[PATCH] ext4: fix race between ext4_sync_parent() and rename()

[PATCH] ext4: fix race between ext4_sync_parent() and rename()

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

'igrab(d_inode(dentry->d_parent))' without holding dentry->d_lock is
broken because without d_lock, d_parent can be concurrently changed due
to a rename().  Then if the old directory is immediately deleted, old
d_parent->inode can be NULL.  That causes a NULL dereference in igrab().

To fix this, use dget_parent() to safely grab a reference to the parent
dentry, which pins the inode.  This also eliminates the need to use
d_find_any_alias() other than for the initial inode, as we no longer
throw away the dentry at each step.

This is an extremely hard race to hit, but it is possible.  Adding a
udelay() in between the reads of ->d_parent and its ->d_inode makes it
reproducible on a no-journal filesystem using the following program:

    #include <fcntl.h>
    #include <unistd.h>

    int main()
    {
        if (fork()) {
            for (;;) {
                mkdir("dir1", 0700);
                int fd = open("dir1/file", O_RDWR|O_CREAT|O_SYNC);
                write(fd, "X", 1);
                close(fd);
            }
        } else {
            mkdir("dir2", 0700);
            for (;;) {
                rename("dir1/file", "dir2/file");
                rmdir("dir1");
            }
        }
    }

Fixes: d59729f4e794 ("ext4: fix races in ext4_sync_parent()")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ext4/fsync.c | 28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index e10206e7f4bbe7..093c359952cdba 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -44,30 +44,28 @@
  */
 static int ext4_sync_parent(struct inode *inode)
 {
-	struct dentry *dentry = NULL;
-	struct inode *next;
+	struct dentry *dentry, *next;
 	int ret = 0;
 
 	if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
 		return 0;
-	inode = igrab(inode);
+	dentry = d_find_any_alias(inode);
+	if (!dentry)
+		return 0;
 	while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
 		ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
-		dentry = d_find_any_alias(inode);
-		if (!dentry)
-			break;
-		next = igrab(d_inode(dentry->d_parent));
+
+		next = dget_parent(dentry);
 		dput(dentry);
-		if (!next)
-			break;
-		iput(inode);
-		inode = next;
+		dentry = next;
+		inode = dentry->d_inode;
+
 		/*
 		 * The directory inode may have gone through rmdir by now. But
 		 * the inode itself and its blocks are still allocated (we hold
-		 * a reference to the inode so it didn't go through
-		 * ext4_evict_inode()) and so we are safe to flush metadata
-		 * blocks and the inode.
+		 * a reference to the inode via its dentry), so it didn't go
+		 * through ext4_evict_inode()) and so we are safe to flush
+		 * metadata blocks and the inode.
 		 */
 		ret = sync_mapping_buffers(inode->i_mapping);
 		if (ret)
@@ -76,7 +74,7 @@ static int ext4_sync_parent(struct inode *inode)
 		if (ret)
 			break;
 	}
-	iput(inode);
+	dput(dentry);
 	return ret;
 }
 
-- 
2.26.2

Re: [PATCH] ext4: fix race between ext4_sync_parent() and rename()

[Sasha Levin]

Hi

[This is an automated email]

This commit has been processed because it contains a "Fixes:" tag
fixing commit: d59729f4e794 ("ext4: fix races in ext4_sync_parent()").

The bot has tested the following trees: v5.6.11, v5.4.39, v4.19.121, v4.14.179, v4.9.222, v4.4.222.

v5.6.11: Build OK!
v5.4.39: Build OK!
v4.19.121: Build OK!
v4.14.179: Build OK!
v4.9.222: Build OK!
v4.4.222: Failed to apply! Possible dependencies:
    6ae4c5a69877 ("ext4: cleanup ext4_sync_parent()")
    78d962510796 ("ext4: respect the nobarrier mount option in nojournal mode")


NOTE: The patch will not be queued to stable trees until it is upstream.

How should we proceed with this patch?

-- 
Thanks
Sasha

Re: [PATCH] ext4: fix race between ext4_sync_parent() and rename()

[Eric Biggers]

On Wed, May 06, 2020 at 11:31:40AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> 'igrab(d_inode(dentry->d_parent))' without holding dentry->d_lock is
> broken because without d_lock, d_parent can be concurrently changed due
> to a rename().  Then if the old directory is immediately deleted, old
> d_parent->inode can be NULL.  That causes a NULL dereference in igrab().
> 
> To fix this, use dget_parent() to safely grab a reference to the parent
> dentry, which pins the inode.  This also eliminates the need to use
> d_find_any_alias() other than for the initial inode, as we no longer
> throw away the dentry at each step.
> 
> This is an extremely hard race to hit, but it is possible.  Adding a
> udelay() in between the reads of ->d_parent and its ->d_inode makes it
> reproducible on a no-journal filesystem using the following program:
> 
>     #include <fcntl.h>
>     #include <unistd.h>
> 
>     int main()
>     {
>         if (fork()) {
>             for (;;) {
>                 mkdir("dir1", 0700);
>                 int fd = open("dir1/file", O_RDWR|O_CREAT|O_SYNC);
>                 write(fd, "X", 1);
>                 close(fd);
>             }
>         } else {
>             mkdir("dir2", 0700);
>             for (;;) {
>                 rename("dir1/file", "dir2/file");
>                 rmdir("dir1");
>             }
>         }
>     }
> 
> Fixes: d59729f4e794 ("ext4: fix races in ext4_sync_parent()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>


Any comments on this patch?

- Eric

Re: [PATCH] ext4: fix race between ext4_sync_parent() and rename()

[Eric Biggers]

On Wed, May 06, 2020 at 11:31:40AM -0700, Eric Biggers wrote:
>  		/*
>  		 * The directory inode may have gone through rmdir by now. But
>  		 * the inode itself and its blocks are still allocated (we hold
> -		 * a reference to the inode so it didn't go through
> -		 * ext4_evict_inode()) and so we are safe to flush metadata
> -		 * blocks and the inode.
> +		 * a reference to the inode via its dentry), so it didn't go
> +		 * through ext4_evict_inode()) and so we are safe to flush
> +		 * metadata blocks and the inode.
>  		 */
>  		ret = sync_mapping_buffers(inode->i_mapping);

Just realized that in this comment, the closing parenthesis I added after
"dentry" shouldn't be there.  Ted, feel free to fix this if you're so inclined.

- Eric