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Subject: Re: [PATCH] sysctl: Limit the size of I/Os to PAGE_SIZE
From:   Josef Bacik <josef@toxicpanda.com>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>
Cc:     linux-abi@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Alexey Dobriyan <adobriyan@gmail.com>
References: <20210513160649.2280429-1-willy@infradead.org>
 <47a34aa5-ad1a-6259-d9cb-f85f314f9ffb@toxicpanda.com>
Message-ID: <b761c108-25c4-634a-8aed-d88cc00aedfe@toxicpanda.com>
Date:   Thu, 13 May 2021 12:15:34 -0400
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On 5/13/21 12:12 PM, Josef Bacik wrote:
> On 5/13/21 12:06 PM, Matthew Wilcox (Oracle) wrote:
>> We currently allow a read or a write that is up to KMALLOC_MAX_SIZE.
>> This has caused problems when cat decides to do a 64kB read and
>> so we allocate a 64kB buffer for the sysctl handler to store into.
>> The immediate problem was fixed by switching to kvmalloc(), but it's
>> ridiculous to allocate so much memory to read what is likely to be a
>> few bytes.
>>
>> sysfs limits reads and writes to PAGE_SIZE, and I feel we should do the
>> same for sysctl.  The largest sysctl anyone's been able to come up with
>> is 433 bytes for /proc/sys/dev/cdrom/info
>>
>> This will allow simplifying the BPF sysctl code later, but I'll leave
>> that for someone who understands it better.
>>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>> ---
>>   fs/proc/proc_sysctl.c | 15 +++++++++------
>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
>> index dea0f5ee540c..a97a8a4ff270 100644
>> --- a/fs/proc/proc_sysctl.c
>> +++ b/fs/proc/proc_sysctl.c
>> @@ -562,11 +562,14 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, 
>> struct iov_iter *iter,
>>       if (!table->proc_handler)
>>           goto out;
>> -    /* don't even try if the size is too large */
>> +    /* reads may return short values; large writes must fail now */
>> +    if (count >= PAGE_SIZE) {
>> +        if (write)
>> +            goto out;
>> +        count = PAGE_SIZE;
>> +    }
>>       error = -ENOMEM;
>> -    if (count >= KMALLOC_MAX_SIZE)
>> -        goto out;
>> -    kbuf = kvzalloc(count + 1, GFP_KERNEL);
>> +    kbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
>>       if (!kbuf)
>>           goto out;
> 
> Below here we have
> 
> kbuf[count] = '\0';
> 

Nevermind I just re-read it and it's

if (write)
	kbuf[count] = '\0';

I'm stupid, you can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef