Subject: Re: Safeguards against incorrect fd flags for fsync()
From: Mark Dilger <hornschnorter@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
References: <20191009062640.GB21379@paquier.xyz>
 <82db297a-74f7-ca23-545b-3c531d692e4d@gmail.com>
 <20191125022858.GE37821@paquier.xyz>
 <107de5d3-6972-7566-04d1-4776b677b462@gmail.com>
Message-ID: <724f2e0a-9fd5-6e8a-5188-0fad8eb91bf7@gmail.com>
Date: Sun, 24 Nov 2019 20:18:38 -0800
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On 11/24/19 6:53 PM, Mark Dilger wrote:
> 
> 
> On 11/24/19 6:28 PM, Michael Paquier wrote:
>> On Thu, Nov 07, 2019 at 01:57:57PM -0800, Mark Dilger wrote:
>>> The code and comments don't clearly indicate what you have said in the
>>> email, that you are verifying directories are opened read-only and 
>>> files are
>>> opened either read-write or write-only.  I'd recommend changing the 
>>> comments
>>> a bit to make that clearer.
>>
>> Thanks for the suggestions, sounds fine to me.
>>
>>> I would also rearrange the code a little, as it is slightly clearer 
>>> to read:
>>>
>>>     if (x)
>>>         /* directory stuff */
>>>     else
>>>         /* file stuff */
>>>
>>> than as you have it:
>>>
>>>     if (!x)
>>>         /* file stuff */
>>>     else
>>>         /* directory stuff */
>>
>> The check order in the former patch is consistent with what's done at
>> the top of fsync_fname_ext(), still I can see your point.  So let's do
>> as you suggest.
>>
>>> I'm a little uncertain about ignoring fstat errors as you do, but 
>>> left that
>>> part of the logic alone.  I understand that any fstat error will 
>>> likely be
>>> immediately followed by another error when the fsync is attempted, but
>>> relying on that seems vaguely similar to the security vulnerability of
>>> checking permissions and then opening a file as two separate operations.
>>> Not sure the analogy actually holds for fstat before fsync, though.
>>
>> The only possible error which could be expected here would be a ENOENT
>> so we could filter after that, but fsync() would most likely complain
>> about that so it sounds better to let it do its work with its own
>> logging, which would be more helpful for the user, if of course we
>> have fsync=on in postgresql.conf.
>>
>>> Attached is a revised version of the patch.  Perhaps you can check 
>>> what I've
>>> done and tell me if I've broken it.
>>
>> Thanks for the review.  I was wondering why I did not do that as well
>> for file_utils.c, just to find out that fsync_fname() is the only
>> entry point in file_utils.c.  Anyway, the patch had a problem
>> regarding fcntl() which is not available on Windows (see for example
>> pg_set_noblock in noblock.c).  Performing the sanity check will allow
>> to catch any problems for all platforms we support, so let's just skip
>> it for Windows.  For this reason it is better as well to update errno
>> to 0 after the fstat() call.  Who knows...  Attached is an updated
>> version, with your changes included.  How does that look?
> 
> That looks great, thank you, but I have not tested it yet.  I'll go do
> that now....

Ok, it passes all regression tests, and I played around with
intentionally breaking the code to open file descriptors in
the wrong mode.  The assertion appears to work as intended.

I'd say this is ready for commit.

-- 
Mark Dilger