[ +kfence folks ]
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
