[cfe-dev] [analyzer] Constrain the size of unknown memory regions
Balázs Benics via cfe-dev
cfe-dev at lists.llvm.org
Mon Mar 16 08:17:14 PDT 2020
I'm impressed.
ANALYZER_ASSERT(ANALYZER_EXTENT(buff) == size);
>
That looks like something I've dreamed about, thank you.
I don't know whether it's a good idea. Not only we have too little
> positive experience with array bound checking for now, but also there
> are other powerful tools to deal with buffer overflows, like ASan/MSan
> and, well, C++ containers. It's unclear to me how often would it be
> impossible
I totally agree, and that is why I was looking into eg.:
`ArrayBoundCheckerV2` and it's buggy behavior.
This problem raised by implementing taint warnings for the
`CStringChecker`, which meant that it would warn for cases like I mentioned
in my first email.
Which would result in false positives without any mechanism to silence
them, since we can not tell the analyzer about the implicit assumptions
which are not present in the code.
How should we ship this `analyzer_assert.h` with the analyzer?
Artem Dergachev <noqnoqneo at gmail.com> ezt írta (időpont: 2020. márc. 16.,
H, 15:41):
> Wait, so you're not trying to do this in a checker, but trying to
> introduce, like, a mechanism for the users to use or something? Ok, got
> it. Interesting, yeah.
>
> Well, you could always add a magic function like
> __clang_analyzer_getExtent() that would work exactly like
> clang_analyzer_getExtent() from the debug.ExprInspection checker but
> will be on by default and double-underscored so that to avoid
> conflicting with user code (note that defining double-underscored
> declarations outside of the standard library is UB). Then give them a
> header:
>
> // analyzer_helper.h
> #ifdef __clang_analyzer__
> size_t __clang_analyzer_getExtent(void *x);
> #define ANALYZER_ASSERT assert
> #define ANALYZER_EXTENT(x) __clang_analyzer_getExtent(x)
> #else
> #define ANALYZER_ASSERT
> #define ANALYZER_EXTENT(x)
> #endif
>
> And then tell them to write code like this:
>
> ANALYZER_ASSERT(ANALYZER_EXTENT(buff) == size);
>
> I don't know whether it's a good idea. Not only we have too little
> positive experience with array bound checking for now, but also there
> are other powerful tools to deal with buffer overflows, like ASan/MSan
> and, well, C++ containers. It's unclear to me how often would it be
> impossible
>
> Another possible approach is to introduce annotations and teach the
> analyzer to understand them, eg.:
>
> int g_user_function(
> __attribute__((buffer_ptr("buff"))) int *buff,
> __attribute__((buffer_size("buff"))) int size);
>
> Or like this:
>
> struct BoundedBuff {
> __attribute__((buffer_ptr("buff"))) int *buff;
> __attribute__((buffer_size("buff"))) int size);
> };
>
> That's slightly less flexible but also less ugly (when you hide those
> behind macros as well).
>
> > I think that no '/annotations/' should introduce UB even if that
> > assumption expressed via an annotation is turned out to be _invalid_.
>
> Well, that's a fairly common thing to happen when annotations are
> introduced for optimizations rather than for hardening. Say, if you
> violate __builtin_assume it's a UB. If you violate a `restrict`
> qualifier contract it's a UB. Even if you violate a `const` qualifier
> contract it's a UB. Interestingly, iirc in-language assertions in C++23
> contracts are currently expected to introduce UB when violated. That
> said, i definitely agree with you in your case.
>
>
> On 3/16/20 5:06 PM, Balázs Benics wrote:
> > Hi Artem and Csaba,
> > Thank you for your response.
> >
> > Sorry for the long description of the problem in my original email,
> > but I wanted to provide as much context as I could.
> > Provided that we have to analyze code like in that example, the
> > analyzer (correctly) assumes that the pointer points to an unknown
> > memory region.
> > However, the user knows that the function will be called with a valid
> > buffer, which would be capable of holding at least/most n elements.
> > For now, we can not tell this assumption to the analyzer. There is no
> > way in standard C++ to express such notion precisely. Asserts would
> > not be powerful enough, as I earlier stated.
> > So we have to tell this kind of assumption to the analyzer in a
> > different way. But in what way?
> >
> > Some sort of annotation or special analyzer assert? I'm not sure, but
> > none of these look promising, really.
> >
> > I think we should find a way to properly analyze the following two cases:
> >
> > * `int f_user_function(int *points_to_at_least_5_elements);`
> > The user knows that this function must be called with a pointer
> > pointing to an array capable of holding at least 5 elements.
> > We should be able to tell this assumption to the analyzer, to
> > analyze its body according to this assumption.
> >
> > * `int g_user_function(int *buff, int size);`
> > There is a connection between the `buff`and `size`, which denotes
> > similar properties that were described in the previous bullet
> > point, but the analyzer would not know.
> >
> > Regards Balazs.
> >
> > Artem Dergachev <noqnoqneo at gmail.com <mailto:noqnoqneo at gmail.com>> ezt
> > írta (időpont: 2020. márc. 16., H, 3:08):
> >
> > I think you're looking for SymbolExtent. It is *the* symbol that
> > denotes
> > the [otherwise completely] unknown size of a region. The helper
> > function
> > for obtaining either a known extent or a SymbolExtent is currently
> > known
> > as getDynamicSize() (but i'd rather rename it back to "extent" - see
> > also https://reviews.llvm.org/D69726).
> >
> > ArrayBoundChecker already has the code that you're looking for.
> >
> > On 3/12/20 3:20 PM, Balázs Benics via cfe-dev wrote:
> > > Hi, checker devs
> > >
> > > TLDR:
> > > How to constrain the size of unknown memory regions, eg. pointed by
> > > 'raw' char pointers?
> > >
> > > longer version:
> > > Working on taint analysis I'm facing with the following problem:
> > >
> > > void strncpy_bounded_tainted_buffer(char *src, char *dst) {
> > > // assert(strlen(src) >= 10 && "src must have at leas 10
> > elements");
> > > int n;
> > > scanf("%d", &n);
> > > if (0 < n && n < 10) {
> > > strncpy(dst, src, n); // Should we warn or not?
> > > }
> > > }
> > >
> > >
> > >
> > > In this example we analyze a function taking two raw pointers,
> > so we
> > > don't know how big those arrays are.
> > > We will check the `strncpy` call, whether it will access /(read and
> > > write)/ only valid memory.
> > > We will check the pointers (src and dst) separately by checking if
> > > /`/&src[0]` and `&src[n-1]` would be in bound of the memory region
> > > pointed by the pointer. Since the analyzer don't know (both
> > states are
> > > non-null), we should check if the `length` parameter is tainted,
> > and
> > > if so, we should still issue a warning telling that "String copy
> > > function might overflow the given buffer since untrusted data is
> > used
> > > to specify the buffer size."
> > > Obviously, if the `length` parameter is not tainted, we will assume
> > > (conservatively) that the access would be valid.
> > >
> > >
> > > How should tell the analyzer that the array which is pointed by the
> > > pointer holds at least/most N elements?
> > > For example in the code above, express something similar via an
> > > assertion, like saying that `src` points to a c-string, which
> > has at
> > > least 10 + 1 element underlying storage.
> > > Although this assertion using `strlen` seems like a solution,
> > > unfortunately not applicable for example to the `dst` buffer,
> > which is
> > > most likely uninitialized - so not a c-string, in other words
> > calling
> > > `strlen` would be undefined behavior.
> > >
> > > The only (hacky) option which came in my mind was to abuse the
> > > standard regarding pointer arithmetic.
> > >
> > > assert(&src[n] - &src[-1]);
> > >
> > > The standard is clear about that pointer arithmetic is only
> > applicable
> > > for pointers pointing to elements of the same array OR to a
> > > hypothetical ONE past/before element of that array.
> > > http://eel.is/c++draft/expr.add#4.2
> > >
> > > This assertion would be undefined behavior if the size of the array
> > > pointed by `src` would be smaller than `n`.
> > >
> > > IMO this looks really ugly.
> > > I think that no '/annotations/' should introduce UB even if that
> > > assumption expressed via an annotation is turned out to be
> > _invalid_.
> > >
> > >
> > > What would be the right approach to guide (to constrain the size
> > of a
> > > memory region) the analyzer?
> > > How can the analyzer inference such constraint?
> > >
> > > Thanks Balazs.
> > >
> > > _______________________________________________
> > > cfe-dev mailing list
> > > cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
>
>
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