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[RFC] fs/kcore: change copy_to_user to copy_in_user

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  [RFC] fs/kcore: change copy_to_user to copy_in_user yalin wang <yalin.wang2010@gmail.com> - 2015-08-20 11:00 +0200
    Re: [RFC] fs/kcore: change copy_to_user to copy_in_user Frans Klaver <fransklaver@gmail.com> - 2015-08-20 12:00 +0200
      Re: [RFC] fs/kcore: change copy_to_user to copy_in_user yalin wang <yalin.wang2010@gmail.com> - 2015-08-20 12:50 +0200
    Re: [RFC] fs/kcore: change copy_to_user to copy_in_user Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-21 02:10 +0200

#1210305 — [RFC] fs/kcore: change copy_to_user to copy_in_user

the copy_to_user() here expect can fix the fault on both kernel and
user address, this is not true on other platforms except x86,
change to user copy_in_user() so that can detect the page fault,
work as expected.

Signed-off-by: yalin wang <yalin.wang2010@gmail.com>
---
 fs/proc/kcore.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 92e6726..4f28deb 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -515,8 +515,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 		} else {
 			if (kern_addr_valid(start)) {
 				unsigned long n;
-
-				n = copy_to_user(buffer, (char *)start, tsz);
+				if ((start + tsz < tsz) ||
+						(start + tsz) > TASK_SIZE)
+					return -EFAULT;
+				set_fs(KERNEL_DS);
+				n = copy_in_user(buffer, (char *)start, tsz);
+				set_fs(USER_DS);
 				/*
 				 * We cannot distinguish between fault on source
 				 * and fault on destination. When this happens
-- 
1.9.1

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#1210361

On Thu, Aug 20, 2015 at 10:59 AM, yalin wang <yalin.wang2010@gmail.com> wrote:
> the copy_to_user() here expect can fix the fault on both kernel and
> user address, this is not true on other platforms except x86,
> change to user copy_in_user() so that can detect the page fault,
> work as expected.

Could you rephrase this into multiple sentences in comprehensible
English? What is the expected behavior, what is the unexpected
behavior and what can people do to trigger it?


> Signed-off-by: yalin wang <yalin.wang2010@gmail.com>
> ---
>  fs/proc/kcore.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 92e6726..4f28deb 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -515,8 +515,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>                 } else {
>                         if (kern_addr_valid(start)) {
>                                 unsigned long n;
> -
> -                               n = copy_to_user(buffer, (char *)start, tsz);
> +                               if ((start + tsz < tsz) ||
> +                                               (start + tsz) > TASK_SIZE)
> +                                       return -EFAULT;
> +                               set_fs(KERNEL_DS);
> +                               n = copy_in_user(buffer, (char *)start, tsz);
> +                               set_fs(USER_DS);
>                                 /*
>                                  * We cannot distinguish between fault on source
>                                  * and fault on destination. When this happens
> --
> 1.9.1
>
> --
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#1210406

> On Aug 20, 2015, at 17:58, Frans Klaver <fransklaver@gmail.com> wrote:
> 
> On Thu, Aug 20, 2015 at 10:59 AM, yalin wang <yalin.wang2010@gmail.com> wrote:
>> the copy_to_user() here expect can fix the fault on both kernel and
>> user address, this is not true on other platforms except x86,
>> change to user copy_in_user() so that can detect the page fault,
>> work as expected.
> 
> Could you rephrase this into multiple sentences in comprehensible
> English? What is the expected behavior, what is the unexpected
> behavior and what can people do to trigger it?
> 
ok, i will send a V2 patch.

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#1210807

On Thu, Aug 20, 2015 at 1:59 AM, yalin wang <yalin.wang2010@gmail.com> wrote:
> -
> -                               n = copy_to_user(buffer, (char *)start, tsz);
> +                               if ((start + tsz < tsz) ||
> +                                               (start + tsz) > TASK_SIZE)
> +                                       return -EFAULT;

This is wrong. You apparently want to have

        if (!access_ok(start, tsz))
                return -EFAULT;

> +                               set_fs(KERNEL_DS);
> +                               n = copy_in_user(buffer, (char *)start, tsz);
> +                               set_fs(USER_DS);

.. and this is actually worse and even less portable than what we have
now, in that it's actively wrong on platforms that may have a user
address and a kernel address with the same value (ie they have
explicitly separate kernel/user address spaces).

Now, that's admittedly unusual, but I think sparc32 actually can do that.

Anyway, I absolutely detest this patch. It replaces one piece of code
that admittedly doesn't work on all architectures because kernel
memory is accessed without testing, with another hack that happens to
work on other architectures and is fragile and prone to be a security
issue.

In other words, I think the end result is _worse_ than the current situation.

You probably want to use "probe_kernel_read()" and do it into a
temporary buffer, and then just do the copy_to_user() from the
temporary buffer. Sure, it's less efficient, but at least it's not
actively wrong and a possible security problem in the long run.

                    Linus
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