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    SubjectRe: [PATCH net] vhost: Use kzalloc() to allocate vhost_msg_node
    On Fri, Apr 27, 2018 at 6:25 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
    >>> >> The struct vhost_msg within struct vhost_msg_node is copied to userspace,
    >>> >> so it should be allocated with kzalloc() to ensure all structure padding
    >>> >> is zeroed.
    >>> >>
    >>> >> Signed-off-by: Kevin Easton <kevin@guarana.org>
    >>> >> Reported-by: syzbot+87cfa083e727a224754b@syzkaller.appspotmail.com
    >>> >
    >>> > Does it help if a patch naming the padding is applied,
    >>> > and then we init just the relevant field?
    >>> > Just curious.
    >>>
    >>> Yes, it would help.
    >>
    >> I think it's slightly better that way then. node has a lot of internal
    >> stuff we don't care to init. Would you mind taking my patch and building
    >> on top of that then?
    >
    >
    > But it's asking for more information leaks in future. This looks like
    > work for compiler.


    Modern compilers are perfectly capable of doing this:

    #include <memory.h>
    #include <unistd.h>
    int main()
    {
    int x[10];
    memset(&x, 0, sizeof(x));
    x[0] = 0;
    x[2] = 2;
    x[3] = 3;
    x[4] = 4;
    x[5] = 5;
    x[6] = 6;
    x[7] = 7;
    x[8] = 8;
    x[9] = 9;
    write(0, x, sizeof(x));
    return 0;
    }

    gcc 7.2 -O3

    0000000000000540 <main>:
    540: sub $0x38,%rsp
    544: mov $0x28,%edx
    549: xor %edi,%edi
    54b: movdqa 0x1cd(%rip),%xmm0 # 720 <_IO_stdin_used+0x10>
    553: mov %rsp,%rsi
    556: movq $0x0,(%rsp)
    55e: movups %xmm0,0x8(%rsp)
    563: movdqa 0x1c5(%rip),%xmm0 # 730 <_IO_stdin_used+0x20>
    56b: movups %xmm0,0x18(%rsp)
    570: callq 520 <write@plt>
    575: xor %eax,%eax
    577: add $0x38,%rsp
    57b: retq
    57c: nopl 0x0(%rax)


    But they will not put a security hole next time fields are shuffled.




    >>> >> ---
    >>> >> drivers/vhost/vhost.c | 2 +-
    >>> >> 1 file changed, 1 insertion(+), 1 deletion(-)
    >>> >>
    >>> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
    >>> >> index f3bd8e9..1b84dcff 100644
    >>> >> --- a/drivers/vhost/vhost.c
    >>> >> +++ b/drivers/vhost/vhost.c
    >>> >> @@ -2339,7 +2339,7 @@ EXPORT_SYMBOL_GPL(vhost_disable_notify);
    >>> >> /* Create a new message. */
    >>> >> struct vhost_msg_node *vhost_new_msg(struct vhost_virtqueue *vq, int type)
    >>> >> {
    >>> >> - struct vhost_msg_node *node = kmalloc(sizeof *node, GFP_KERNEL);
    >>> >> + struct vhost_msg_node *node = kzalloc(sizeof *node, GFP_KERNEL);
    >>> >> if (!node)
    >>> >> return NULL;
    >>> >> node->vq = vq;
    >>> >> --
    >>> >> 2.8.1

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