Messages in this thread | | | From | (Eric W. Biederman) | Date | Thu, 17 Nov 2016 17:55:16 -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 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.
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.
Eric
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