Re: [merged] exec-open-code-copy_string_kernel.patch removed from -mm tree

Re: [merged] exec-open-code-copy_string_kernel.patch removed from -mm tree

[Vegard Nossum]

On 2020-06-05 22:19, akpm@linux-foundation.org wrote:
> The patch titled
>       Subject: exec: open code copy_string_kernel
> has been removed from the -mm tree.  Its filename was
>       exec-open-code-copy_string_kernel.patch
> 
> This patch was dropped because it was merged into mainline or a subsystem tree
> 
> ------------------------------------------------------
> From: Christoph Hellwig <hch@lst.de>
> Subject: exec: open code copy_string_kernel
> 
> Currently copy_string_kernel is just a wrapper around copy_strings that
> simplifies the calling conventions and uses set_fs to allow passing a
> kernel pointer.  But due to the fact the we only need to handle a single
> kernel argument pointer, the logic can be sigificantly simplified while
> getting rid of the set_fs.
> 
> Link: http://lkml.kernel.org/r/20200501104105.2621149-3-hch@lst.de
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>   fs/exec.c |   45 +++++++++++++++++++++++++++++++++++----------
>   1 file changed, 35 insertions(+), 10 deletions(-)
> 
> --- a/fs/exec.c~exec-open-code-copy_string_kernel
> +++ a/fs/exec.c
> @@ -592,17 +592,42 @@ out:
>    */
>   int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
>   {
> -	int r;
> -	mm_segment_t oldfs = get_fs();
> -	struct user_arg_ptr argv = {
> -		.ptr.native = (const char __user *const  __user *)&arg,
> -	};
> -
> -	set_fs(KERNEL_DS);
> -	r = copy_strings(1, argv, bprm);
> -	set_fs(oldfs);
> +	int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */;
> +	unsigned long pos = bprm->p;
>   
> -	return r;
> +	if (len == 0)
> +		return -EFAULT;

Just a quick question, how can len ever be 0 here when len was set to
strnlen() + 1? Should the test be different?

The old version (i.e. copy_strings()) seems to return -EFAULT when
strnlen() returns 0.


Vegard

> +	if (!valid_arg_len(bprm, len))
> +		return -E2BIG;
> +
> +	/* We're going to work our way backwards. */
> +	arg += len;
> +	bprm->p -= len;
> +	if (IS_ENABLED(CONFIG_MMU) && bprm->p < bprm->argmin)
> +		return -E2BIG;
> +
> +	while (len > 0) {
> +		unsigned int bytes_to_copy = min_t(unsigned int, len,
> +				min_not_zero(offset_in_page(pos), PAGE_SIZE));
> +		struct page *page;
> +		char *kaddr;
> +
> +		pos -= bytes_to_copy;
> +		arg -= bytes_to_copy;
> +		len -= bytes_to_copy;
> +
> +		page = get_arg_page(bprm, pos, 1);
> +		if (!page)
> +			return -E2BIG;
> +		kaddr = kmap_atomic(page);
> +		flush_arg_page(bprm, pos & PAGE_MASK, page);
> +		memcpy(kaddr + offset_in_page(pos), arg, bytes_to_copy);
> +		flush_kernel_dcache_page(page);
> +		kunmap_atomic(kaddr);
> +		put_arg_page(page);
> +	}
> +
> +	return 0;
>   }
>   EXPORT_SYMBOL(copy_string_kernel);
>   

Re: [merged] exec-open-code-copy_string_kernel.patch removed from -mm tree

[Christoph Hellwig]

On Tue, Jun 16, 2020 at 03:14:44PM +0200, Vegard Nossum wrote:
>
> On 2020-06-05 22:19, akpm@linux-foundation.org wrote:
>> The patch titled
>>       Subject: exec: open code copy_string_kernel
>> has been removed from the -mm tree.  Its filename was
>>       exec-open-code-copy_string_kernel.patch
>>
>> This patch was dropped because it was merged into mainline or a subsystem tree
>>
>> ------------------------------------------------------
>> From: Christoph Hellwig <hch@lst.de>
>> Subject: exec: open code copy_string_kernel
>>
>> Currently copy_string_kernel is just a wrapper around copy_strings that
>> simplifies the calling conventions and uses set_fs to allow passing a
>> kernel pointer.  But due to the fact the we only need to handle a single
>> kernel argument pointer, the logic can be sigificantly simplified while
>> getting rid of the set_fs.
>>
>> Link: http://lkml.kernel.org/r/20200501104105.2621149-3-hch@lst.de
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>
>>   fs/exec.c |   45 +++++++++++++++++++++++++++++++++++----------
>>   1 file changed, 35 insertions(+), 10 deletions(-)
>>
>> --- a/fs/exec.c~exec-open-code-copy_string_kernel
>> +++ a/fs/exec.c
>> @@ -592,17 +592,42 @@ out:
>>    */
>>   int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
>>   {
>> -	int r;
>> -	mm_segment_t oldfs = get_fs();
>> -	struct user_arg_ptr argv = {
>> -		.ptr.native = (const char __user *const  __user *)&arg,
>> -	};
>> -
>> -	set_fs(KERNEL_DS);
>> -	r = copy_strings(1, argv, bprm);
>> -	set_fs(oldfs);
>> +	int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */;
>> +	unsigned long pos = bprm->p;
>>   -	return r;
>> +	if (len == 0)
>> +		return -EFAULT;
>
> Just a quick question, how can len ever be 0 here when len was set to
> strnlen() + 1? Should the test be different?
>
> The old version (i.e. copy_strings()) seems to return -EFAULT when
> strnlen() returns 0.

So, the nasty part here is that strnlen_user has different semantics
from strnlen:

 - strlen excludes the terminating null byte and never returns error
   codes
 - strnlen_user includes the terminating null byte, and a 0 return
   means it could not access the user address (a condition that can't
   happen for strlen).

Now with that back to your original question:  I think then len == 0
check can just be removed without replacement.