[PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Alexei Starovoitov]

Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
the result is typically passed to print("%s", buf) and extra bytes
after zero don't cause any harm.
In bpf the result of bpf_get_current_comm() is used as the part of
map key and was causing spurious hash map mismatches.
Use strlcpy() to guarantee zero-terminated string.
bpf verifier checks that output buffer is zero-initialized,
so even for short task names the output buffer don't have junk bytes.
Note it's not a security concern, since kprobe+bpf is root only.

Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid, comm accessors")
Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
Targeting net-next, since it's too late for net.
I think it makes sense for stable as well.

 kernel/bpf/helpers.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 4504ca66118d..50da680c479f 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size, u64 r3, u64 r4, u64 r5)
 	if (!task)
 		return -EINVAL;
 
-	memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
+	strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
 	return 0;
 }
 
-- 
2.8.0.rc1

Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[David Miller]

From: Alexei Starovoitov <ast@fb.com>
Date: Wed, 9 Mar 2016 20:02:33 -0800

> Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
> the result is typically passed to print("%s", buf) and extra bytes
> after zero don't cause any harm.
> In bpf the result of bpf_get_current_comm() is used as the part of
> map key and was causing spurious hash map mismatches.
> Use strlcpy() to guarantee zero-terminated string.
> bpf verifier checks that output buffer is zero-initialized,
> so even for short task names the output buffer don't have junk bytes.
> Note it's not a security concern, since kprobe+bpf is root only.
> 
> Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid, comm accessors")
> Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
> Targeting net-next, since it's too late for net.
> I think it makes sense for stable as well.

Applied and queued up for -stable, thanks.

Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Daniel Borkmann]

On 03/10/2016 05:02 AM, Alexei Starovoitov wrote:
> Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
> the result is typically passed to print("%s", buf) and extra bytes
> after zero don't cause any harm.
> In bpf the result of bpf_get_current_comm() is used as the part of
> map key and was causing spurious hash map mismatches.
> Use strlcpy() to guarantee zero-terminated string.
> bpf verifier checks that output buffer is zero-initialized,

Sorry for late reply, more below:

> so even for short task names the output buffer don't have junk bytes.
> Note it's not a security concern, since kprobe+bpf is root only.
>
> Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid, comm accessors")
> Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
[...]
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 4504ca66118d..50da680c479f 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size, u64 r3, u64 r4, u64 r5)
>   	if (!task)
>   		return -EINVAL;
>
> -	memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
> +	strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));

If I see this correctly, __set_task_comm() makes sure comm is always zero
terminated, so that seems good, but isn't it already sufficient when switching
to strlcpy() to simply use:

     strlcpy(buf, task->comm, size);

The min_t() seems unnecessary work to me, why do we still need it? size
is guaranteed to be > 0 through the eBPF verifier, so strlcpy() should take
care of the rest.

Thanks,
Daniel

Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Alexei Starovoitov]

On 3/11/16 2:24 AM, Daniel Borkmann wrote:
> On 03/10/2016 05:02 AM, Alexei Starovoitov wrote:
>> Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
>> the result is typically passed to print("%s", buf) and extra bytes
>> after zero don't cause any harm.
>> In bpf the result of bpf_get_current_comm() is used as the part of
>> map key and was causing spurious hash map mismatches.
>> Use strlcpy() to guarantee zero-terminated string.
>> bpf verifier checks that output buffer is zero-initialized,
>
> Sorry for late reply, more below:
>
>> so even for short task names the output buffer don't have junk bytes.
>> Note it's not a security concern, since kprobe+bpf is root only.
>>
>> Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid,
>> comm accessors")
>> Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
>> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> [...]
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 4504ca66118d..50da680c479f 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size,
>> u64 r3, u64 r4, u64 r5)
>>       if (!task)
>>           return -EINVAL;
>>
>> -    memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
>> +    strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
>
> If I see this correctly, __set_task_comm() makes sure comm is always zero
> terminated, so that seems good, but isn't it already sufficient when
> switching
> to strlcpy() to simply use:
>
>      strlcpy(buf, task->comm, size);
>
> The min_t() seems unnecessary work to me, why do we still need it? size
> is guaranteed to be > 0 through the eBPF verifier, so strlcpy() should take
> care of the rest.

that's one clever optimization. yep. we can drop min_t.
btw I wanted to add memset to __set_task_comm, keep memcpy in
bpf_get_current_comm and optimize perf_event_comm_event
(which doing: memset+strlcpy and can be replaced with memcpy),
but figured that such 'fix' is not suitable for stable.
I guess we can do in the next cycle? strlen is not cheap.
Especially since it turned out that bpf_get_current_comm() is
used very often in the hot path in bcc/tools.

Also for the next cycle I'm planning to extend verifier to
allow uninitialized stack to be passed to functions like
bpf_get_current_comm() and they would have to zero it in
error cases. Then we can save few more cycles from the programs.

Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Daniel Borkmann]

On 03/11/2016 06:20 PM, Alexei Starovoitov wrote:
> On 3/11/16 2:24 AM, Daniel Borkmann wrote:
>> On 03/10/2016 05:02 AM, Alexei Starovoitov wrote:
>>> Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
>>> the result is typically passed to print("%s", buf) and extra bytes
>>> after zero don't cause any harm.
>>> In bpf the result of bpf_get_current_comm() is used as the part of
>>> map key and was causing spurious hash map mismatches.
>>> Use strlcpy() to guarantee zero-terminated string.
>>> bpf verifier checks that output buffer is zero-initialized,
>>
>> Sorry for late reply, more below:
>>
>>> so even for short task names the output buffer don't have junk bytes.
>>> Note it's not a security concern, since kprobe+bpf is root only.
>>>
>>> Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid,
>>> comm accessors")
>>> Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
>>> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>> [...]
>>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>>> index 4504ca66118d..50da680c479f 100644
>>> --- a/kernel/bpf/helpers.c
>>> +++ b/kernel/bpf/helpers.c
>>> @@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size,
>>> u64 r3, u64 r4, u64 r5)
>>>       if (!task)
>>>           return -EINVAL;
>>>
>>> -    memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
>>> +    strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
>>
>> If I see this correctly, __set_task_comm() makes sure comm is always zero
>> terminated, so that seems good, but isn't it already sufficient when
>> switching
>> to strlcpy() to simply use:
>>
>>      strlcpy(buf, task->comm, size);
>>
>> The min_t() seems unnecessary work to me, why do we still need it? size
>> is guaranteed to be > 0 through the eBPF verifier, so strlcpy() should take
>> care of the rest.
>
> that's one clever optimization. yep. we can drop min_t.
> btw I wanted to add memset to __set_task_comm, keep memcpy in
> bpf_get_current_comm and optimize perf_event_comm_event
> (which doing: memset+strlcpy and can be replaced with memcpy),
> but figured that such 'fix' is not suitable for stable.
> I guess we can do in the next cycle? strlen is not cheap.
> Especially since it turned out that bpf_get_current_comm() is
> used very often in the hot path in bcc/tools.

Would strscpy() help in this case (see 30035e45753b ("string: provide
strscpy()"))?

> Also for the next cycle I'm planning to extend verifier to
> allow uninitialized stack to be passed to functions like
> bpf_get_current_comm() and they would have to zero it in
> error cases. Then we can save few more cycles from the programs.

That would be useful also for other helpers indeed.

Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Alexei Starovoitov]

On 3/11/16 10:02 AM, Daniel Borkmann wrote:
> Would strscpy() help in this case (see 30035e45753b ("string: provide
> strscpy()"))?

I've looked at it too, but 990486c8af04 scared me a little,
it's not easily backport-able and mainly I don't think
it's faster than strlcpy for small strings like comm.
memcpy going to be faster for sure.