linux-ext4 - Re: [PATCH 3/3] ext4: correctly detect when an xattr value has an invalid size

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Message-ID: <20161128235035.GC145516@google.com>
Date:   Mon, 28 Nov 2016 15:50:35 -0800
From:   Eric Biggers <ebiggers@...gle.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Subject: Re: [PATCH 3/3] ext4: correctly detect when an xattr value has an
 invalid size

On Mon, Nov 28, 2016 at 12:50:02PM -0700, Andreas Dilger wrote:
> On Nov 26, 2016, at 11:39 PM, Eric Biggers <ebiggers@...gle.com> wrote:
> > 
> > It was possible for an xattr value to have a very large size, which
> > would then pass validation on 32-bit architectures due to a pointer
> > wraparound.  Fix this by validating the size in a way which avoids
> > pointer wraparound.
> 
> It isn't actually possible for a valid xattr value to be very large.
> At most 65536 bytes even with large blocks, so it might be easier to
> directly check that e_value_size is not too large rather than trying
> to deal with values of 0xfffffffe bytes or similar?
> 

I suppose we could do something like

	EXT4_XATTR_SIZE(size) > end - value || size > EXT4_MAX_BLOCK_SIZE

instead of

	size > end - value || EXT4_XATTR_SIZE(size) > end - value

But I don't think it's really any better.

Eric
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