linux-kernel - Re: [RFC PATCH v2 3/3] kasan: add interceptors for all string functions

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Date:   Tue, 2 Apr 2019 19:14:02 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Daniel Axtens <dja@...ens.net>
Cc:     linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [RFC PATCH v2 3/3] kasan: add interceptors for all string
 functions

On 4/2/19 12:43 PM, Christophe Leroy wrote:
> Hi Dmitry, Andrey and others,
> 
> Do you have any comments to this series ?
> 

I don't see justification for adding all these non-instrumented functions. We need only some subset of these functions
and only on powerpc so far. Arches that don't use str*() that early simply doesn't need not-instrumented __str*() variant.

Also I don't think that auto-replace str* to __str* for all not instrumented files is a good idea, as this will reduce KASAN coverage.
E.g. we don't instrument slub.c but there is no reason to use non-instrumented __str*() functions there.

And finally, this series make bug reporting slightly worse. E.g. let's look at strcpy():

+char *strcpy(char *dest, const char *src)
+{
+	size_t len = __strlen(src) + 1;
+
+	check_memory_region((unsigned long)src, len, false, _RET_IP_);
+	check_memory_region((unsigned long)dest, len, true, _RET_IP_);
+
+	return __strcpy(dest, src);
+}

If src is not-null terminated string we might not see proper out-of-bounds report from KASAN only a crash in __strlen().
Which might make harder to identify where 'src' comes from, where it was allocated and what's the size of allocated area.

> I'd like to know if this approach is ok or if it is better to keep doing as in https://patchwork.ozlabs.org/patch/1055788/
>
I think the patch from link is a better solution to the problem.