Re: mm: NULL ptr deref handling mmaping of special mappings

H. Peter Anvin
2014-05-16T22:56:28Z

On 05/16/2014 03:40 PM, Andy Lutomirski wrote:
>
> My current draft is here:
>
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=vdso/cleanups
>
> On 64-bit userspace, it results in:
>
> 7fffa1dfd000-7fffa1dfe000 r-xp 00000000 00:00 0 [vdso]
> 7fffa1dfe000-7fffa1e00000 r--p 00000000 00:00 0 [vvar]
> ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0
> [vsyscall]
>
> On 32-bit userspace, it results in:
>
> f7748000-f7749000 r-xp 00000000 00:00 0 [vdso]
> f7749000-f774b000 r--p 00000000 00:00 0 [vvar]
> ffd94000-ffdb5000 rw-p 00000000 00:00 0 [stack]
>
> Is this good for CRIU? Another approach would be to name both of
> these things "vdso", since they are sort of both the vdso, but that
> might be a bit confusing -- [vvar] is not static text the way that
> [vdso] is.
>
> If I backport this for 3.15 (which might be nasty -- I would argue
> that the code change is actually a cleanup, but it's fairly
> intrusive), then [vvar] will be *before* [vdso], not after it. I'd be
> very hesitant to name both of them "[vdso]" in that case, since there
> is probably code that assumes that the beginning of "[vdso]" is a DSO.
>
> Note that it is *not* safe to blindly read from "[vvar]". On some
> configurations you *will* get SIGBUS if you try to read from some of
> the vvar pages. (That's what started this whole thread.) Some pages
> in "[vvar]" may have strange caching modes, so SIGBUS might not be the
> only surprising thing about poking at it.
>

mremap() should work on these pages, right?

-hpa