possible array out of bounds access in sys/netinet/sctp_output.c

[Michael Tüxen]

On Nov 27, 2011, at 5:56 PM, Davide Italiano wrote:

> On Sun, Nov 27, 2011 at 5:24 PM, Jilles Tjoelker <jilles at stack.nl> wrote:
>> On Sun, Nov 27, 2011 at 03:45:36PM +0000, Alexander Best wrote:
>>> i've been playing with clang tot and noticed the following error:
>> 
>>> /usr/local/bin/clang -c -O3 -pipe -fno-inline-functions -fno-strict-aliasing -march=core2 -std=c99 -g -fdiagnostics-show-option -fformat-extensions -Wall  -Wcast-qual -Winline -Wmissing-include-dirs  -Wmissing-prototypes -Wnested-externs -Wpointer-arith  -Wredundant-decls -Wstrict-prototypes -Wundef  -Wno-pointer-sign -nostdinc  -I. -I/usr/git-freebsd-head/sys -I/usr/git-freebsd-head/sys/contrib/altq -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h  -fno-omit-frame-pointer -mno-aes -mno-avx -mcmodel=kernel -mno-red-zone -mno-mmx -msoft-float  -fno-asynchronous-unwind-tables -ffreestanding -Wno-error=tautological-compare -Wno-error=shift-count-negative  -Wno-error=shift-count-overflow -Wno-error=shift-overflow -Wno-error=conversion  -Wno-error=empty-body -Wno-error=gnu-designator -Wno-error=format  -Wno-error=format-invalid-specifier -Wno-error=format-extra-args -Werror  /usr/git-freebsd-head/sys/netinet/sctp_output.c
>>> clang: warning: argument unused during compilation: '-fformat-extensions'
>>> /usr/git-freebsd-head/sys/netinet/sctp_output.c:4685:2: error: array index 1 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
>>>         sup_addr->addr_type[1] = htons(SCTP_IPV6_ADDRESS);
>>>         ^                   ~
>>> /usr/git-freebsd-head/sys/netinet/sctp_header.h:84:2: note: array 'addr_type' declared here
>>>         uint16_t addr_type[SCTP_ARRAY_MIN_LEN]; /* array of supported address
>>>         ^
>>> 1 error generated.
>>> *** Error code 1
>>> 
>>> Stop in /usr/obj/usr/git-freebsd-head/sys/GENERIC.
>>> *** Error code 1
>>> 
>>> Stop in /usr/git-freebsd-head.
>>> *** Error code 1
>>> 
>>> Stop in /usr/git-freebsd-head.
>> 
>>> this is from a GENERIC kernel build (so INET + INET6) for amd64. is this a
>>> false positive, or is length(sup_addr->addr_type) really == 1, thus making
>>> sup_addr->addr_type[1] an illegal access?
>> 
>> This is the fairly common construct of a variable-length array at the
>> end of a struct. With C89, this was not allowed but defining one element
>> and allocating more elements worked in most implementations. C99
>> recognized this need and created a way to do it, which looks like
>> uint16_t addr_type[];. This adds any necessary padding and allows access
>> to however many elements have been allocated. Also, if it is not at the
>> end of a struct it is an error.
>> 
>> Using this new construct requires code changes because some code such as
>> fairly close to the error message relies on the size of the one element
>> already in the struct.
>> 
>> --
>> Jilles Tjoelker
>> _______________________________________________
>> freebsd-current at freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-current
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>> 
> 
> 
> I looked at sctp_send_initiate() and it seems that independently from
> the number of types supported (IPV6/IPV4 or both) two elements are
> allocated in the array sup_addr->addr_type[0] . In case only a type is
> supported one of the elements is simply padding.
> (http://fxr.watson.org/fxr/source/netinet/sctp_output.c#L4670) .
> 
> So, this should fix the issue, but maybe I'm wrong so feel free to correct me.
> 
> http://davit.altervista.org/sctp_header_types.diff
> 
> I defined a new macro mainly because SCTP_ARRAY_MIN_LEN is used in
> another place, i.e. in the field name of struct sctp_host_name_param,
> defined in sctp_header.h). Thanks to arundel@ for testing.
The problem with your fix is that the size of the structure is used in
the code and there are also changes required.

When looking at the code to avoid the warning I realized that the
supported address types parameters will have a wrong length if
the kernel is build with either INET or INET6, but not both.

I've committed a fix in
http://svn.freebsd.org/changeset/base/228031

Best regards
Michael
> 
> Regards
> 
> Davide
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