Re: [Xen-devel] [PATCH] tools: Fix wild memory allocations from c/s 250f0b4 and 85d78b4

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/05/15 15:34, Boris Ostrovsky wrote:
> On 05/18/2015 10:09 AM, Andrew Cooper wrote:
>> On 18/05/15 15:00, Boris Ostrovsky wrote:
>>> On 05/18/2015 08:57 AM, Andrew Cooper wrote:
>>>> These changesets cause the respective libxc functions to
>>>> unconditonally
>>>> dereference their max_cpus/nodes parameters as part of initial memory
>>>> allocations.  It will fail at obtaining the correct number of
>>>> cpus/nodes from
>>>> Xen, as the guest handles will not be NULL.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
>>>> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
>>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>>>>
>>>> ---
>>>> Spotted by XenServers Coverity run.
>>>> ---
>>>>    tools/libxl/libxl.c               |    4 ++--
>>>>    tools/misc/xenpm.c                |    4 ++--
>>>>    tools/python/xen/lowlevel/xc/xc.c |    4 ++--
>>>>    3 files changed, 6 insertions(+), 6 deletions(-)
>>> xenpm bug is already fixed (commit
>>> b315cd9cce5b6da7ca89b2d7bad3fb01e7716044 n the staging tree).
>>>
>>> I am not sure I understand why Coverity complains about other spots.
>>> For example, in libxl_get_cpu_topology() num_cpus can be left
>>> uninitialized only if xc_cputopoinfo(ctx->xch, &num_cpus, NULL) fails,
>>> in which case we go to 'GC_FREE;  return ret;', so it's not ever used.
>> xc_cputopoinfo(ctx->xch, &num_cpus, NULL) unconditionally dereferences
>> and reads &num_cpus, and performs a memory allocation based on the
>> result.
>
> Ah, OK. xc_cputopoinf() (or, rather, the hypervisor) actually doesn't
> use the value of dereferenced num_cpus in this case but obviously
> Coverity can't know about this.
>
> So Coverity cross-checks routines to see how callers use the arguments?

xc_cputopoinfo(ctx->xch, &num_cpus, NULL) dereferences &num_cpus as part
of its DECLARE_HYPERCALL_BUFFER()s.  All of this happens before getting
anywhere near the hypervisor.

~Andrew

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