linux-kernel - Re: [PATCH] KEYS: Don't write out to userspace while holding key semaphore

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Date:   Thu, 5 Mar 2020 16:30:18 -0500
From:   Waiman Long <longman@...hat.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mimi Zohar <zohar@...ux.ibm.com>, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Chris von Recklinghausen <crecklin@...hat.com>
Subject: Re: [PATCH] KEYS: Don't write out to userspace while holding key
 semaphore

On 3/5/20 4:16 PM, Eric Biggers wrote:
> On Thu, Mar 05, 2020 at 04:06:40PM -0500, Waiman Long wrote:
>> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
>> index 9b898c969558..564a4d187329 100644
>> --- a/security/keys/keyctl.c
>> +++ b/security/keys/keyctl.c
>> @@ -846,14 +846,36 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>>  can_read_key:
>>  	ret = -EOPNOTSUPP;
>>  	if (key->type->read) {
>> -		/* Read the data with the semaphore held (since we might sleep)
>> +		/*
>> +		 * Read the data with the semaphore held (since we might sleep)
>>  		 * to protect against the key being updated or revoked.
>> +		 *
>> +		 * Allocating a temporary buffer to hold the keys before
>> +		 * transferring them to user buffer to avoid potential
>> +		 * deadlock involving page fault and mmap_sem.
>>  		 */
>> +		char *tmpbuf = kmalloc(buflen, GFP_KERNEL);
> This is passing an arbitrarily large size from userspace into kmalloc().
>
> - Eric
>
That is true. Is there a limit of how much key data are expected to be
returned from the kernel? We could set an internal limit here on how
large the buffer can be.

Cheers,
Longman