[PATCH v6 2/2] proc: restrict /proc/pid/mem

[Kees Cook]

On Tue, Jun 18, 2024 at 03:39:44PM -0700, Jeff Xu wrote:
> Hi
> 
> Thanks for the patch !
> 
> On Thu, Jun 13, 2024 at 6:40 AM Adrian Ratiu <adrian.ratiu at collabora.com> wrote:
> >
> > Prior to v2.6.39 write access to /proc/<pid>/mem was restricted,
> > after which it got allowed in commit 198214a7ee50 ("proc: enable
> > writing to /proc/pid/mem"). Famous last words from that patch:
> > "no longer a security hazard". :)
> >
> > Afterwards exploits started causing drama like [1]. The exploits
> > using /proc/*/mem can be rather sophisticated like [2] which
> > installed an arbitrary payload from noexec storage into a running
> > process then exec'd it, which itself could include an ELF loader
> > to run arbitrary code off noexec storage.
> >
> > One of the well-known problems with /proc/*/mem writes is they
> > ignore page permissions via FOLL_FORCE, as opposed to writes via
> > process_vm_writev which respect page permissions. These writes can
> > also be used to bypass mode bits.
> >
> > To harden against these types of attacks, distrbutions might want
> > to restrict /proc/pid/mem accesses, either entirely or partially,
> > for eg. to restrict FOLL_FORCE usage.
> >
> > Known valid use-cases which still need these accesses are:
> >
> > * Debuggers which also have ptrace permissions, so they can access
> > memory anyway via PTRACE_POKEDATA & co. Some debuggers like GDB
> > are designed to write /proc/pid/mem for basic functionality.
> >
> > * Container supervisors using the seccomp notifier to intercept
> > syscalls and rewrite memory of calling processes by passing
> > around /proc/pid/mem file descriptors.
> >
> > There might be more, that's why these params default to disabled.
> >
> > Regarding other mechanisms which can block these accesses:
> >
> > * seccomp filters can be used to block mmap/mprotect calls with W|X
> > perms, but they often can't block open calls as daemons want to
> > read/write their runtime state and seccomp filters cannot check
> > file paths, so plain write calls can't be easily blocked.
> >
> > * Since the mem file is part of the dynamic /proc/<pid>/ space, we
> > can't run chmod once at boot to restrict it (and trying to react
> > to every process and run chmod doesn't scale, and the kernel no
> > longer allows chmod on any of these paths).
> >
> > * SELinux could be used with a rule to cover all /proc/*/mem files,
> > but even then having multiple ways to deny an attack is useful in
> > case one layer fails.
> >
> > Thus we introduce four kernel parameters to restrict /proc/*/mem
> > access: open-read, open-write, write and foll_force. All these can
> > be independently set to the following values:
> >
> > all     => restrict all access unconditionally.
> > ptracer => restrict all access except for ptracer processes.
> >
> > If left unset, the existing behaviour is preserved, i.e. access
> > is governed by basic file permissions.
> >
> > Examples which can be passed by bootloaders:
> >
> > proc_mem.restrict_foll_force=all
> > proc_mem.restrict_open_write=ptracer
> > proc_mem.restrict_open_read=ptracer
> > proc_mem.restrict_write=all
> >
> > These knobs can also be enabled via Kconfig like for eg:
> >
> > CONFIG_PROC_MEM_RESTRICT_WRITE_PTRACE_DEFAULT=y
> > CONFIG_PROC_MEM_RESTRICT_FOLL_FORCE_PTRACE_DEFAULT=y
> >
> > Each distribution needs to decide what restrictions to apply,
> > depending on its use-cases. Embedded systems might want to do
> > more, while general-purpouse distros might want a more relaxed
> > policy, because for e.g. foll_force=all and write=all both break
> > break GDB, so it might be a bit excessive.
> >
> > Based on an initial patch by Mike Frysinger <vapier at chromium.org>.
> >
> It is noteworthy that ChromeOS has benefited from blocking
> /proc/pid/mem write since 2017 [1], owing to the patch implemented by
> Mike Frysinger.
> 
> It is great that upstream can consider this patch, ChromeOS will use
> the solution once it is accepted.
> 
> > Link: https://lwn.net/Articles/476947/ [1]
> > Link: https://issues.chromium.org/issues/40089045 [2]
> > Cc: Guenter Roeck <groeck at chromium.org>
> > Cc: Doug Anderson <dianders at chromium.org>
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Jann Horn <jannh at google.com>
> > Cc: Andrew Morton <akpm at linux-foundation.org>
> > Cc: Randy Dunlap <rdunlap at infradead.org>
> > Cc: Christian Brauner <brauner at kernel.org>
> > Cc: Jeff Xu <jeffxu at google.com>
> > Co-developed-by: Mike Frysinger <vapier at chromium.org>
> > Signed-off-by: Mike Frysinger <vapier at chromium.org>
> > Signed-off-by: Adrian Ratiu <adrian.ratiu at collabora.com>
> 
> Reviewed-by: Jeff Xu <jeffxu at chromium.org>
> Tested-by: Jeff Xu <jeffxu at chromium.org>
> [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/764773

Thanks for the testing! What settings did you use? I think Chrome OS was
effectively doing this?

PROC_MEM_RESTRICT_OPEN_READ_OFF=y
CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_ALL=y
CONFIG_PROC_MEM_RESTRICT_WRITE_ALL=y
CONFIG_PROC_MEM_RESTRICT_FOLL_FORCE_ALL=y

Though I don't see the FOLL_FORCE changes in the linked Chrome OS patch,
but I suspect it's unreachable with
CONFIG_PROC_MEM_RESTRICT_OPEN_WRITE_ALL=y.

-Kees

-- 
Kees Cook