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. I think revoking the fd would be great. Does that mechanism exist? -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
