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Subject: Re: [PATCH][v2] proc: use vmalloc for our kernel buffer
To:     Christoph Hellwig <hch@lst.de>
Cc:     viro@ZenIV.linux.org.uk, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, kernel-team@fb.com,
        willy@infradead.org
References: <20200813145305.805730-1-josef@toxicpanda.com>
 <20200813153356.857625-1-josef@toxicpanda.com>
 <20200813153722.GA13844@lst.de>
From:   Josef Bacik <josef@toxicpanda.com>
Message-ID: <974e469e-e73d-6c3e-9167-fad003f1dfb9@toxicpanda.com>
Date:   Thu, 13 Aug 2020 11:40:00 -0400
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On 8/13/20 11:37 AM, Christoph Hellwig wrote:
> On Thu, Aug 13, 2020 at 11:33:56AM -0400, Josef Bacik wrote:
>> Since
>>
>>    sysctl: pass kernel pointers to ->proc_handler
>>
>> we have been pre-allocating a buffer to copy the data from the proc
>> handlers into, and then copying that to userspace.  The problem is this
>> just blind kmalloc()'s the buffer size passed in from the read, which in
>> the case of our 'cat' binary was 64kib.  Order-4 allocations are not
>> awesome, and since we can potentially allocate up to our maximum order,
>> use vmalloc for these buffers.
>>
>> Fixes: 32927393dc1c ("sysctl: pass kernel pointers to ->proc_handler")
>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>> ---
>> v1->v2:
>> - Make vmemdup_user_nul actually do the right thing...sorry about that.
>>
>>   fs/proc/proc_sysctl.c  |  6 +++---
>>   include/linux/string.h |  1 +
>>   mm/util.c              | 27 +++++++++++++++++++++++++++
>>   3 files changed, 31 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
>> index 6c1166ccdaea..207ac6e6e028 100644
>> --- a/fs/proc/proc_sysctl.c
>> +++ b/fs/proc/proc_sysctl.c
>> @@ -571,13 +571,13 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
>>   		goto out;
>>   
>>   	if (write) {
>> -		kbuf = memdup_user_nul(ubuf, count);
>> +		kbuf = vmemdup_user_nul(ubuf, count);
> 
> Given that this can also do a kmalloc and thus needs to be paired
> with kvfree shouldn't it be kvmemdup_user_nul?
> 

There's an existing vmemdup_user that does kvmalloc, so I followed the existing 
naming convention.  Do you want me to change them both?  Thanks,

Josef