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    SubjectRe: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file
    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.

    >
    > 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
    >
    >



    --
    Andy Lutomirski
    AMA Capital Management, LLC

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