[PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[Jinghao Jia]

The bpf_sys_bpf() helper function allows an eBPF program to load another
eBPF program from within the kernel. In this case the argument union
bpf_attr pointer (as well as the insns and license pointers inside) is a
kernel address instead of a userspace address (which is the case of a
usual bpf() syscall). To make the memory copying process in the syscall
work in both cases, bpfptr_t [1] was introduced to wrap around the
pointer and distinguish its origin. Specifically, when copying memory
contents from a bpfptr_t, a copy_from_user() is performed in case of a
userspace address and a memcpy() is performed for a kernel address [2].

This can lead to problems because the in-kernel pointer is never checked
for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
with a bad insns pointer, say 0xdeadbeef (which is supposed to point to
the start of the instruction array) in the bpf_attr union, memcpy() is
always happy to dereference the bad pointer to cause a un-handle-able
page fault and in turn an oops. However, this is not supposed to happen
because at that point the eBPF program is already verified and should
not cause a memory error. The same issue in userspace is handled
gracefully by copy_from_user(), which would return -EFAULT in such a
case.

Replace memcpy() with the safer copy_from_kernel_nofault() and
strncpy_from_kernel_nofault().

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44

Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
---
 include/linux/sockptr.h | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
index d45902fb4cad..3b8a41c82516 100644
--- a/include/linux/sockptr.h
+++ b/include/linux/sockptr.h
@@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void *dst, sockptr_t src,
 {
 	if (!sockptr_is_kernel(src))
 		return copy_from_user(dst, src.user + offset, size);
-	memcpy(dst, src.kernel + offset, size);
-	return 0;
+	return copy_from_kernel_nofault(dst, src.kernel + offset, size);
 }
 
 static inline int copy_from_sockptr(void *dst, sockptr_t src, size_t size)
@@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t src, size_t len)
 
 static inline long strncpy_from_sockptr(char *dst, sockptr_t src, size_t count)
 {
-	if (sockptr_is_kernel(src)) {
-		size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
-
-		memcpy(dst, src.kernel, len);
-		return len;
-	}
+	if (sockptr_is_kernel(src))
+		return strncpy_from_kernel_nofault(dst, src.kernel, count);
 	return strncpy_from_user(dst, src.user, count);
 }
 

base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f
-- 
2.35.1

Re: [PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[Yonghong Song]

On 7/27/22 6:29 AM, Jinghao Jia wrote:
> The bpf_sys_bpf() helper function allows an eBPF program to load another
> eBPF program from within the kernel. In this case the argument union
> bpf_attr pointer (as well as the insns and license pointers inside) is a
> kernel address instead of a userspace address (which is the case of a
> usual bpf() syscall). To make the memory copying process in the syscall
> work in both cases, bpfptr_t [1] was introduced to wrap around the
> pointer and distinguish its origin. Specifically, when copying memory
> contents from a bpfptr_t, a copy_from_user() is performed in case of a
> userspace address and a memcpy() is performed for a kernel address [2].
> 
> This can lead to problems because the in-kernel pointer is never checked
> for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
> with a bad insns pointer, say 0xdeadbeef (which is supposed to point to
> the start of the instruction array) in the bpf_attr union, memcpy() is
> always happy to dereference the bad pointer to cause a un-handle-able
> page fault and in turn an oops. However, this is not supposed to happen
> because at that point the eBPF program is already verified and should
> not cause a memory error. The same issue in userspace is handled
> gracefully by copy_from_user(), which would return -EFAULT in such a
> case.
> 
> Replace memcpy() with the safer copy_from_kernel_nofault() and
> strncpy_from_kernel_nofault().
> 
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44
> 
> Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
> ---
>   include/linux/sockptr.h | 11 +++--------
>   1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
> index d45902fb4cad..3b8a41c82516 100644
> --- a/include/linux/sockptr.h
> +++ b/include/linux/sockptr.h
> @@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void *dst, sockptr_t src,
>   {
>   	if (!sockptr_is_kernel(src))
>   		return copy_from_user(dst, src.user + offset, size);
> -	memcpy(dst, src.kernel + offset, size);
> -	return 0;
> +	return copy_from_kernel_nofault(dst, src.kernel + offset, size);
>   }

The subject and commit message mentioned it is bpf_sys_bpf() helper
might have issues. But the patch itself tries to modify 
copy_from_sockptr_offset() and strncpy_from_sockptr(), why?

>   
>   static inline int copy_from_sockptr(void *dst, sockptr_t src, size_t size)
> @@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t src, size_t len)
>   
>   static inline long strncpy_from_sockptr(char *dst, sockptr_t src, size_t count)
>   {
> -	if (sockptr_is_kernel(src)) {
> -		size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
> -
> -		memcpy(dst, src.kernel, len);
> -		return len;
> -	}
> +	if (sockptr_is_kernel(src))
> +		return strncpy_from_kernel_nofault(dst, src.kernel, count);
>   	return strncpy_from_user(dst, src.user, count);
>   }
>   
> 
> base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f

Re: [PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[Jinghao Jia]

On 7/28/22 10:52 AM, Yonghong Song wrote:
>
>
> On 7/27/22 6:29 AM, Jinghao Jia wrote:
>> The bpf_sys_bpf() helper function allows an eBPF program to load another
>> eBPF program from within the kernel. In this case the argument union
>> bpf_attr pointer (as well as the insns and license pointers inside) is a
>> kernel address instead of a userspace address (which is the case of a
>> usual bpf() syscall). To make the memory copying process in the syscall
>> work in both cases, bpfptr_t [1] was introduced to wrap around the
>> pointer and distinguish its origin. Specifically, when copying memory
>> contents from a bpfptr_t, a copy_from_user() is performed in case of a
>> userspace address and a memcpy() is performed for a kernel address [2].
>>
>> This can lead to problems because the in-kernel pointer is never checked
>> for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
>> with a bad insns pointer, say 0xdeadbeef (which is supposed to point to
>> the start of the instruction array) in the bpf_attr union, memcpy() is
>> always happy to dereference the bad pointer to cause a un-handle-able
>> page fault and in turn an oops. However, this is not supposed to happen
>> because at that point the eBPF program is already verified and should
>> not cause a memory error. The same issue in userspace is handled
>> gracefully by copy_from_user(), which would return -EFAULT in such a
>> case.
>>
>> Replace memcpy() with the safer copy_from_kernel_nofault() and
>> strncpy_from_kernel_nofault().
>>
>> [1]: 
>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h
>> [2]: 
>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44
>>
>> Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
>> ---
>>   include/linux/sockptr.h | 11 +++--------
>>   1 file changed, 3 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
>> index d45902fb4cad..3b8a41c82516 100644
>> --- a/include/linux/sockptr.h
>> +++ b/include/linux/sockptr.h
>> @@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void 
>> *dst, sockptr_t src,
>>   {
>>       if (!sockptr_is_kernel(src))
>>           return copy_from_user(dst, src.user + offset, size);
>> -    memcpy(dst, src.kernel + offset, size);
>> -    return 0;
>> +    return copy_from_kernel_nofault(dst, src.kernel + offset, size);
>>   }
>
> The subject and commit message mentioned it is bpf_sys_bpf() helper
> might have issues. But the patch itself tries to modify 
> copy_from_sockptr_offset() and strncpy_from_sockptr(), why?
>

Hi Yonghong,

Sorry for the confusion. The problem happens when bpf_sys_bpf() helper 
is called with a bad kernel address but the dereference takes place in 
the copy_from_sockptr_offset() and strncpy_from_sockptr() functions.

Let's assume we are doing a BPF_PROG_LOAD operation using bpf_sys_bpf() 
and our insns pointer inside the union bpf_attr argument is set to NULL 
(could be any other bad address). The helper calls __sys_bpf() which 
would then call bpf_prog_load() to load the program. bpf_prog_load() is 
responsible for copying the eBPF instructions to the newly allocated 
memory for the program, which uses the bpfptr_t API [1]. Internally, all 
bpfptr_t operations are backed by the corresponding sockptr_t 
operations. In other words, the code that performs the copy (and 
therefore the deref of the pointer) is inside copy_from_sockptr_offset() 
and strncpy_from_sockptr().

[1]: 
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/syscall.c#n2566

Best,
Jinghao
>>     static inline int copy_from_sockptr(void *dst, sockptr_t src, 
>> size_t size)
>> @@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t 
>> src, size_t len)
>>     static inline long strncpy_from_sockptr(char *dst, sockptr_t src, 
>> size_t count)
>>   {
>> -    if (sockptr_is_kernel(src)) {
>> -        size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
>> -
>> -        memcpy(dst, src.kernel, len);
>> -        return len;
>> -    }
>> +    if (sockptr_is_kernel(src))
>> +        return strncpy_from_kernel_nofault(dst, src.kernel, count);
>>       return strncpy_from_user(dst, src.user, count);
>>   }
>>
>> base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f

Re: [PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[Yonghong Song]

On 7/28/22 11:01 AM, Jinghao Jia wrote:
> 
> On 7/28/22 10:52 AM, Yonghong Song wrote:
>>
>>
>> On 7/27/22 6:29 AM, Jinghao Jia wrote:
>>> The bpf_sys_bpf() helper function allows an eBPF program to load another
>>> eBPF program from within the kernel. In this case the argument union
>>> bpf_attr pointer (as well as the insns and license pointers inside) is a
>>> kernel address instead of a userspace address (which is the case of a
>>> usual bpf() syscall). To make the memory copying process in the syscall
>>> work in both cases, bpfptr_t [1] was introduced to wrap around the
>>> pointer and distinguish its origin. Specifically, when copying memory
>>> contents from a bpfptr_t, a copy_from_user() is performed in case of a
>>> userspace address and a memcpy() is performed for a kernel address [2].
>>>
>>> This can lead to problems because the in-kernel pointer is never checked
>>> for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
>>> with a bad insns pointer, say 0xdeadbeef (which is supposed to point to
>>> the start of the instruction array) in the bpf_attr union, memcpy() is
>>> always happy to dereference the bad pointer to cause a un-handle-able
>>> page fault and in turn an oops. However, this is not supposed to happen
>>> because at that point the eBPF program is already verified and should
>>> not cause a memory error. The same issue in userspace is handled
>>> gracefully by copy_from_user(), which would return -EFAULT in such a
>>> case.
>>>
>>> Replace memcpy() with the safer copy_from_kernel_nofault() and
>>> strncpy_from_kernel_nofault().
>>>
>>> [1]: 
>>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h 
>>>
>>> [2]: 
>>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44 
>>>
>>>
>>> Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
>>> ---
>>>   include/linux/sockptr.h | 11 +++--------
>>>   1 file changed, 3 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
>>> index d45902fb4cad..3b8a41c82516 100644
>>> --- a/include/linux/sockptr.h
>>> +++ b/include/linux/sockptr.h
>>> @@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void 
>>> *dst, sockptr_t src,
>>>   {
>>>       if (!sockptr_is_kernel(src))
>>>           return copy_from_user(dst, src.user + offset, size);
>>> -    memcpy(dst, src.kernel + offset, size);
>>> -    return 0;
>>> +    return copy_from_kernel_nofault(dst, src.kernel + offset, size);
>>>   }
>>
>> The subject and commit message mentioned it is bpf_sys_bpf() helper
>> might have issues. But the patch itself tries to modify 
>> copy_from_sockptr_offset() and strncpy_from_sockptr(), why?
>>
> 
> Hi Yonghong,
> 
> Sorry for the confusion. The problem happens when bpf_sys_bpf() helper 
> is called with a bad kernel address but the dereference takes place in 
> the copy_from_sockptr_offset() and strncpy_from_sockptr() functions.
> 
> Let's assume we are doing a BPF_PROG_LOAD operation using bpf_sys_bpf() 
> and our insns pointer inside the union bpf_attr argument is set to NULL 
> (could be any other bad address). The helper calls __sys_bpf() which 
> would then call bpf_prog_load() to load the program. bpf_prog_load() is 
> responsible for copying the eBPF instructions to the newly allocated 
> memory for the program, which uses the bpfptr_t API [1]. Internally, all 
> bpfptr_t operations are backed by the corresponding sockptr_t 
> operations. In other words, the code that performs the copy (and 
> therefore the deref of the pointer) is inside copy_from_sockptr_offset() 
> and strncpy_from_sockptr().

Thanks for explanation. It would be great if you can put the above
details in the commit message (esp. call stack) which leads to
the kernel panic(?).

> 
> [1]: 
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/syscall.c#n2566 
> 
> 
> Best,
> Jinghao
>>>     static inline int copy_from_sockptr(void *dst, sockptr_t src, 
>>> size_t size)
>>> @@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t 
>>> src, size_t len)
>>>     static inline long strncpy_from_sockptr(char *dst, sockptr_t src, 
>>> size_t count)
>>>   {
>>> -    if (sockptr_is_kernel(src)) {
>>> -        size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
>>> -
>>> -        memcpy(dst, src.kernel, len);
>>> -        return len;
>>> -    }
>>> +    if (sockptr_is_kernel(src))
>>> +        return strncpy_from_kernel_nofault(dst, src.kernel, count);
>>>       return strncpy_from_user(dst, src.user, count);
>>>   }

I think we should not modify copy_from_sockptr() and 
strncpy_from_sockptr(). These two functions are used by networking
as well and nofault version should not be called for calls in
networking stack.

So I suggest you directly modify copy_from_bpfptr() and 
strncpy_from_bpfptr() since these two functions indeed might
have invalid kernel address and may cause fault.

>>>
>>> base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f

Re: [PATCH] BPF: Fix potential bad pointer dereference in bpf_sys_bpf

[Jinghao Jia]

On 7/28/22 7:16 PM, Yonghong Song wrote:
>
>
> On 7/28/22 11:01 AM, Jinghao Jia wrote:
>>
>> On 7/28/22 10:52 AM, Yonghong Song wrote:
>>>
>>>
>>> On 7/27/22 6:29 AM, Jinghao Jia wrote:
>>>> The bpf_sys_bpf() helper function allows an eBPF program to load 
>>>> another
>>>> eBPF program from within the kernel. In this case the argument union
>>>> bpf_attr pointer (as well as the insns and license pointers inside) 
>>>> is a
>>>> kernel address instead of a userspace address (which is the case of a
>>>> usual bpf() syscall). To make the memory copying process in the 
>>>> syscall
>>>> work in both cases, bpfptr_t [1] was introduced to wrap around the
>>>> pointer and distinguish its origin. Specifically, when copying memory
>>>> contents from a bpfptr_t, a copy_from_user() is performed in case of a
>>>> userspace address and a memcpy() is performed for a kernel address 
>>>> [2].
>>>>
>>>> This can lead to problems because the in-kernel pointer is never 
>>>> checked
>>>> for validity. If an eBPF syscall program tries to call bpf_sys_bpf()
>>>> with a bad insns pointer, say 0xdeadbeef (which is supposed to 
>>>> point to
>>>> the start of the instruction array) in the bpf_attr union, memcpy() is
>>>> always happy to dereference the bad pointer to cause a un-handle-able
>>>> page fault and in turn an oops. However, this is not supposed to 
>>>> happen
>>>> because at that point the eBPF program is already verified and should
>>>> not cause a memory error. The same issue in userspace is handled
>>>> gracefully by copy_from_user(), which would return -EFAULT in such a
>>>> case.
>>>>
>>>> Replace memcpy() with the safer copy_from_kernel_nofault() and
>>>> strncpy_from_kernel_nofault().
>>>>
>>>> [1]: 
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpfptr.h 
>>>>
>>>> [2]: 
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/sockptr.h#n44 
>>>>
>>>>
>>>> Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
>>>> ---
>>>>   include/linux/sockptr.h | 11 +++--------
>>>>   1 file changed, 3 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
>>>> index d45902fb4cad..3b8a41c82516 100644
>>>> --- a/include/linux/sockptr.h
>>>> +++ b/include/linux/sockptr.h
>>>> @@ -46,8 +46,7 @@ static inline int copy_from_sockptr_offset(void 
>>>> *dst, sockptr_t src,
>>>>   {
>>>>       if (!sockptr_is_kernel(src))
>>>>           return copy_from_user(dst, src.user + offset, size);
>>>> -    memcpy(dst, src.kernel + offset, size);
>>>> -    return 0;
>>>> +    return copy_from_kernel_nofault(dst, src.kernel + offset, size);
>>>>   }
>>>
>>> The subject and commit message mentioned it is bpf_sys_bpf() helper
>>> might have issues. But the patch itself tries to modify 
>>> copy_from_sockptr_offset() and strncpy_from_sockptr(), why?
>>>
>>
>> Hi Yonghong,
>>
>> Sorry for the confusion. The problem happens when bpf_sys_bpf() 
>> helper is called with a bad kernel address but the dereference takes 
>> place in the copy_from_sockptr_offset() and strncpy_from_sockptr() 
>> functions.
>>
>> Let's assume we are doing a BPF_PROG_LOAD operation using 
>> bpf_sys_bpf() and our insns pointer inside the union bpf_attr 
>> argument is set to NULL (could be any other bad address). The helper 
>> calls __sys_bpf() which would then call bpf_prog_load() to load the 
>> program. bpf_prog_load() is responsible for copying the eBPF 
>> instructions to the newly allocated memory for the program, which 
>> uses the bpfptr_t API [1]. Internally, all bpfptr_t operations are 
>> backed by the corresponding sockptr_t operations. In other words, the 
>> code that performs the copy (and therefore the deref of the pointer) 
>> is inside copy_from_sockptr_offset() and strncpy_from_sockptr().
>
> Thanks for explanation. It would be great if you can put the above
> details in the commit message (esp. call stack) which leads to
> the kernel panic(?).
>
>>
>> [1]: 
>> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/syscall.c#n2566 
>>
>>
>> Best,
>> Jinghao
>>>>     static inline int copy_from_sockptr(void *dst, sockptr_t src, 
>>>> size_t size)
>>>> @@ -93,12 +92,8 @@ static inline void *memdup_sockptr_nul(sockptr_t 
>>>> src, size_t len)
>>>>     static inline long strncpy_from_sockptr(char *dst, sockptr_t 
>>>> src, size_t count)
>>>>   {
>>>> -    if (sockptr_is_kernel(src)) {
>>>> -        size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
>>>> -
>>>> -        memcpy(dst, src.kernel, len);
>>>> -        return len;
>>>> -    }
>>>> +    if (sockptr_is_kernel(src))
>>>> +        return strncpy_from_kernel_nofault(dst, src.kernel, count);
>>>>       return strncpy_from_user(dst, src.user, count);
>>>>   }
>
> I think we should not modify copy_from_sockptr() and 
> strncpy_from_sockptr(). These two functions are used by networking
> as well and nofault version should not be called for calls in
> networking stack.
>
> So I suggest you directly modify copy_from_bpfptr() and 
> strncpy_from_bpfptr() since these two functions indeed might
> have invalid kernel address and may cause fault.
>

Thanks a lot for the feedback! I do agree that the changes on sockptr 
functions might be problematic. I will roll out a V2 based on your comments.

--Jinghao
>>>>
>>>> base-commit: d295daf505758f9a0e4d05f4ee3bfdfb4192c18f