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? -Kees -- Kees Cook Chrome OS Security -- 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
