Re: [PATCH v3 3/3] KEYS: Use kvmalloc() to better handle large buffer allocation

[Waiman Long <longman@redhat.com>]

On 3/13/20 12:43 PM, Eric Biggers wrote:
> On Fri, Mar 13, 2020 at 11:21:02AM -0400, Waiman Long wrote:
>> For large multi-page temporary buffer allocation, the security/keys
>> subsystem don't need contiguous physical pages. It will work perfectly
>> fine with virtually mapped pages.
>>
>> Replace the kmalloc() call by kvmalloc() and provide a __kvzfree()
>> helper function to clear and free the kvmalloc'ed buffer. This will
>> reduce the chance of memory allocation failure just because of highly
>> fragmented pages.
>>
>> Suggested-by: David Howells <dhowells@redhat.com>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  security/keys/internal.h | 14 ++++++++++++++
>>  security/keys/keyctl.c   | 10 +++++-----
>>  2 files changed, 19 insertions(+), 5 deletions(-)
>>
>> diff --git a/security/keys/internal.h b/security/keys/internal.h
>> index ba3e2da14cef..855b11eb73ee 100644
>> --- a/security/keys/internal.h
>> +++ b/security/keys/internal.h
>> @@ -16,6 +16,8 @@
>>  #include <linux/keyctl.h>
>>  #include <linux/refcount.h>
>>  #include <linux/compat.h>
>> +#include <linux/mm.h>
>> +#include <linux/vmalloc.h>
>>  
>>  struct iovec;
>>  
>> @@ -349,4 +351,16 @@ static inline void key_check(const struct key *key)
>>  
>>  #endif
>>  
>> +/*
>> + * Helper function to clear and free a kvmalloc'ed memory object.
>> + */
>> +static inline void __kvzfree(const void *addr, size_t len)
>> +{
>> +	if (is_vmalloc_addr(addr)) {
>> +		memset((void *)addr, 0, len);
>> +		vfree(addr);
>> +	} else {
>> +		kzfree(addr);
>> +	}
>> +}
> Since this takes the length as a parameter, it can be simplified to:
>
> static inline void __kvzfree(const void *addr, size_t len)
> {
> 	if (addr) {
> 		memset((void *)addr, 0, len);
> 		kvfree(addr);
> 	}
> }
Yes, that will work too.
>>  			if (!tmpbuf || unlikely(ret > tmpbuflen)) {
>>  				if (unlikely(tmpbuf))
>> -					kzfree(tmpbuf);
>> +					__kvzfree(tmpbuf, tmpbuflen);
> Both kzfree() and __kvzfree() handle a NULL pointer, so there's no need for the
> NULL check first.
>
I would like to keep this one because of the unlikely annotation.


>> @@ -920,7 +920,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>>  				ret = -EFAULT;
>>  		}
>>  		if (tmpbuf)
>> -			kzfree(tmpbuf);
>> +			__kvzfree(tmpbuf, tmpbuflen);
> Likewise here.  No need for the NULL check.

Yes, that tmpbuf check is not really necessary, but it doesn't harm either.

My plan is to send out a mm patch to officially add the kvzfree()
function to mm/util.c. I will remove the tmpbuf check at that time if
you don't mind.

Cheers,
Longman