On Wed, Oct 02, 2013 at 11:35:45AM -0700, Kees Cook wrote: > On Wed, Oct 2, 2013 at 11:22 AM, Djalal Harouni <tixxdz@xxxxxxxxxx> wrote: > > On Wed, Oct 02, 2013 at 10:48:55AM -0700, Kees Cook 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.) > > Kees for 0400 on /proc/*/maps, it was reported that it could break glibc > > I didn't mean maps should be 0400. The maps file is already handled > differently (pinning mm at open time). I didn't think it was one of > the problematic files. Kees currently all these files can be used to leak data, except for /proc/*/{mem,environ} These are the only one that pin the mm at open time. But I'm not sure that this solution will work for /proc/*/maps since they need vma info which will be perhaps freed if task execv, Need to check it. > Regardless, glibc uses /proc/self/maps, which would be fine here, right? I did not touch /proc/self/maps and others, but I'm planning to fix them if this solution is accepted. I'll do the same thing as in /proc/*/stat for maps, let it be 0444, and try to delay the check to ->read(). So during ->read() perform ptrace_may_access() on currenct's cred and process_allow_access() on file's opener cred. This should work. > -Kees > > -- > Kees Cook > Chrome OS Security -- 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
