linux-ext4 - Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d

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Date:   Fri, 24 Jan 2020 22:35:22 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Daniel Rosenberg <drosen@...gle.com>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash()

On Thu, Jan 23, 2020 at 08:12:34PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Since ->d_compare() and ->d_hash() can be called in RCU-walk mode,
> ->d_parent and ->d_inode can be concurrently modified, and in
> particular, ->d_inode may be changed to NULL.  For ext4_d_hash() this
> resulted in a reproducible NULL dereference if a lookup is done in a
> directory being deleted, e.g. with:
> 
> 	int main()
> 	{
> 		if (fork()) {
> 			for (;;) {
> 				mkdir("subdir", 0700);
> 				rmdir("subdir");
> 			}
> 		} else {
> 			for (;;)
> 				access("subdir/file", 0);
> 		}
> 	}
> 
> ... or by running the 't_encrypted_d_revalidate' program from xfstests.
> Both repros work in any directory on a filesystem with the encoding
> feature, even if the directory doesn't actually have the casefold flag.
> 
> I couldn't reproduce a crash in ext4_d_compare(), but it appears that a
> similar crash is possible there.
> 
> Fix these bugs by reading ->d_parent and ->d_inode using READ_ONCE() and
> falling back to the case sensitive behavior if the inode is NULL.
> 
> Reported-by: Al Viro <viro@...iv.linux.org.uk>
> Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
> Cc: <stable@...r.kernel.org> # v5.2+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Thanks, applied.

						- Ted