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

[Jan Beulich]

>>> On 23.04.15 at 17:33, <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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?

I'm afraid not one you'd like - we've seen ftrace initialization fail for
quite some time on our Xen kernels, but in a way only affecting
ftrace itself. Said change converted that failure to an oops. (The
ftrace init failure itself is because the traditional Xen kernel creates
1:1 mappings for pages that are part of kernel image as read-only,
to avoid having to special case embedded regions [GDT, page tables]
that must be r/o under Xen. I think the pv-ops kernel behaves
differently, which made me recognize this as a problem wider than
just for our specific Xen case only on second thought.)

Jan

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