Re: Runtime failure running sh:qemu in -next due to 'sh: fix copy_from_user()'

[Al Viro]

On Fri, Sep 16, 2016 at 12:12:18PM -0700, Guenter Roeck wrote:
> Hi,
> 
> I see the following runtime failure when running a 'sh' image with qemu in -next.

> Bisect points to commit 6e050503a150 ("sh: fix copy_from_user()"). Bisect log is
> attached.

Does reverting it recover the thing?

The change in question is
        if (__copy_size && __access_ok(__copy_from, __copy_size))
-               return __copy_user(to, from, __copy_size);
+               __copy_size = __copy_user(to, from, __copy_size);
+
+       if (unlikely(__copy_size))
+               memset(to + (n - __copy_size), 0, __copy_size);
 
        return __copy_size;

so the only difference is zeroing the tail of destination; return value
remains the same in all cases (what used to be return foo(); becomes
__copy_size = foo(); /* operations not modifying __copy_size */
return __copy_size;) and that memset is 100% legitimate -
copy_from_user(to, from, n) returning m means that the last m bytes of
[to .. to + n - 1] have not been copied into and must be zeroed.

If it affects anything at all, we have a serious problem somewhere in the
caller.