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![Jann Horn](/assets/img/avatar_default.png)
By checking the effective credentials instead of the real UID / permitted capabilities, ensure that the calling process actually intended to use its credentials. To ensure that all ptrace checks use the correct caller credentials (e.g. in case out-of-tree code or newly added code omits the PTRACE_MODE_*CREDS flag), use two new flags and require one of them to be set. The problem was that when a privileged task had temporarily dropped its privileges, e.g. by calling setreuid(0, user_uid), with the intent to perform following syscalls with the credentials of a user, it still passed ptrace access checks that the user would not be able to pass. While an attacker should not be able to convince the privileged task to perform a ptrace() syscall, this is a problem because the ptrace access check is reused for things in procfs. In particular, the following somewhat interesting procfs entries only rely on ptrace access checks: /proc/$pid/stat - uses the check for determining whether pointers should be visible, useful for bypassing ASLR /proc/$pid/maps - also useful for bypassing ASLR /proc/$pid/cwd - useful for gaining access to restricted directories that contain files with lax permissions, e.g. in this scenario: lrwxrwxrwx root root /proc/13020/cwd -> /root/foobar drwx------ root root /root drwxr-xr-x root root /root/foobar -rw-r--r-- root root /root/foobar/secret Therefore, on a system where a root-owned mode 6755 binary changes its effective credentials as described and then dumps a user-specified file, this could be used by an attacker to reveal the memory layout of root's processes or reveal the contents of files he is not allowed to access (through /proc/$pid/cwd). [akpm@linux-foundation.org: fix warning] Signed-off-by: Jann Horn <jann@thejh.net> Acked-by: Kees Cook <keescook@chromium.org> Cc: Casey Schaufler <casey@schaufler-ca.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: James Morris <james.l.morris@oracle.com> Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Willy Tarreau <w@1wt.eu> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
176 lines
4 KiB
C
176 lines
4 KiB
C
#include <linux/proc_fs.h>
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#include <linux/nsproxy.h>
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#include <linux/ptrace.h>
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#include <linux/namei.h>
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#include <linux/file.h>
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#include <linux/utsname.h>
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#include <net/net_namespace.h>
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#include <linux/ipc_namespace.h>
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#include <linux/pid_namespace.h>
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#include <linux/user_namespace.h>
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#include "internal.h"
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static const struct proc_ns_operations *ns_entries[] = {
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#ifdef CONFIG_NET_NS
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&netns_operations,
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#endif
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#ifdef CONFIG_UTS_NS
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&utsns_operations,
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#endif
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#ifdef CONFIG_IPC_NS
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&ipcns_operations,
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#endif
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#ifdef CONFIG_PID_NS
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&pidns_operations,
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#endif
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#ifdef CONFIG_USER_NS
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&userns_operations,
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#endif
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&mntns_operations,
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};
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static const char *proc_ns_get_link(struct dentry *dentry,
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struct inode *inode,
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struct delayed_call *done)
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{
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const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns_ops;
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struct task_struct *task;
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struct path ns_path;
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void *error = ERR_PTR(-EACCES);
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if (!dentry)
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return ERR_PTR(-ECHILD);
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task = get_proc_task(inode);
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if (!task)
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return error;
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if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
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error = ns_get_path(&ns_path, task, ns_ops);
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if (!error)
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nd_jump_link(&ns_path);
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}
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put_task_struct(task);
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return error;
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}
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static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int buflen)
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{
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struct inode *inode = d_inode(dentry);
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const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns_ops;
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struct task_struct *task;
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char name[50];
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int res = -EACCES;
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task = get_proc_task(inode);
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if (!task)
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return res;
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if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
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res = ns_get_name(name, sizeof(name), task, ns_ops);
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if (res >= 0)
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res = readlink_copy(buffer, buflen, name);
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}
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put_task_struct(task);
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return res;
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}
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static const struct inode_operations proc_ns_link_inode_operations = {
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.readlink = proc_ns_readlink,
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.get_link = proc_ns_get_link,
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.setattr = proc_setattr,
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};
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static int proc_ns_instantiate(struct inode *dir,
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struct dentry *dentry, struct task_struct *task, const void *ptr)
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{
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const struct proc_ns_operations *ns_ops = ptr;
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struct inode *inode;
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struct proc_inode *ei;
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inode = proc_pid_make_inode(dir->i_sb, task);
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if (!inode)
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goto out;
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ei = PROC_I(inode);
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inode->i_mode = S_IFLNK|S_IRWXUGO;
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inode->i_op = &proc_ns_link_inode_operations;
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ei->ns_ops = ns_ops;
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d_set_d_op(dentry, &pid_dentry_operations);
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d_add(dentry, inode);
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/* Close the race of the process dying before we return the dentry */
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if (pid_revalidate(dentry, 0))
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return 0;
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out:
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return -ENOENT;
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}
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static int proc_ns_dir_readdir(struct file *file, struct dir_context *ctx)
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{
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struct task_struct *task = get_proc_task(file_inode(file));
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const struct proc_ns_operations **entry, **last;
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if (!task)
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return -ENOENT;
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if (!dir_emit_dots(file, ctx))
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goto out;
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if (ctx->pos >= 2 + ARRAY_SIZE(ns_entries))
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goto out;
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entry = ns_entries + (ctx->pos - 2);
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last = &ns_entries[ARRAY_SIZE(ns_entries) - 1];
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while (entry <= last) {
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const struct proc_ns_operations *ops = *entry;
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if (!proc_fill_cache(file, ctx, ops->name, strlen(ops->name),
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proc_ns_instantiate, task, ops))
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break;
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ctx->pos++;
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entry++;
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}
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out:
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put_task_struct(task);
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return 0;
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}
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const struct file_operations proc_ns_dir_operations = {
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.read = generic_read_dir,
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.iterate = proc_ns_dir_readdir,
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};
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static struct dentry *proc_ns_dir_lookup(struct inode *dir,
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struct dentry *dentry, unsigned int flags)
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{
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int error;
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struct task_struct *task = get_proc_task(dir);
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const struct proc_ns_operations **entry, **last;
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unsigned int len = dentry->d_name.len;
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error = -ENOENT;
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if (!task)
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goto out_no_task;
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last = &ns_entries[ARRAY_SIZE(ns_entries)];
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for (entry = ns_entries; entry < last; entry++) {
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if (strlen((*entry)->name) != len)
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continue;
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if (!memcmp(dentry->d_name.name, (*entry)->name, len))
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break;
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}
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if (entry == last)
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goto out;
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error = proc_ns_instantiate(dir, dentry, task, *entry);
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out:
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put_task_struct(task);
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out_no_task:
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return ERR_PTR(error);
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}
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const struct inode_operations proc_ns_dir_inode_operations = {
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.lookup = proc_ns_dir_lookup,
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.getattr = pid_getattr,
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.setattr = proc_setattr,
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};
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