linux-ext4 - Re: ext4_has_free_blocks always checks cap_sys

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Date:	Fri, 24 Oct 2008 12:00:36 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Eric Paris <eparis@...hat.com>, linux-ext4@...r.kernel.org,
	selinux@...ho.nsa.gov, sds@...ho.nsa.gov, esandeen@...hat.com,
	tytso@....edu, dwalsh@...hat.com,
	linux-security-module@...r.kernel.org
Subject: Re: ext4_has_free_blocks always checks cap_sys_resource and makes
	SELinux unhappy


在 2008-10-24五的 11:56 -0500，Eric Sandeen写道：
> Eric Paris wrote:
> > I'm running an ext4 root filesystem and regularly get SELinux denials
> > like:
> > 
> > Oct 16 08:32:55 localhost kernel: type=1400 audit(1224160369.076:5):
> > avc: denied  { sys_resource } for  pid=1624 comm="dbus-daemon"
> > capability=24 scontext=system_u:system_r:system_dbusd_t:s0
> > tcontext=system_u:system_r:system_dbusd_t:s0 tclass=capability
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=467216
> > 
> > Since this doesn't happen with people who have ext3 filesystems but
> > everything else the same it lead me to look at ext4.  I see that
> > ext?_has_free_blocks() has changed since ext3 and now we always check
> > for capable(CAP_SYS_RESOUCE).  If a process actually has the capability
> > in pE (as many root processes would) but doesn't have the capability in
> > SELinux policy we will get a denial.
> > 
> > I can think of a couple ways to fix this:
> > 
> > the first (and one I like) is to change ext4 to stop checking
> > CAP_SYS_RESOURCE all the time.  It's not really 'pretty' but I think you
> > would actually get a better performing function.  Just always calculate
> > root_blocks and if we don't have enough room then then do the whole
> > check to see if are root and recalculate without root_blocks.  I'd guess
> > that a great majority of the time operations will succeed even with a
> > non-zero root_blocks and I would guess that most process aren't going to
> > be root processes and so we would be calculating root_blocks anyway.
> > This would (like ext3) only cause these denials when it was filled up.
> > We've been living with that forever, so I don't see a problem there...
> 
> Thanks Eric, I'll look into this.  It seems that ext4_has_free_blocks is
> now overly complex; it used to return how many blocks are available, if
> that number is < nblocks, but the single caller now only checks
> success/failure for having nblocks free.  I'll see if I can simplify it
> and delay the cap check as you suggest.
> 

Most functionality in ext4_has_free_blocks()  is duplicated in
ext4_claim_free_blocks(). I guess the ext4_has_free_blocks() could be
simplified a bit, or the two functional merge into one.

The delay cap check sounds right thing to me.

Mingming

> -Eric
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