Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 17 Nov 2016 18:35:57 -0600 | Subject | Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file |
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Andy Lutomirski <luto@amacapital.net> writes:
> On Thu, Nov 17, 2016 at 3:55 PM, Eric W. Biederman > <ebiederm@xmission.com> wrote: >> Andy Lutomirski <luto@amacapital.net> writes: >> >>> On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman >>> <ebiederm@xmission.com> wrote: >>>> >>>> It is the reasonable expectation that if an executable file is not >>>> readable there will be no way for a user without special privileges to >>>> read the file. This is enforced in ptrace_attach but if we are >>>> already attached there is no enforcement if a readonly executable >>>> is exec'd. >>>> >>>> Therefore do the simple thing and if there is a non-dumpable >>>> executable that we are tracing without privilege fail to exec it. >>>> >>>> Fixes: v1.0 >>>> Cc: stable@vger.kernel.org >>>> Reported-by: Andy Lutomirski <luto@amacapital.net> >>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> >>>> --- >>>> fs/exec.c | 8 +++++++- >>>> 1 file changed, 7 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/fs/exec.c b/fs/exec.c >>>> index fdec760bfac3..de107f74e055 100644 >>>> --- a/fs/exec.c >>>> +++ b/fs/exec.c >>>> @@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm) >>>> { >>>> int retval; >>>> >>>> + /* Fail if the tracer can't read the executable */ >>>> + if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) && >>>> + !ptracer_capable(current, bprm->mm->user_ns)) >>>> + return -EPERM; >>>> + >>> >>> At the very least, I think that BINPRM_FLAGS_ENFORCE_NONDUMP needs to >>> check capable_wrt_inode_uidgid too. Otherwise we risk breaking: >>> >>> $ gcc whatever.c >>> $ chmod 400 a.out >>> $ strace a.out >> >> It is an invariant that if you have caps in mm->user_ns you will >> also be capable_write_inode_uidgid of all files that a process exec's. > > I meant to check whether you *are* the owner, too.
I don't follow. BINPRM_FLAGS_ENFORCE_NONDUMP is only set if the caller of exec does not have inode_permission(inode, MAY_READ).
Which in your example would have guaranteed that BINPRM_FLAGS_ENFORCE_NONDUMP would have be unset.
The ptracer_capable thing is only asking in this instance if we can ignore the nondumpable status because we have CAP_SYS_PTRACE over a user namespace that includes all of the files that would_dump was called on (mm->user_ns).
ptrace_access_vm in the replacement patch has essentially the same permission check. It is just at PTRACE_PEEKTEXT, PTRACE_PEEKDATA, PTRACE_POKETEXT, or PTRACE_POKEDATA time.
So I am curious if you are seeing something that is worth fixing.
>> My third patch winds up changing mm->user_ns to maintain this invariant. >> >> It is also true that Willy convinced me while this check is trivial it >> will break historic uses so I have replaced this patch with: >> "ptrace: Don't allow accessing an undumpable mm. > > I think that's better.
Eric
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