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Date: Fri, 24 Jan 2020 13:04:25 +0800 From: Gao Xiang <hsiangkao@....com> 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() Hi Eric, 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> > --- > fs/ext4/dir.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c > index 8964778aabefb..0129d14629881 100644 > --- a/fs/ext4/dir.c > +++ b/fs/ext4/dir.c > @@ -671,9 +671,11 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len, > 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); I'm not sure if we really need READ_ONCE d_parent here (p.s. d_parent won't be NULL anyway), and d_seq will guard all its validity. If I'm wrong, correct me kindly... Otherwise, it looks good to me... Reviewed-by: Gao Xiang <gaoxiang25@...wei.com> Thanks, Gao Xiang > + 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); > @@ -686,10 +688,11 @@ static int ext4_d_hash(const struct dentry *dentry, struct qstr *str) > { > 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); > -- > 2.25.0 >
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