On Mon, Nov 09, 2015 at 12:55:54PM -0800, Andrew Morton wrote: > On Sun, 8 Nov 2015 13:08:36 +0100 Jann Horn <[email protected]> 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.
I agree.
> Can we do
>
> #define PTRACE_foo (PTRACE_MODE_READ|PTRACE_MODE_FSCREDS)
>
> to avoid all that?
Hm. All combinations of the PTRACE_MODE_*CREDS flags with
PTRACE_MODE_{READ,ATTACH} plus optionally PTRACE_MODE_NOAUDIT
make sense, I think. So your suggestion would be to create
four new #defines
PTRACE_MODE_{READ,ATTACH}_{FSCREDS,REALCREDS} and then let
callers OR in the PTRACE_MODE_NOAUDIT flag if needed?
> > --- 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.
OK, will fix that. I thought it would be appropriate to put it in the
header since that one-line comment was already there.
> > +++ 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?
Oh. Sorry about that. I only added PTRACE_MODE_REALCREDS in this iteration
of the patch and forgot to re-test afterwards. It is supposed to be the
other way around, so that you need to set exactly one. s/!=/==/
> > + WARN(1, "denying ptrace access check without
> > PTRACE_MODE_*CREDS\n");
>
> This warning cannot be triggered by malicious userspace, I trust?
Yeah, the ptrace access check flags should come from kernelspace only.
My patch modifies all callers of mm_access / ptrace_may_access so that
exactly one of the new flags is added, and the mode argument is always
a constant.
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