[PATCH] lkdtm: use the return value of strim()

[PATCH] lkdtm: use the return value of strim()

[GONG, Ruiqi]

From: "GONG, Ruiqi" <gongruiqi1@huawei.com>

Make use of the return value of strim() to achieve left-trim as well as
right-trim, which prevents the following unusual fail case:

 # echo " EXCEPTION" > /sys/kernel/debug/provoke-crash/DIRECT
 sh: write error: Invalid argument

Link: https://github.com/KSPP/linux/issues/337
Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
---
 drivers/misc/lkdtm/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index 0772e4a4757e..812c96461ab2 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -242,7 +242,7 @@ static ssize_t lkdtm_debugfs_entry(struct file *f,
 	}
 	/* NULL-terminate and remove enter */
 	buf[count] = '\0';
-	strim(buf);
+	buf = strim(buf);
 
 	crashtype = find_crashtype(buf);
 	free_page((unsigned long)buf);
@@ -318,7 +318,7 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
 	}
 	/* NULL-terminate and remove enter */
 	buf[count] = '\0';
-	strim(buf);
+	buf = strim(buf);
 
 	crashtype = find_crashtype(buf);
 	free_page((unsigned long) buf);
-- 
2.25.1

Re: [PATCH] lkdtm: use the return value of strim()

[Kees Cook]

On Thu, Aug 17, 2023 at 10:21:17PM +0800, GONG, Ruiqi wrote:
> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
> 
> Make use of the return value of strim() to achieve left-trim as well as
> right-trim, which prevents the following unusual fail case:
> 
>  # echo " EXCEPTION" > /sys/kernel/debug/provoke-crash/DIRECT
>  sh: write error: Invalid argument
> 
> Link: https://github.com/KSPP/linux/issues/337
> Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
> ---
>  drivers/misc/lkdtm/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index 0772e4a4757e..812c96461ab2 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -242,7 +242,7 @@ static ssize_t lkdtm_debugfs_entry(struct file *f,
>  	}
>  	/* NULL-terminate and remove enter */
>  	buf[count] = '\0';
> -	strim(buf);
> +	buf = strim(buf);
>  
>  	crashtype = find_crashtype(buf);
>  	free_page((unsigned long)buf);

Will free_page() still work in this case, though? The address won't
match the allocation any more...

-Kees

> @@ -318,7 +318,7 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
>  	}
>  	/* NULL-terminate and remove enter */
>  	buf[count] = '\0';
> -	strim(buf);
> +	buf = strim(buf);
>  
>  	crashtype = find_crashtype(buf);
>  	free_page((unsigned long) buf);
> -- 
> 2.25.1
> 

-- 
Kees Cook

Re: [PATCH] lkdtm: use the return value of strim()

[GONG, Ruiqi]

On 2023/08/18 0:55, Kees Cook wrote:
> On Thu, Aug 17, 2023 at 10:21:17PM +0800, GONG, Ruiqi wrote:
>> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
>>
>> Make use of the return value of strim() to achieve left-trim as well as
>> right-trim, which prevents the following unusual fail case:
>>
>>  # echo " EXCEPTION" > /sys/kernel/debug/provoke-crash/DIRECT
>>  sh: write error: Invalid argument
>>
>> Link: https://github.com/KSPP/linux/issues/337
>> Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
>> ---
>>  drivers/misc/lkdtm/core.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
>> index 0772e4a4757e..812c96461ab2 100644
>> --- a/drivers/misc/lkdtm/core.c
>> +++ b/drivers/misc/lkdtm/core.c
>> @@ -242,7 +242,7 @@ static ssize_t lkdtm_debugfs_entry(struct file *f,
>>  	}
>>  	/* NULL-terminate and remove enter */
>>  	buf[count] = '\0';
>> -	strim(buf);
>> +	buf = strim(buf);
>>  
>>  	crashtype = find_crashtype(buf);
>>  	free_page((unsigned long)buf);
> 
> Will free_page() still work in this case, though? The address won't
> match the allocation any more...

Yes I noticed that, but I was under the impression that it's fine to do
free_page(paddr + offset_in_page) since the offset is within the page,
and its corresponding struct page can still be found when being freed.
Please let me know if this thought is wrong and I will submit another
version.

> 
> -Kees
> 
>> @@ -318,7 +318,7 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
>>  	}
>>  	/* NULL-terminate and remove enter */
>>  	buf[count] = '\0';
>> -	strim(buf);
>> +	buf = strim(buf);
>>  
>>  	crashtype = find_crashtype(buf);
>>  	free_page((unsigned long) buf);
>> -- 
>> 2.25.1
>>
> 

Re: [PATCH] lkdtm: use the return value of strim()

[Heiko Carstens]

On Fri, Aug 18, 2023 at 10:39:06AM +0800, GONG, Ruiqi wrote:
> 
> On 2023/08/18 0:55, Kees Cook wrote:
> > On Thu, Aug 17, 2023 at 10:21:17PM +0800, GONG, Ruiqi wrote:
> >> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
> >>
> >> Make use of the return value of strim() to achieve left-trim as well as
> >> right-trim, which prevents the following unusual fail case:
> >>
> >>  # echo " EXCEPTION" > /sys/kernel/debug/provoke-crash/DIRECT
> >>  sh: write error: Invalid argument
...
> >>  	/* NULL-terminate and remove enter */
> >>  	buf[count] = '\0';
> >> -	strim(buf);
> >> +	buf = strim(buf);
> >>  
> >>  	crashtype = find_crashtype(buf);
> >>  	free_page((unsigned long)buf);
> > 
> > Will free_page() still work in this case, though? The address won't
> > match the allocation any more...
> 
> Yes I noticed that, but I was under the impression that it's fine to do
> free_page(paddr + offset_in_page) since the offset is within the page,
> and its corresponding struct page can still be found when being freed.
> Please let me know if this thought is wrong and I will submit another
> version.

The question is rather why you think this patch makes any sense at
all. Nobody is doing what you described above - hence there is no
problem. The comments in the code even say strim() is used to remove the
trailing '\n'. If anybody passes a string with whitespace at the beginning
then that's just a user error.

There really is no point in patches like this.

Re: [PATCH] lkdtm: use the return value of strim()

[Greg Kroah-Hartman]

On Thu, Aug 17, 2023 at 10:21:17PM +0800, GONG, Ruiqi wrote:
> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
> 
> Make use of the return value of strim() to achieve left-trim as well as
> right-trim, which prevents the following unusual fail case:
> 
>  # echo " EXCEPTION" > /sys/kernel/debug/provoke-crash/DIRECT
>  sh: write error: Invalid argument

And that is correct, don't use leading whitespace, the kernel is not
there to parse it away :)

thanks,

greg k-h