Re: [PATCH] proc: do not allow negative offsets on/proc/<pid>/environ

From: Oleg Nesterov
Date: Mon Jul 23 2012 - 11:52:26 EST

Hi Djalal,

On 07/23, Djalal Harouni wrote:
> Hi Oleg,
> On Sun, Jul 22, 2012 at 10:00:49PM +0200, Oleg Nesterov wrote:
> >
> > Probablt the patch makes sense, but I can't understand the changelog...
> >
> > > Allowing negative offsets on /proc/<pid>/environ can turn it to act like
> > > /proc/<pid>/mem. A negative offset will pass the
> > > fs/read_write.c:lseek_execute() and the environ_read() checks and will
> > > point to another VMA.
> >
> > which VMA?
> It depends on the offset. Please see below.
> > environ_read() can only read the memory from [env_start, env_end], and
> > it should check *ppos anyway to ensure it doesn't read something else.
> Yes I agree, but currently that's not the case, there are no checks on *ppos.
There is, unless I missed something, just it is buggy, no?

> So if you pass a negative offset you will be able to read from an arbitrary
> address.
> [...snip...]
> inside environ_read() there is only a one check:
> int this_len = mm->env_end - (mm->env_start + src);
> if (this_len <= 0)
> break;
> Here 'src' is 'src = *ppos' the negative offset converted to unsigned long
> and (mm->env_start + src) can overflow and point to another VMA.
> int this_len = mm->env_end - (mm->env_start + src)
> 'this_len' will be positive and we pass that check.

OK, thanks, but doesn't this mean that this check should be fixed
to avoid the overflow, no matter what *ppos is?

With or without FMODE_UNSIGNED_OFFSET change. And perhaps it is
possible to trigger the overflow even with the positive *ppos,

> I also don't like the truncation of the result to 'int this_len'


> BTW should I resend the patch with a better changelog entry ?

Up to you, but I think this makes sense ;)

> I'll also add another patch to check the offsets inside environ_read().

Yes, agreed, but please see above.

Please correct me, but afaics this patch should come 1st and fix the bug.
FMODE_UNSIGNED_OFFSET change can be considered as a cleanup after that.

What do you think?


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