Re: [PATCH] mm/backing-dev.c: check user buffer length before copy data to the related user buffer.

[Chen Gang <gang.chen@asianux.com>]

On 08/21/2013 07:29 AM, Andrew Morton wrote:
> On Tue, 20 Aug 2013 11:23:24 +0800 Chen Gang <gang.chen@asianux.com> wrote:
> 
>> '*lenp' may be less than "sizeof(kbuf)", need check it before the next
>> copy_to_user().
>>
>> pdflush_proc_obsolete() is called by sysctl which 'procname' is
>> "nr_pdflush_threads", if the user passes buffer length less than
>> "sizeof(kbuf)", it will cause issue.
>>
>> ...
>>
>> --- a/mm/backing-dev.c
>> +++ b/mm/backing-dev.c
>> @@ -649,7 +649,7 @@ int pdflush_proc_obsolete(struct ctl_table *table, int write,
>>  {
>>  	char kbuf[] = "0\n";
>>  
>> -	if (*ppos) {
>> +	if (*ppos || *lenp < sizeof(kbuf)) {
>>  		*lenp = 0;
>>  		return 0;
>>  	}
> 
> Well sort-of.  If userspace opens /proc/sys/vm/nr_pdflush_threads and
> then does a series of one-byte reads, the kernel should return "0" on the
> first read, "\n" on the second and then EOF.
> 

Excuse me for my English, I guess your meaning is

  "this patch is OK, but can be improvement"

Is it correct ?

> However this usually doesn't work in /proc anyway :(
> 
> akpm3:/tmp> cat /proc/sys/vm/max_map_count         
> 65530
> akpm3:/tmp> dd if=/proc/sys/vm/max_map_count of=foo bs=1
> 1+0 records in
> 1+0 records out
> 1 byte (1 B) copied, 0.00011963 s, 8.4 kB/s
> akpm3:/tmp> wc foo
> 0 1 1 foo
> 

Also excuse me for my English, I guess your meaning is:

  for "dd if=/proc/sys/vm/* of=/tmp/foo bs=1",
  need return "1 byte (1 B) copied", not 2 bytes (memory overflow), or 0 which meaningless.



If both of my guesses are correct, is the diff below better ?

----------------------------diff begin----------------------------------

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e04454c..d3be432 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -647,19 +647,19 @@ EXPORT_SYMBOL(wait_iff_congested);
 int pdflush_proc_obsolete(struct ctl_table *table, int write,
 			void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-	char kbuf[] = "0\n";
+	char kbuf[2] = {'0', '\n'};
 
 	if (*ppos) {
 		*lenp = 0;
 		return 0;
 	}
 
-	if (copy_to_user(buffer, kbuf, sizeof(kbuf)))
+	if (copy_to_user(buffer, kbuf, min(*lenp, sizeof(kbuf))))
 		return -EFAULT;
 	printk_once(KERN_WARNING "%s exported in /proc is scheduled for removal\n",
 			table->procname);
 
-	*lenp = 2;
+	*lenp = min(*lenp, sizeof(kbuf));
 	*ppos += *lenp;
 	return 2;
 }

----------------------------diff end------------------------------------


The diff above is not tested, if OK, I will send related patch after
finish the related test.


Thanks.
-- 
Chen Gang

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