linux-kernel - Re: [PATCH] ext4: fix use-after-free in dx

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Message-ID: <20190508082926.GC19198@codeaurora.org>
Date:   Wed, 8 May 2019 13:59:26 +0530
From:   Sahitya Tummala <stummala@...eaurora.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: fix use-after-free in dx_release()

On Wed, May 08, 2019 at 01:09:47AM -0600, Andreas Dilger wrote:
> On May 8, 2019, at 12:13 AM, Sahitya Tummala <stummala@...eaurora.org> wrote:
> > 
> > The buffer_head (frames[0].bh) and it's corresping page can be
> > potentially free'd once brelse() is done inside the for loop
> > but before the for loop exits in dx_release(). It can be free'd
> > in another context, when the page cache is flushed via
> > drop_caches_sysctl_handler(). This results into below data abort
> > when accessing info->indirect_levels in dx_release().
> > 
> > Unable to handle kernel paging request at virtual address ffffffc17ac3e01e
> > Call trace:
> > dx_release+0x70/0x90
> > ext4_htree_fill_tree+0x2d4/0x300
> > ext4_readdir+0x244/0x6f8
> > iterate_dir+0xbc/0x160
> > SyS_getdents64+0x94/0x174
> > 
> > Signed-off-by: Sahitya Tummala <stummala@...eaurora.org>
> 
> The patch looks reasonable, but there is a danger that it may be
> "optimized" back to the pre-patch form again.  It probably makes
> sense to include a comment like:
> 
> 	/* save local copy, "info" may be freed after brelse() */

Thanks for reviewing it. Sure, I will add the comment.

> 
> Looks fine otherwise.
> 
> Reviewed-by: Andreas Dilger <adilger@...ger.ca>
> 
> > ---
> > fs/ext4/namei.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > index 4181c9c..7e6c298 100644
> > --- a/fs/ext4/namei.c
> > +++ b/fs/ext4/namei.c
> > @@ -871,12 +871,14 @@ static void dx_release(struct dx_frame *frames)
> > {
> > 	struct dx_root_info *info;
> > 	int i;
> > +	unsigned int indirect_levels;
> > 
> > 	if (frames[0].bh == NULL)
> > 		return;
> > 
> > 	info = &((struct dx_root *)frames[0].bh->b_data)->info;
> > -	for (i = 0; i <= info->indirect_levels; i++) {
> > +	indirect_levels = info->indirect_levels;
> > +	for (i = 0; i <= indirect_levels; i++) {
> > 		if (frames[i].bh == NULL)
> > 			break;
> > 		brelse(frames[i].bh);
> > --
> > Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
> > 
> 
> 
> Cheers, Andreas
> 
> 
> 
> 
> 



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