From e3def70ce83c07314bb2614865a08cf775563b3c Mon Sep 17 00:00:00 2001 From: Vasiliy Kulikov Date: Wed, 5 Oct 2011 11:43:53 +1100 Subject: [PATCH] proc: force dcache drop on unauthorized access The patch "proc: fix races against execve() of /proc/PID/fd**" is still a partial fix for a setxid problem. link(2) is a yet another way to identify whether a specific fd is opened by a privileged process. By calling link(2) against /proc/PID/fd/* an attacker may identify whether the fd number is valid for PID by analysing link(2) return code. Both getattr() and link() can be used by the attacker iff the dentry is present in the dcache. In this case ->lookup() is not called and the only way to check ptrace permissions is either operation handler or ->revalidate(). The easiest solution to prevent any unauthorized access to /proc/PID/fd*/ files is to force the dentry drop on each unauthorized access attempt. If an attacker keeps opened fd of /proc/PID/fd/ and dcache contains a specific dentry for some /proc/PID/fd/XXX, any future attemp to use the dentry by the attacker would lead to the dentry drop as a result of a failed ptrace check in ->revalidate(). Then the attacker cannot spawn a dentry for the specific fd number because of ptrace check in ->lookup(). The dentry drop can be still observed by an attacker by analysing information from /proc/slabinfo, which is addressed in the successive patch. Signed-off-by: Vasiliy Kulikov Cc: Cyrill Gorcunov Cc: Al Viro Cc: Christoph Lameter Cc: Pekka Enberg Cc: Matt Mackall Cc: Alexey Dobriyan Signed-off-by: Andrew Morton --- fs/proc/base.c | 42 ++++++------------------------------------ 1 file changed, 6 insertions(+), 36 deletions(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index d4f4913f00db..754e748b51c9 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -1652,46 +1652,12 @@ out: return error; } -static int proc_pid_fd_link_getattr(struct vfsmount *mnt, struct dentry *dentry, - struct kstat *stat) -{ - struct inode *inode = dentry->d_inode; - struct task_struct *task = get_proc_task(inode); - int rc; - - if (task == NULL) - return -ESRCH; - - rc = -EACCES; - if (lock_trace(task)) - goto out_task; - - generic_fillattr(inode, stat); - unlock_trace(task); - rc = 0; -out_task: - put_task_struct(task); - return rc; -} - static const struct inode_operations proc_pid_link_inode_operations = { .readlink = proc_pid_readlink, .follow_link = proc_pid_follow_link, .setattr = proc_setattr, }; -static const struct inode_operations proc_fdinfo_link_inode_operations = { - .setattr = proc_setattr, - .getattr = proc_pid_fd_link_getattr, -}; - -static const struct inode_operations proc_fd_link_inode_operations = { - .readlink = proc_pid_readlink, - .follow_link = proc_pid_follow_link, - .setattr = proc_setattr, - .getattr = proc_pid_fd_link_getattr, -}; - /* building an inode */ @@ -2000,6 +1966,11 @@ static int tid_fd_revalidate(struct dentry *dentry, struct nameidata *nd) task = get_proc_task(inode); fd = proc_fd(inode); + if (!ptrace_may_access(task, PTRACE_MODE_READ)) { + put_task_struct(task); + task = NULL; + } + if (task) { files = get_files_struct(task); if (files) { @@ -2072,7 +2043,7 @@ static struct dentry *proc_fd_instantiate(struct inode *dir, spin_unlock(&files->file_lock); put_files_struct(files); - inode->i_op = &proc_fd_link_inode_operations; + inode->i_op = &proc_pid_link_inode_operations; inode->i_size = 64; ei->op.proc_get_link = proc_fd_link; d_set_d_op(dentry, &tid_fd_dentry_operations); @@ -2254,7 +2225,6 @@ static struct dentry *proc_fdinfo_instantiate(struct inode *dir, ei->fd = fd; inode->i_mode = S_IFREG | S_IRUSR; inode->i_fop = &proc_fdinfo_file_operations; - inode->i_op = &proc_fdinfo_link_inode_operations; d_set_d_op(dentry, &tid_fd_dentry_operations); d_add(dentry, inode); /* Close the race of the process dying before we return the dentry */ -- 2.39.2