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    SubjectRe: <Query> Looking more details and reasons for using orig_add_limit.
    Hi Prasad,

    On 15/02/17 05:52, Sodagudi Prasad wrote:
    > When any sys call is made from user space orig_addr_limit will be zero and after
    > that driver is calling set_fs(KERNEL_DS) and then copy_to_user() to user space
    > memory.

    Don't do this, its exactly the case PAN+UAO and the code you pointed to are
    designed to catch. Accessing userspace needs doing carefully, setting USER_DS
    and using the put_user()/copy_to_user() accessors are the required steps.

    Which driver is doing this? Is it in mainline?


    > If there is permission fault for user space address the above condition
    > is leading to kernel crash. Because orig_add_limit is having KERNEL_DS as set_fs
    > called before copy_to_user().
    >
    > 1) So I would like to understand that, is that user space pointer leading to
    > permission fault not correct(condition_1) in this scenario?

    The correct thing has happened here. To access user space set_fs(USER_DS) first.
    (and set it back to whatever it was afterwards).


    > 2) Are there any corner cases where these if conditions (condition_1 and
    > condition2) would lead to kernel crash ?

    If you do this on behalf of a user space process the kernel will try to clean up
    as best it can and carry on. If you access user space from an interrupt handler
    or from a kernel thread you can expect the kernel to panic().


    > 3) What are all scenarios these if conditions (condition_1 and condition2)
    > would like to take care?

    I'm not sure I understand this question. PAN prevents general kernel code from
    accessing user space, you have to use the accessors. When you have UAO too, it
    can enforce the set_fs() limit as PAN will generate permission faults when the
    accessors touch the kernel/user-space after setting the other set_fs() limit.

    I hope this helps!


    Thanks,

    James

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