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ext4: fix race between ext4_sync_parent() and rename()
'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: d59729f4e7
("ext4: fix races in ext4_sync_parent()")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200506183140.541194-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
parent
80dd4978dd
commit
08adf452e6
1 changed files with 13 additions and 15 deletions
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@ -44,30 +44,28 @@
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*/
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static int ext4_sync_parent(struct inode *inode)
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{
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struct dentry *dentry = NULL;
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struct inode *next;
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struct dentry *dentry, *next;
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int ret = 0;
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if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
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return 0;
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inode = igrab(inode);
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dentry = d_find_any_alias(inode);
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if (!dentry)
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return 0;
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while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
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ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
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dentry = d_find_any_alias(inode);
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if (!dentry)
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break;
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next = igrab(d_inode(dentry->d_parent));
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next = dget_parent(dentry);
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dput(dentry);
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if (!next)
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break;
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iput(inode);
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inode = next;
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dentry = next;
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inode = dentry->d_inode;
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/*
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* The directory inode may have gone through rmdir by now. But
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* the inode itself and its blocks are still allocated (we hold
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* a reference to the inode so it didn't go through
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* ext4_evict_inode()) and so we are safe to flush metadata
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* blocks and the inode.
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* a reference to the inode via its dentry), so it didn't go
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* through ext4_evict_inode()) and so we are safe to flush
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* metadata blocks and the inode.
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*/
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ret = sync_mapping_buffers(inode->i_mapping);
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if (ret)
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@ -76,7 +74,7 @@ static int ext4_sync_parent(struct inode *inode)
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if (ret)
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break;
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}
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iput(inode);
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dput(dentry);
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return ret;
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}
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