On Wed, Oct 02, 2013 at 11:00:26AM -0700, Andy Lutomirski wrote: > On Wed, Oct 2, 2013 at 10:48 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Wed, Oct 2, 2013 at 9:51 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > >> On Wed, Oct 2, 2013 at 3:37 PM, Djalal Harouni <tixxdz@xxxxxxxxxx> wrote: > >>> On Tue, Oct 01, 2013 at 06:40:41PM -0700, Andy Lutomirski wrote: > >>>> On 10/01/2013 01:26 PM, Djalal Harouni wrote: > >>>> > /proc/<pid>/* entries varies at runtime, appropriate permission checks > >>>> > need to happen during each system call. > >>>> > > >>>> > Currently some of these sensitive entries are protected by performing > >>>> > the ptrace_may_access() check. However even with that the /proc file > >>>> > descriptors can be passed to a more privileged process > >>>> > (e.g. a suid-exec) which will pass the classic ptrace_may_access() > >>>> > check. In general the ->open() call will be issued by an unprivileged > >>>> > process while the ->read(),->write() calls by a more privileged one. > >>>> > > >>>> > Example of these files are: > >>>> > /proc/*/syscall, /proc/*/stack etc. > >>>> > > >>>> > And any open(/proc/self/*) then suid-exec to read()/write() /proc/self/* > >>>> > > >>>> > > >>>> > These files are protected during read() by the ptrace_may_access(), > >>>> > however the file descriptor can be passed to a suid-exec which can be > >>>> > used to read data and bypass ASLR. Of course this was discussed several > >>>> > times on LKML. > >>>> > >>>> Can you elaborate on what it is that you're fixing? That is, can you > >>>> give a concrete example of what process opens what file and passes the > >>>> fd to what process? > >>> Yes, the references were already given in this email: > >>> https://lkml.org/lkml/2013/8/31/209 > >>> > >>> This has been discussed several times on lkml: > >>> https://lkml.org/lkml/2013/8/28/544 > >>> > >>> https://lkml.org/lkml/2013/8/28/564 (check Kees's references) > >>> > >>> > >>>> I'm having trouble following your description. > >>> Process open a /proc file and pass the fd to a more privilaged process > >>> that will pass the ptrace_may_access() check, while the original process > >>> that opened that file should fail at the ptrace_may_access() > >> > >> So we're talking about two kinds of attacks, right? > > > > Correct. > > > >> Type 1: Unprivileged process does something like open("/proc/1/maps", > >> O_RDONLY) and then passes the resulting fd to something privileged. > > > > ... and then leaks contents back to unprivileged process. > > > >> Type 2: Unprivileged process does something like > >> open("/proc/self/maps", O_RDONLY) and then forks. The parent calls > >> execve on something privileged. > > > > ... and then parent snoops on file contents for the privileged child. > > > > (Type 2 is solved currently, IIUC. Type 1 could be reduced in scope by > > changing these file modes back to 0400.) > > > >> Can we really not get away with fixing type 1 by preventing these > >> files from being opened in the first place and type 2 by revoking all > >> of these fds when a privilege-changing exec happens? > > > > Type 1 can be done via exec as well. Instead of using a priv exec to > > read an arbitrary process, read it could read its own. > > Right. > > > > > I think revoking the fd would be great. Does that mechanism exist? > > There's this thing that never got merged. > > http://thread.gmane.org/gmane.linux.kernel/1523331 > > But doing it more directly should be reasonably straightforward. Either: > > (a) when a process execs and privileges change, find all the old proc > inodes, mark them dead, and unlink them, or Will take a look at it. > (b) add self_exec_id to all the proc file private_data entries (or > somewhere else). Then just make sure that they're unchanged. I think > the bug last time around was because the self_exec_id and struct pid > weren't being compared together. The bug was about self_exec_id not beeing unique. self_exec_id stuff must be unique during life time as it's done currently in grsecurity with exec_id. -- Djalal Harouni http://opendz.org -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
