Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

[Pablo Neira Ayuso]

On Fri, Dec 01, 2017 at 01:46:07AM +0100, Jann Horn 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 8801eff2c494 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 8801eff2c3c0
> which belongs to the cache kmalloc-2048 of size 2048
> The buggy address is located 212 bytes inside of
> 2048-byte region [8801eff2c3c0, 8801eff2cbc0)
> [...]
> ==

Applied, thanks.

Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

[Willem de Bruijn]

On Thu, Nov 30, 2017 at 11:08 PM, Jann Horn  wrote:
> On Fri, Dec 1, 2017 at 5:04 AM, Willem de Bruijn
>  wrote:
>> On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn  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 8801eff2c494 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 8801eff2c3c0
>>> which belongs to the cache kmalloc-2048 of size 2048
>>> The buggy address is located 212 bytes inside of
>>> 2048-byte region [8801eff2c3c0, 8801eff2cbc0)
>>> [...]
>>> ==
>>>
>>> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
>>> Signed-off-by: Jann Horn 
>>> ---
>>>  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

Argh, of course. Thanks.

Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

[Jann Horn]

On Fri, Dec 1, 2017 at 5:04 AM, Willem de Bruijn
 wrote:
> On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn  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 8801eff2c494 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 8801eff2c3c0
>> which belongs to the cache kmalloc-2048 of size 2048
>> The buggy address is located 212 bytes inside of
>> 2048-byte region [8801eff2c3c0, 8801eff2cbc0)
>> [...]
>> ==
>>
>> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
>> Signed-off-by: Jann Horn 
>> ---
>>  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

Re: [PATCH] netfilter: add overflow checks in xt_bpf.c

[Willem de Bruijn]

On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn  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 8801eff2c494 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 8801eff2c3c0
> which belongs to the cache kmalloc-2048 of size 2048
> The buggy address is located 212 bytes inside of
> 2048-byte region [8801eff2c3c0, 8801eff2cbc0)
> [...]
> ==
>
> Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
> Signed-off-by: Jann Horn 
> ---
>  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.

> @@ -55,6 +58,9 @@ static int __bpf_mt_check_path(const char *path, struct 
> bpf_prog **ret)
> mm_segment_t oldfs = get_fs();
> int retval, fd;
>
> +   if (strnlen(path, XT_BPF_PATH_MAX) == XT_BPF_PATH_MAX)
> +   return -EINVAL;
> +

Good catch. It looks like this code needs a more thorough revision.

https://lkml.kernel.org/r/<20171201034859.gn21...@zeniv.linux.org.uk>

[PATCH] netfilter: add overflow checks in xt_bpf.c

[Jann Horn]

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 8801eff2c494 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 8801eff2c3c0
which belongs to the cache kmalloc-2048 of size 2048
The buggy address is located 212 bytes inside of
2048-byte region [8801eff2c3c0, 8801eff2cbc0)
[...]
==

Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
Signed-off-by: Jann Horn 
---
 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;
 
@@ -55,6 +58,9 @@ static int __bpf_mt_check_path(const char *path, struct 
bpf_prog **ret)
mm_segment_t oldfs = get_fs();
int retval, fd;
 
+   if (strnlen(path, XT_BPF_PATH_MAX) == XT_BPF_PATH_MAX)
+   return -EINVAL;
+
set_fs(KERNEL_DS);
fd = bpf_obj_get_user(path, 0);
set_fs(oldfs);
-- 
2.15.0.531.g2ccb3012c9-goog