On Sun, 8 Nov 2015 13:08:36 +0100 Jann Horn <jann@xxxxxxxxx> wrote: > 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). I'll await reviewer input on this one. Meanwhile, a bunch of minor(ish) things... > --- a/fs/proc/array.c > +++ b/fs/proc/array.c > @@ -395,7 +395,8 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, > > state = *get_task_state(task); > vsize = eip = esp = 0; > - permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT); > + permitted = ptrace_may_access(task, > + PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT | PTRACE_MODE_FSCREDS); There's lots of ugliness in the patch to do with fitting code into 80 cols. Can we do #define PTRACE_foo (PTRACE_MODE_READ|PTRACE_MODE_FSCREDS) to avoid all that? > --- a/include/linux/ptrace.h > +++ b/include/linux/ptrace.h > @@ -57,7 +57,22 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead); > #define PTRACE_MODE_READ 0x01 > #define PTRACE_MODE_ATTACH 0x02 > #define PTRACE_MODE_NOAUDIT 0x04 > -/* Returns true on success, false on denial. */ > +#define PTRACE_MODE_FSCREDS 0x08 > +#define PTRACE_MODE_REALCREDS 0x10 > +/** > + * ptrace_may_access - check whether the caller is permitted to access > + * a target task. > + * @task: target task > + * @mode: selects type of access and caller credentials > + * > + * Returns true on success, false on denial. > + * > + * One of the flags PTRACE_MODE_FSCREDS and PTRACE_MODE_REALCREDS must > + * be set in @mode to specify whether the access was requested through > + * a filesystem syscall (should use effective capabilities and fsuid > + * of the caller) or through an explicit syscall such as > + * process_vm_writev or ptrace (and should use the real credentials). > + */ > extern bool ptrace_may_access(struct task_struct *task, unsigned int mode); It is unconventional to put the kernedoc in the header - people have been trained to look for it in the .c file. > +++ b/kernel/ptrace.c > @@ -219,6 +219,13 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode) > static int __ptrace_may_access(struct task_struct *task, unsigned int mode) > { > const struct cred *cred = current_cred(), *tcred; > + kuid_t caller_uid; > + kgid_t caller_gid; > + > + if (!(mode & PTRACE_MODE_FSCREDS) != !(mode & PTRACE_MODE_REALCREDS)) { So setting either one of these and not the other is an error. How come? > + WARN(1, "denying ptrace access check without PTRACE_MODE_*CREDS\n"); This warning cannot be triggered by malicious userspace, I trust? -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html