On Wed, Oct 2, 2013 at 11:07 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Wed, Oct 2, 2013 at 11:00 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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 > > > > (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. > > > > (a) is probably nicer. I don't know if it'll break things. Linus > > seemed to think that the Chrome sandbox was sensitive to this stuff, > > but I don't know why. > > I agree, (a) seems much cleaner. Hm, I don't think Chrome does > anything with these sensitive files (maps, stack, syscall, etc). But > let's ask Julien. :) > > Julien, do you see any problem with Chrome's sandbox behavior if these > proc files would be unavailable across privilege changes? There is nothing that currently jumps to mind in Chromium. However, anything that breaks "file descriptors are capabilities" inevitably ends-up breaking something. For instance, I could easily imagine breakage because a process uses PR_SET_DUMPABLE (more so than, say, transitions to uid 0) while its /proc entries are being monitored by another part of the same application. Please cc:me on patches. Julien -- 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
