From 605c912bb843c024b1ed173dc427cd5c08e5d54d Mon Sep 17 00:00:00 2001 From: Artem Bityutskiy Date: Fri, 28 Jun 2013 14:15:15 +0300 Subject: UBIFS: fix a horrid bug Al Viro pointed me to the fact that '->readdir()' and '->llseek()' have no mutual exclusion, which means the 'ubifs_dir_llseek()' can be run while we are in the middle of 'ubifs_readdir()'. This means that 'file->private_data' can be freed while 'ubifs_readdir()' uses it, and this is a very bad bug: not only 'ubifs_readdir()' can return garbage, but this may corrupt memory and lead to all kinds of problems like crashes an security holes. This patch fixes the problem by using the 'file->f_version' field, which '->llseek()' always unconditionally sets to zero. We set it to 1 in 'ubifs_readdir()' and whenever we detect that it became 0, we know there was a seek and it is time to clear the state saved in 'file->private_data'. I tested this patch by writing a user-space program which runds readdir and seek in parallell. I could easily crash the kernel without these patches, but could not crash it with these patches. Cc: stable@vger.kernel.org Reported-by: Al Viro Tested-by: Artem Bityutskiy Signed-off-by: Artem Bityutskiy Signed-off-by: Al Viro --- fs/ubifs/dir.c | 30 +++++++++++++++++++++++++++--- 1 file changed, 27 insertions(+), 3 deletions(-) diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c index 8e587afd773..605af512aec 100644 --- a/fs/ubifs/dir.c +++ b/fs/ubifs/dir.c @@ -365,6 +365,24 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir) */ return 0; + if (file->f_version == 0) { + /* + * The file was seek'ed, which means that @file->private_data + * is now invalid. This may also be just the first + * 'ubifs_readdir()' invocation, in which case + * @file->private_data is NULL, and the below code is + * basically a no-op. + */ + kfree(file->private_data); + file->private_data = NULL; + } + + /* + * 'generic_file_llseek()' unconditionally sets @file->f_version to + * zero, and we use this for detecting whether the file was seek'ed. + */ + file->f_version = 1; + /* File positions 0 and 1 correspond to "." and ".." */ if (pos == 0) { ubifs_assert(!file->private_data); @@ -438,6 +456,14 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir) file->f_pos = pos = key_hash_flash(c, &dent->key); file->private_data = dent; cond_resched(); + + if (file->f_version == 0) + /* + * The file was seek'ed meanwhile, lets return and start + * reading direntries from the new position on the next + * invocation. + */ + return 0; } out: @@ -448,15 +474,13 @@ out: kfree(file->private_data); file->private_data = NULL; + /* 2 is a special value indicating that there are no more direntries */ file->f_pos = 2; return 0; } -/* If a directory is seeked, we have to free saved readdir() state */ static loff_t ubifs_dir_llseek(struct file *file, loff_t offset, int whence) { - kfree(file->private_data); - file->private_data = NULL; return generic_file_llseek(file, offset, whence); } -- cgit v1.2.3