Re: regression from your recent change to x86's copy_user_handle_tail()

[Linus Torvalds]

On Tue, Apr 21, 2015 at 11:33 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> while the description of commit cae2a173fe certainly makes sense, the
> change itself ignores the __probe_kernel_write() code path, for which
> the destination address is expected to be in kernel space but accesses
> may still fault. I.e. the use of plain memset() causes
> __probe_kernel_write() to oops rather than return an error. Shouldn't
> the "(unsigned long)to >= TASK_SIZE_MAX" be relaxed to take the
> effect of set_fs() into account?

Hmm. I think you're right.  So something like

    --- a/arch/x86/lib/usercopy_64.c
    +++ b/arch/x86/lib/usercopy_64.c
    @@ -82,7 +82,7 @@ copy_user_handle_tail(char *to, char *from, unsigned len)
            clac();

            /* If the destination is a kernel buffer, we always clear the end */
    -       if ((unsigned long)to >= TASK_SIZE_MAX)
    +       if (!__addr_ok(to))
                    memset(to, 0, len);
            return len;
     }

which will effectively say "only if we copy from user mode to kernel
mode" because if we use "set_fs(KERNEL_DS)" then kernel addresses will
also be __addr_ok..

Did you have a test-case for this? I guess we're talking odd ftrace
uses or kgdb?

                        Linus
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