linux-ext4 - Re: [PATCH] ext2/super: Fix a possible sleep-in-atomic bug in parse

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Message-ID: <20171007022839.GP21978@ZenIV.linux.org.uk>
Date:   Sat, 7 Oct 2017 03:28:39 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jia-Ju Bai <baijiaju1990@....com>, Jan Kara <jack@...e.com>,
        Sagi Grimberg <sagi@...mberg.me>, james.smart@...adcom.com,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext2/super: Fix a possible sleep-in-atomic bug in
 parse_options

On Sat, Oct 07, 2017 at 03:02:17AM +0100, Al Viro wrote:
> > I do wonder if we shouldn't just use something like
> > 
> >  "skip leading zeroes, copy to size-limited stack location instead"
> > 
> > because the input length really *is* limited once you skip leading
> > zeroes (and whatever base marker we have). We might have at most a
> > 64-bit value in octal, so 22 bytes max.
> > 
> > But I guess just changing the two GFP_KERNEL's to GFP_ATOMIC is much simpler.
> 
> 	There's match_strdup() as well...
> 
> 	FWIW, ext2 side also looks fishy; it might be cleaner if we
> collected new state into some object and applied it only after the last
> possible failure exit.  The entire "restore the original state" logics
> would go away...

	I'm not saying that the bug had been introduced by conversion to
spinlock, BTW - it was racy back when ext2_remount() relied upon BKL.
I hadn't considered the atomicity issues back then - mea culpa...