On Fri, Dec 1, 2017 at 5:04 AM, Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
> On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
>> Check whether inputs from userspace are too long (explicit length field too
>> big or string not null-terminated) to avoid out-of-bounds reads.
>>
>> As far as I can tell, this can at worst lead to very limited kernel heap
>> memory disclosure or oopses.
>>
>> This bug can be triggered by an unprivileged user even if the xt_bpf module
>> is not loaded: iptables is available in network namespaces, and the xt_bpf
>> module can be autoloaded.
>>
>> Triggering the bug with a classic BPF filter with fake length 0x1000 causes
>> the following KASAN report:
>>
>> ==================================================================
>> BUG: KASAN: slab-out-of-bounds in bpf_prog_create+0x84/0xf0
>> Read of size 32768 at addr ffff8801eff2c494 by task test/4627
>>
>> CPU: 0 PID: 4627 Comm: test Not tainted 4.15.0-rc1+ #1
>> [...]
>> Call Trace:
>> dump_stack+0x5c/0x85
>> print_address_description+0x6a/0x260
>> kasan_report+0x254/0x370
>> ? bpf_prog_create+0x84/0xf0
>> memcpy+0x1f/0x50
>> bpf_prog_create+0x84/0xf0
>> bpf_mt_check+0x90/0xd6 [xt_bpf]
>> [...]
>> Allocated by task 4627:
>> kasan_kmalloc+0xa0/0xd0
>> __kmalloc_node+0x47/0x60
>> xt_alloc_table_info+0x41/0x70 [x_tables]
>> [...]
>> The buggy address belongs to the object at ffff8801eff2c3c0
>> which belongs to the cache kmalloc-2048 of size 2048
>> The buggy address is located 212 bytes inside of
>> 2048-byte region [ffff8801eff2c3c0, ffff8801eff2cbc0)
>> [...]
>> ==================================================================
>>
>> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
>> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
>> ---
>> net/netfilter/xt_bpf.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>>
>> diff --git a/net/netfilter/xt_bpf.c b/net/netfilter/xt_bpf.c
>> index 041da0d9c06f..1f7fbd3c7e5a 100644
>> --- a/net/netfilter/xt_bpf.c
>> +++ b/net/netfilter/xt_bpf.c
>> @@ -27,6 +27,9 @@ static int __bpf_mt_check_bytecode(struct sock_filter *insns, __u16 len,
>> {
>> struct sock_fprog_kern program;
>>
>> + if (len > XT_BPF_MAX_NUM_INSTR)
>> + return -EINVAL;
>> +
>> program.len = len;
>> program.filter = insns;
>
> Next, this calls bpf_prog_create, which calls bpf_check_basics_ok to verify len.
Irrelevant:
- see the KASAN splat in the commit message
- bpf_check_basics_ok checks against BPF_MAXINSNS (4096), but a check against
XT_BPF_MAX_NUM_INSTR (64) is needed because that's the size of the
member in the
input struct
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