linux-kernel - Re: [PATCH v4] mm/vmalloc: randomize vmalloc() allocations

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Date:   Mon, 15 Mar 2021 16:35:10 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        linux-hardening@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH v4] mm/vmalloc: randomize vmalloc() allocations

> On 14.3.2021 19.23, Uladzislau Rezki wrote:
> > Also, using vmaloc test driver i can trigger a kernel BUG:
> > 
> > <snip>
> > [   24.627577] kernel BUG at mm/vmalloc.c:1272!
> 
> It seems that most tests indeed fail. Perhaps the vmalloc subsystem isn't
> very robust in face of fragmented virtual memory. What could be done to fix
> that?
> 
Your patch is broken in context of checking "vend" when you try to
allocate next time after first attempt. Passed "vend" is different
there comparing what is checked later to figure out if an allocation
failed or not:

<snip>
    if (unlikely(addr == vend))
        goto overflow;
<snip>

>
> In this patch, I could retry __alloc_vmap_area() with the whole region after
> failure of both [random, vend] and [vstart, random] but I'm not sure that
> would help much. Worth a try of course.
> 
There is no need in your second [vstart, random]. If a first bigger range
has not been successful, the smaller one will never be success anyway. The
best way to go here is to repeat with real [vsart:vend], if it still fails
on a real range, then it will not be possible to accomplish an allocation
request with given parameters.

>
> By the way, some of the tests in test_vmalloc.c don't check for vmalloc()
> failure, for example in full_fit_alloc_test().
> 
Where?

--
Vlad Rezki