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

[zhongbaisong]

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?

Thanks,

.

> 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@xxxxxxxxxx>
> > ---
> >   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.
>
> >       if (!data)
> >           return ERR_PTR(-ENOMEM);
>
> Thanks,
> Daniel