sys/vm/mmap_min_addr
    65536
    $ cat install_special_mapping.s
    section .bss
    resb BSS_SIZE
    section .text
    global _start
    _start:
    mov eax, __NR_pause
    int 0x80
    $ nasm -D__NR_pause=29 -DBSS_SIZE=0xfffed000 -f elf -o install_special_mapping.o install_special_mapping.s
    $ ld -m elf_i386 -Ttext=0x10000 -Tbss=0x11000 -o install_special_mapping install_special_mapping.o
    $ ./install_special_mapping &
    [1] 14303
    $ cat /proc/14303/maps
    0000f000-00010000 r-xp 00000000 00:00 0 [vdso]
    00010000-00011000 r-xp 00001000 00:19 2453665 /home/taviso/install_special_mapping
    00011000-ffffe000 rwxp 00000000 00:00 0 [stack]

    It's worth noting that Red Hat are shipping with mmap_min_addr set to 4096.

    Signed-off-by: Tavis Ormandy <taviso@google.com>
    Acked-by: Kees Cook <kees@ubuntu.com>
    Acked-by: Robert Swiecki <swiecki@google.com>
    ---

    fs/exec.c | 7 +++++++
    mm/mmap.c | 5 +++++
    2 files changed, 12 insertions(+), 0 deletions(-)

    diff --git a/fs/exec.c b/fs/exec.c
    index d68c378..7e8c4b6 100644
    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -275,7 +275,14 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
    vma->vm_flags = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
    vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
    INIT_LIST_HEAD(&vma->anon_vma_chain);
    +
    + err = security_file_mmap(NULL, 0, 0, 0, vma->vm_start, 1);
    +
    + if (err)
    + goto err;
    +
    err = insert_vm_struct(mm, vma);
    +
    if (err)
    goto err;

    diff --git a/mm/mmap.c b/mm/mmap.c
    index b179abb..1de3f4b 100644
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -2479,6 +2479,11 @@ int install_special_mapping(struct mm_struct *mm,
    vma->vm_ops = &special_mapping_vmops;
    vma->vm_private_data = pages;

    + if (security_file_mmap(NULL, 0, 0, 0, vma->vm_start, 1)) {
    + kmem_cache_free(vm_area_cachep, vma);
    + return -EPERM;
    + }
    +
    if (unlikely(insert_vm_struct(mm, vma))) {
    kmem_cache_free(vm_area_cachep, vma);
    return -ENOMEM;

    --
    -------------------------------------
    taviso@cmpxchg8b.com | pgp encrypted mail preferred
    -------------------------------------------------------

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