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From:   Jann Horn <jannh@google.com>
Date:   Thu, 7 Feb 2019 11:26:39 +0100
Message-ID: <CAG48ez1gXgsBG6bYGG5+B4Dqkhk_iVaYLqt63RaxURxE0yt9eA@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] mm: add probe_user_read()
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Mike Rapoport <rppt@linux.ibm.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linuxppc-dev@lists.ozlabs.org, Linux-MM <linux-mm@kvack.org>
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On Thu, Feb 7, 2019 at 10:22 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
> In powerpc code, there are several places implementing safe
> access to user data. This is sometimes implemented using
> probe_kernel_address() with additional access_ok() verification,
> sometimes with get_user() enclosed in a pagefault_disable()/enable()
> pair, etc. :
>     show_user_instructions()
>     bad_stack_expansion()
>     p9_hmi_special_emu()
>     fsl_pci_mcheck_exception()
>     read_user_stack_64()
>     read_user_stack_32() on PPC64
>     read_user_stack_32() on PPC32
>     power_pmu_bhrb_to()
>
> In the same spirit as probe_kernel_read(), this patch adds
> probe_user_read().
>
> probe_user_read() does the same as probe_kernel_read() but
> first checks that it is really a user address.
>
> The patch defines this function as a static inline so the "size"
> variable can be examined for const-ness by the check_object_size()
> in __copy_from_user_inatomic()
>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>


> ---
>  v3: Moved 'Returns:" comment after description.
>      Explained in the commit log why the function is defined static inline
>
>  v2: Added "Returns:" comment and removed probe_user_address()
>
>  include/linux/uaccess.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 37b226e8df13..ef99edd63da3 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -263,6 +263,40 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
>  #define probe_kernel_address(addr, retval)             \
>         probe_kernel_read(&retval, addr, sizeof(retval))
>
> +/**
> + * probe_user_read(): safely attempt to read from a user location
> + * @dst: pointer to the buffer that shall take the data
> + * @src: address to read from
> + * @size: size of the data chunk
> + *
> + * Safely read from address @src to the buffer at @dst.  If a kernel fault
> + * happens, handle that and return -EFAULT.
> + *
> + * We ensure that the copy_from_user is executed in atomic context so that
> + * do_page_fault() doesn't attempt to take mmap_sem.  This makes
> + * probe_user_read() suitable for use within regions where the caller
> + * already holds mmap_sem, or other locks which nest inside mmap_sem.
> + *
> + * Returns: 0 on success, -EFAULT on error.
> + */
> +
> +#ifndef probe_user_read
> +static __always_inline long probe_user_read(void *dst, const void __user *src,
> +                                           size_t size)
> +{
> +       long ret;
> +
> +       if (!access_ok(src, size))
> +               return -EFAULT;

If this happens in code that's running with KERNEL_DS, the access_ok()
is a no-op. If this helper is only intended for accessing real
userspace memory, it would be more robust to add
set_fs(USER_DS)/set_fs(oldfs) around this thing. Looking at the
functions you're referring to in the commit message, e.g.
show_user_instructions() does an explicit `__access_ok(pc,
NR_INSN_TO_PRINT * sizeof(int), USER_DS)` to get the same effect.
(However, __access_ok() looks like it's horribly broken on x86 from
what I can tell, because it's going to use the generic version that
always returns 1...)

> +       pagefault_disable();
> +       ret = __copy_from_user_inatomic(dst, src, size);
> +       pagefault_enable();
> +
> +       return ret ? -EFAULT : 0;
> +}
> +#endif
> +
>  #ifndef user_access_begin
>  #define user_access_begin(ptr,len) access_ok(ptr, len)
>  #define user_access_end() do { } while (0)
> --
> 2.13.3
>
>