diff mbox series

[12/23] bpf: handle the compat string in bpf_trace_copy_string better

Message ID 20200521152301.2587579-13-hch@lst.de (mailing list archive)
State New, archived
Headers show
Series [01/23] maccess: unexport probe_kernel_write and probe_user_write | expand

Commit Message

Christoph Hellwig May 21, 2020, 3:22 p.m. UTC 
User the proper helper for kernel or userspace addresses based on
TASK_SIZE instead of the dangerous strncpy_from_unsafe function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 kernel/trace/bpf_trace.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Comments

Andrii Nakryiko May 21, 2020, 10:10 p.m. UTC | #1 
On Thu, May 21, 2020 at 8:24 AM Christoph Hellwig <hch@lst.de> wrote:
>
> User the proper helper for kernel or userspace addresses based on
> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  kernel/trace/bpf_trace.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>

[...]
Andrew Morton May 28, 2020, 2:04 a.m. UTC | #2 
On Thu, 21 May 2020 17:22:50 +0200 Christoph Hellwig <hch@lst.de> wrote:

> User the proper helper for kernel or userspace addresses based on
> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
> 
> ...
>
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -331,8 +331,11 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
>  	switch (fmt_ptype) {
>  	case 's':
>  #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> -		strncpy_from_unsafe(buf, unsafe_ptr, bufsz);
> -		break;
> +		if ((unsigned long)unsafe_ptr < TASK_SIZE) {
> +			strncpy_from_user_nofault(buf, user_ptr, bufsz);
> +			break;
> +		}
> +		fallthrough;
>  #endif
>  	case 'k':
>  		strncpy_from_kernel_nofault(buf, unsafe_ptr, bufsz);

Another user of strncpy_from_unsafe() has popped up in linux-next's
bpf.  I did the below, but didn't try very hard - it's probably wrong
if CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE=n?

Anyway, please take a look at all the bpf_trace.c changes in
linux-next.


From: Andrew Morton <akpm@linux-foundation.org>
Subject: bpf:bpf_seq_printf(): handle potentially unsafe format string better

User the proper helper for kernel or userspace addresses based on
TASK_SIZE instead of the dangerous strncpy_from_unsafe function.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/trace/bpf_trace.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/kernel/trace/bpf_trace.c~xxx
+++ a/kernel/trace/bpf_trace.c
@@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
 		}
 
 		if (fmt[i] == 's') {
+			void *unsafe_ptr;
+
 			/* try our best to copy */
 			if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
 				err = -E2BIG;
 				goto out;
 			}
 
-			err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
-						  (void *) (long) args[fmt_cnt],
-						  MAX_SEQ_PRINTF_STR_LEN);
+			unsafe_ptr = (void *)(long)args[fmt_cnt];
+			if ((unsigned long)unsafe_ptr < TASK_SIZE) {
+				err = strncpy_from_user_nofault(
+					bufs->buf[memcpy_cnt], unsafe_ptr,
+					MAX_SEQ_PRINTF_STR_LEN);
+			} else {
+				err = -EFAULT;
+			}
 			if (err < 0)
 				bufs->buf[memcpy_cnt][0] = '\0';
 			params[fmt_cnt] = (u64)(long)bufs->buf[memcpy_cnt];
Yonghong Song May 28, 2020, 2:26 a.m. UTC | #3 
On 5/27/20 7:04 PM, Andrew Morton wrote:
> On Thu, 21 May 2020 17:22:50 +0200 Christoph Hellwig <hch@lst.de> wrote:
> 
>> User the proper helper for kernel or userspace addresses based on
>> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
>>
>> ...
>>
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -331,8 +331,11 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
>>   	switch (fmt_ptype) {
>>   	case 's':
>>   #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>> -		strncpy_from_unsafe(buf, unsafe_ptr, bufsz);
>> -		break;
>> +		if ((unsigned long)unsafe_ptr < TASK_SIZE) {
>> +			strncpy_from_user_nofault(buf, user_ptr, bufsz);
>> +			break;
>> +		}
>> +		fallthrough;
>>   #endif
>>   	case 'k':
>>   		strncpy_from_kernel_nofault(buf, unsafe_ptr, bufsz);
> 
> Another user of strncpy_from_unsafe() has popped up in linux-next's
> bpf.  I did the below, but didn't try very hard - it's probably wrong
> if CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE=n?
> 
> Anyway, please take a look at all the bpf_trace.c changes in
> linux-next.
> 
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: bpf:bpf_seq_printf(): handle potentially unsafe format string better
> 
> User the proper helper for kernel or userspace addresses based on
> TASK_SIZE instead of the dangerous strncpy_from_unsafe function.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>   kernel/trace/bpf_trace.c |   13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> --- a/kernel/trace/bpf_trace.c~xxx
> +++ a/kernel/trace/bpf_trace.c
> @@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
>   		}
>   
>   		if (fmt[i] == 's') {
> +			void *unsafe_ptr;
> +
>   			/* try our best to copy */
>   			if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
>   				err = -E2BIG;
>   				goto out;
>   			}
>   
> -			err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
> -						  (void *) (long) args[fmt_cnt],
> -						  MAX_SEQ_PRINTF_STR_LEN);
> +			unsafe_ptr = (void *)(long)args[fmt_cnt];
> +			if ((unsigned long)unsafe_ptr < TASK_SIZE) {
> +				err = strncpy_from_user_nofault(
> +					bufs->buf[memcpy_cnt], unsafe_ptr,
> +					MAX_SEQ_PRINTF_STR_LEN);
> +			} else {
> +				err = -EFAULT;
> +			}

This probably not right.
The pointer stored at args[fmt_cnt] is a kernel pointer,
but it could be an invalid address and we do not want to fault.
Not sure whether it exists or not, we should use 
strncpy_from_kernel_nofault()?

>   			if (err < 0)
>   				bufs->buf[memcpy_cnt][0] = '\0';
>   			params[fmt_cnt] = (u64)(long)bufs->buf[memcpy_cnt];
> _
>
Christoph Hellwig May 28, 2020, 4:39 a.m. UTC | #4 
On Wed, May 27, 2020 at 07:26:30PM -0700, Yonghong Song wrote:
>> --- a/kernel/trace/bpf_trace.c~xxx
>> +++ a/kernel/trace/bpf_trace.c
>> @@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
>>   		}
>>     		if (fmt[i] == 's') {
>> +			void *unsafe_ptr;
>> +
>>   			/* try our best to copy */
>>   			if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
>>   				err = -E2BIG;
>>   				goto out;
>>   			}
>>   -			err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
>> -						  (void *) (long) args[fmt_cnt],
>> -						  MAX_SEQ_PRINTF_STR_LEN);
>> +			unsafe_ptr = (void *)(long)args[fmt_cnt];
>> +			if ((unsigned long)unsafe_ptr < TASK_SIZE) {
>> +				err = strncpy_from_user_nofault(
>> +					bufs->buf[memcpy_cnt], unsafe_ptr,
>> +					MAX_SEQ_PRINTF_STR_LEN);
>> +			} else {
>> +				err = -EFAULT;
>> +			}
>
> This probably not right.
> The pointer stored at args[fmt_cnt] is a kernel pointer,
> but it could be an invalid address and we do not want to fault.
> Not sure whether it exists or not, we should use 
> strncpy_from_kernel_nofault()?

If you know it is a kernel pointer with this series it should be
strncpy_from_kernel_nofault.  But even before the series it should have
been strncpy_from_unsafe_strict.
Yonghong Song May 28, 2020, 5:06 p.m. UTC | #5 
On 5/27/20 9:39 PM, Christoph Hellwig wrote:
> On Wed, May 27, 2020 at 07:26:30PM -0700, Yonghong Song wrote:
>>> --- a/kernel/trace/bpf_trace.c~xxx
>>> +++ a/kernel/trace/bpf_trace.c
>>> @@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
>>>    		}
>>>      		if (fmt[i] == 's') {
>>> +			void *unsafe_ptr;
>>> +
>>>    			/* try our best to copy */
>>>    			if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
>>>    				err = -E2BIG;
>>>    				goto out;
>>>    			}
>>>    -			err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
>>> -						  (void *) (long) args[fmt_cnt],
>>> -						  MAX_SEQ_PRINTF_STR_LEN);
>>> +			unsafe_ptr = (void *)(long)args[fmt_cnt];
>>> +			if ((unsigned long)unsafe_ptr < TASK_SIZE) {
>>> +				err = strncpy_from_user_nofault(
>>> +					bufs->buf[memcpy_cnt], unsafe_ptr,
>>> +					MAX_SEQ_PRINTF_STR_LEN);
>>> +			} else {
>>> +				err = -EFAULT;
>>> +			}
>>
>> This probably not right.
>> The pointer stored at args[fmt_cnt] is a kernel pointer,
>> but it could be an invalid address and we do not want to fault.
>> Not sure whether it exists or not, we should use
>> strncpy_from_kernel_nofault()?
> 
> If you know it is a kernel pointer with this series it should be
> strncpy_from_kernel_nofault.  But even before the series it should have
> been strncpy_from_unsafe_strict.

The use of strncpy_from_unsafe() mimics old bpf_trace_printk() 
implementation which just changed to _strict version:
https://lkml.org/lkml/2020/5/18/1309

Agreed that we should change to strncpy_from_unsafe_strict().
I can submit a patch for this.

Thanks!
diff mbox series

Patch

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 9d4080590f711..737d739230a6b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -331,8 +331,11 @@  static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
 	switch (fmt_ptype) {
 	case 's':
 #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
-		strncpy_from_unsafe(buf, unsafe_ptr, bufsz);
-		break;
+		if ((unsigned long)unsafe_ptr < TASK_SIZE) {
+			strncpy_from_user_nofault(buf, user_ptr, bufsz);
+			break;
+		}
+		fallthrough;
 #endif
 	case 'k':
 		strncpy_from_kernel_nofault(buf, unsafe_ptr, bufsz);