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Subject[PATCH 5.4 170/309] ext4: fix race conditions in ->d_compare() and ->d_hash()
Date
From: Eric Biggers <ebiggers@google.com>

commit ec772f01307a2c06ebf6cdd221e6b518a71ddae7 upstream.

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@zeniv.linux.org.uk>
Fixes: b886ee3e778e ("ext4: Support case-insensitive file name lookups")
Cc: <stable@vger.kernel.org> # v5.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200124041234.159740-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/dir.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -673,9 +673,11 @@ static int ext4_d_compare(const struct d
const char *str, const struct qstr *name)
{
struct qstr qstr = {.name = str, .len = len };
- struct inode *inode = dentry->d_parent->d_inode;
+ const struct dentry *parent = READ_ONCE(dentry->d_parent);
+ const struct inode *inode = READ_ONCE(parent->d_inode);

- if (!IS_CASEFOLDED(inode) || !EXT4_SB(inode->i_sb)->s_encoding) {
+ if (!inode || !IS_CASEFOLDED(inode) ||
+ !EXT4_SB(inode->i_sb)->s_encoding) {
if (len != name->len)
return -1;
return memcmp(str, name->name, len);
@@ -688,10 +690,11 @@ static int ext4_d_hash(const struct dent
{
const struct ext4_sb_info *sbi = EXT4_SB(dentry->d_sb);
const struct unicode_map *um = sbi->s_encoding;
+ const struct inode *inode = READ_ONCE(dentry->d_inode);
unsigned char *norm;
int len, ret = 0;

- if (!IS_CASEFOLDED(dentry->d_inode) || !um)
+ if (!inode || !IS_CASEFOLDED(inode) || !um)
return 0;

norm = kmalloc(PATH_MAX, GFP_ATOMIC);

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