linux-kernel - Re: [PATCH v2] vfs: read file_handle only once in handle_to

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Date:	Tue, 02 Jun 2015 11:23:35 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	"security@...nel.org" <security@...nel.org>
Subject: Re: [PATCH v2] vfs: read file_handle only once in handle_to_path

Hi,

This commit fixes a race condition an unprivileged user can exploit to corrupt
kernel memory. I can't seem to get anyone to grab it for the past 4+ months.

Help please?


Thanks,
Sasha

On 04/30/2015 11:04 AM, Sasha Levin wrote:
> ping?
> 
> On 01/28/2015 03:30 PM, Sasha Levin wrote:
>> We used to read file_handle twice. Once to get the amount of extra bytes, and
>> once to fetch the entire structure.
>>
>> This may be problematic since we do size verifications only after the first
>> read, so if the number of extra bytes changes in userspace between the first
>> and second calls, we'll have an incoherent view of file_handle.
>>
>> Instead, read the constant size once, and copy that over to the final
>> structure without having to re-read it again.
>>
>> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
>> ---
>> Change in v2:
>>  - Use the f_handle pointer rather than size of struct
>>
>>  fs/fhandle.c |    5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/fhandle.c b/fs/fhandle.c
>> index 999ff5c..d59712d 100644
>> --- a/fs/fhandle.c
>> +++ b/fs/fhandle.c
>> @@ -195,8 +195,9 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
>>  		goto out_err;
>>  	}
>>  	/* copy the full handle */
>> -	if (copy_from_user(handle, ufh,
>> -			   sizeof(struct file_handle) +
>> +	*handle = f_handle;
>> +	if (copy_from_user(&handle->f_handle,
>> +			   &ufh->f_handle,
>>  			   f_handle.handle_bytes)) {
>>  		retval = -EFAULT;
>>  		goto out_handle;
>>
> 

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