Re: [PATCH v4] Randomized slab caches for kmalloc()

[Kees Cook]

On Mon, Jun 26, 2023 at 11:18:35AM +0800, GONG, Ruiqi wrote:
> When exploiting memory vulnerabilities, "heap spraying" is a common
> technique targeting those related to dynamic memory allocation (i.e. the
> "heap"), and it plays an important role in a successful exploitation.
> Basically, it is to overwrite the memory area of vulnerable object by
> triggering allocation in other subsystems or modules and therefore
> getting a reference to the targeted memory location. It's usable on
> various types of vulnerablity including use after free (UAF), heap out-
> of-bound write and etc.
> 
> There are (at least) two reasons why the heap can be sprayed: 1) generic
> slab caches are shared among different subsystems and modules, and
> 2) dedicated slab caches could be merged with the generic ones.
> Currently these two factors cannot be prevented at a low cost: the first
> one is a widely used memory allocation mechanism, and shutting down slab
> merging completely via `slub_nomerge` would be overkill.
> 
> To efficiently prevent heap spraying, we propose the following approach:
> to create multiple copies of generic slab caches that will never be
> merged, and random one of them will be used at allocation. The random
> selection is based on the address of code that calls `kmalloc()`, which
> means it is static at runtime (rather than dynamically determined at
> each time of allocation, which could be bypassed by repeatedly spraying
> in brute force). In other words, the randomness of cache selection will
> be with respect to the code address rather than time, i.e. allocations
> in different code paths would most likely pick different caches,
> although kmalloc() at each place would use the same cache copy whenever
> it is executed. In this way, the vulnerable object and memory allocated
> in other subsystems and modules will (most probably) be on different
> slab caches, which prevents the object from being sprayed.
> 
> Meanwhile, the static random selection is further enhanced with a
> per-boot random seed, which prevents the attacker from finding a usable
> kmalloc that happens to pick the same cache with the vulnerable
> subsystem/module by analyzing the open source code. In other words, with
> the per-boot seed, the random selection is static during each time the
> system starts and runs, but not across different system startups.
> 
> The overhead of performance has been tested on a 40-core x86 server by
> comparing the results of `perf bench all` between the kernels with and
> without this patch based on the latest linux-next kernel, which shows
> minor difference. A subset of benchmarks are listed below:
> 
>                 sched/  sched/  syscall/       mem/       mem/
>              messaging    pipe     basic     memcpy     memset
>                  (sec)   (sec)     (sec)   (GB/sec)   (GB/sec)
> 
> control1         0.019   5.459     0.733  15.258789  51.398026
> control2         0.019   5.439     0.730  16.009221  48.828125
> control3         0.019   5.282     0.735  16.009221  48.828125
> control_avg      0.019   5.393     0.733  15.759077  49.684759
> 
> experiment1      0.019   5.374     0.741  15.500992  46.502976
> experiment2      0.019   5.440     0.746  16.276042  51.398026
> experiment3      0.019   5.242     0.752  15.258789  51.398026
> experiment_avg   0.019   5.352     0.746  15.678608  49.766343
> 
> The overhead of memory usage was measured by executing `free` after boot
> on a QEMU VM with 1GB total memory, and as expected, it's positively
> correlated with # of cache copies:
> 
>            control  4 copies  8 copies  16 copies
> 
> total       969.8M    968.2M    968.2M     968.2M
> used         20.0M     21.9M     24.1M      26.7M
> free        936.9M    933.6M    931.4M     928.6M
> available   932.2M    928.8M    926.6M     923.9M
> 
> Co-developed-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
> Signed-off-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
> Signed-off-by: GONG, Ruiqi <gongruiqi@xxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks for the v4; this looks good. :)

-- 
Kees Cook