Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 01, 2022 at 09:05:42PM -0700, Jakub Kicinski wrote:
> On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote:
> > On 2022/11/2 0:45, Daniel Borkmann wrote:
> > > [ +kfence folks ]  
> > 
> > + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov
> > 
> > Do you have any suggestions about this problem?
> 
> + Kees who has been sending similar patches for drivers
> 
> > > On 11/1/22 5:04 AM, Baisong Zhong wrote:  
> > >> Recently, we got a syzkaller problem because of aarch64
> > >> alignment fault if KFENCE enabled.
> > >>
> > >> When the size from user bpf program is an odd number, like
> > >> 399, 407, etc, it will cause skb shard info's alignment access,
> > >> as seen below:
> > >>
> > >> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 
> > >> net/core/skbuff.c:1032
> > >>
> > >> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
> > >>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
> > >>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
> > >>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
> > >>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
> > >>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> > >>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
> > >>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
> > >>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
> > >>   bpf_prog_d3839dd9068ceb51+0x80/0x330
> > >>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
> > >>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
> > >>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
> > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > >>
> > >> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, 
> > >> cache=kmalloc-512
> > >>
> > >> allocated by task 15074 on cpu 0 at 1342.585390s:
> > >>   kmalloc include/linux/slab.h:568 [inline]
> > >>   kzalloc include/linux/slab.h:675 [inline]
> > >>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
> > >>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
> > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > >>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> > >>
> > >> To fix the problem, we round up allocations with kmalloc_size_roundup()
> > >> so that build_skb()'s use of kize() is always alignment and no special
> > >> handling of the memory is needed by KFENCE.
> > >>
> > >> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> > >> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> > >> ---
> > >>   net/bpf/test_run.c | 1 +
> > >>   1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > >> index 13d578ce2a09..058b67108873 100644
> > >> --- a/net/bpf/test_run.c
> > >> +++ b/net/bpf/test_run.c
> > >> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr 
> > >> *kattr, u32 user_size,
> > >>       if (user_size > size)
> > >>           return ERR_PTR(-EMSGSIZE);
> > >> +    size = kmalloc_size_roundup(size);
> > >>       data = kzalloc(size + headroom + tailroom, GFP_USER);  
> > > 
> > > The fact that you need to do this roundup on call sites feels broken, no?
> > > Was there some discussion / consensus that now all k*alloc() call sites
> > > would need to be fixed up? Couldn't this be done transparently in k*alloc()
> > > when KFENCE is enabled? I presume there may be lots of other such occasions
> > > in the kernel where similar issue triggers, fixing up all call-sites feels
> > > like ton of churn compared to api-internal, generic fix.

I hope I answer this in more detail here:
https://lore.kernel.org/lkml/202211010937.4631CB1B0E@keescook/

The problem is that ksize() should never have existed in the first
place. :P Every runtime bounds checker has tripped over it, and with
the addition of the __alloc_size attribute, I had to start ripping
ksize() out: it can't be used to pretend an allocation grew in size.
Things need to either preallocate more or go through *realloc() like
everything else. Luckily, ksize() is rare.

FWIW, the above fix doesn't look correct to me -- I would expect this to
be:

	size_t alloc_size;
	...
	alloc_size = kmalloc_size_roundup(size + headroom + tailroom);
	data = kzalloc(alloc_size, GFP_USER);

-- 
Kees Cook