Re: [PATCH] proc: allow restricting /proc/pid/mem writes

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Kees Cook]

On Mon, Feb 26, 2024 at 09:10:54AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Wed, Feb 21, 2024 at 1:06 PM Adrian Ratiu <adrian.ratiu@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 appeared started causing drama like [1]. The
> > /proc/*/mem exploits 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.
> >
> > As part of hardening against these types of attacks, distrbutions
> > can restrict /proc/*/mem to only allow writes when they makes sense,
> > like in case of debuggers which have ptrace permissions, as they
> > are able to access memory anyway via PTRACE_POKEDATA and friends.
> >
> > Dropping the mode bits disables write access for non-root users.
> > Trying to `chmod` the paths back fails as the kernel rejects it.
> >
> > For users with CAP_DAC_OVERRIDE (usually just root) we have to
> > disable the mem_write callback to avoid bypassing the mode bits.
> >
> > Writes can be used to bypass permissions on memory maps, even if a
> > memory region is mapped r-x (as is a program's executable pages),
> > the process can open its own /proc/self/mem file and write to the
> > pages directly.
> >
> > Even if seccomp filters block mmap/mprotect calls with W|X perms,
> > they often cannot block open calls as daemons want to read/write
> > their own runtime state and seccomp filters cannot check file paths.
> > Write calls also can't be blocked in general via seccomp.
> >
> > 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 on layer fails.
> >
> > [1] https://lwn.net/Articles/476947/
> > [2] https://issues.chromium.org/issues/40089045
> >
> > Based on an initial patch by Mike Frysinger <vapier@chromium.org>.
> >
> > Cc: Guenter Roeck <groeck@chromium.org>
> > Cc: Doug Anderson <dianders@chromium.org>
> > Signed-off-by: Mike Frysinger <vapier@chromium.org>

This should have a "Co-developed-by: Mike..." tag, since you're making
changes and not just passing it along directly.

> > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
> > ---
> > Tested on next-20240220.
> >
> > I would really like to avoid depending on CONFIG_MEMCG which is
> > required for the struct mm_stryct "owner" pointer.
> >
> > Any suggestions how check the ptrace owner without MEMCG?
> > ---
> >  fs/proc/base.c   | 26 ++++++++++++++++++++++++--
> >  security/Kconfig | 13 +++++++++++++
> >  2 files changed, 37 insertions(+), 2 deletions(-)
> 
> Thanks for posting this! This looks reasonable to me, but I'm nowhere
> near an expert on this so I won't add a Reviewed-by tag.
> 
> This feels like the kind of thing that Kees might be interested in
> reviewing, so adding him to the "To" list.

I'd love to make /proc/$pid/mem more strict. A few comments:

> [...]
> +	if (ptracer_capable(current, mm->user_ns) &&

It really looks like you're trying to do a form of ptrace_may_access(),
but _without_ the introspection exception?

Also, using "current" in the write path can lead to problems[1], so this
should somehow use file->f_cred, or limit write access during the open()
instead?

> [...]
> +config SECURITY_PROC_MEM_RESTRICT_WRITES

Instead of a build-time CONFIG, I'd prefer a boot-time config (or a
sysctl, but that's be harder given the perms). That this is selectable
by distro users, etc, and they don't need to rebuild their kernel to
benefit from it.

Jann Horn has tried to restrict access to this file in the past as well,
so he may have some additional advice about it.

-Kees

[1] https://docs.kernel.org/security/credentials.html#open-file-credentials

-- 
Kees Cook

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Kees Cook]

[sorry for the duplicate, fixing Jann's email address]

On Mon, Feb 26, 2024 at 09:10:54AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Wed, Feb 21, 2024 at 1:06 PM Adrian Ratiu <adrian.ratiu@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 appeared started causing drama like [1]. The
> > /proc/*/mem exploits 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.
> >
> > As part of hardening against these types of attacks, distrbutions
> > can restrict /proc/*/mem to only allow writes when they makes sense,
> > like in case of debuggers which have ptrace permissions, as they
> > are able to access memory anyway via PTRACE_POKEDATA and friends.
> >
> > Dropping the mode bits disables write access for non-root users.
> > Trying to `chmod` the paths back fails as the kernel rejects it.
> >
> > For users with CAP_DAC_OVERRIDE (usually just root) we have to
> > disable the mem_write callback to avoid bypassing the mode bits.
> >
> > Writes can be used to bypass permissions on memory maps, even if a
> > memory region is mapped r-x (as is a program's executable pages),
> > the process can open its own /proc/self/mem file and write to the
> > pages directly.
> >
> > Even if seccomp filters block mmap/mprotect calls with W|X perms,
> > they often cannot block open calls as daemons want to read/write
> > their own runtime state and seccomp filters cannot check file paths.
> > Write calls also can't be blocked in general via seccomp.
> >
> > 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 on layer fails.
> >
> > [1] https://lwn.net/Articles/476947/
> > [2] https://issues.chromium.org/issues/40089045
> >
> > Based on an initial patch by Mike Frysinger <vapier@chromium.org>.
> >
> > Cc: Guenter Roeck <groeck@chromium.org>
> > Cc: Doug Anderson <dianders@chromium.org>
> > Signed-off-by: Mike Frysinger <vapier@chromium.org>

This should have a "Co-developed-by: Mike..." tag, since you're making
changes and not just passing it along directly.

> > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
> > ---
> > Tested on next-20240220.
> >
> > I would really like to avoid depending on CONFIG_MEMCG which is
> > required for the struct mm_stryct "owner" pointer.
> >
> > Any suggestions how check the ptrace owner without MEMCG?
> > ---
> >  fs/proc/base.c   | 26 ++++++++++++++++++++++++--
> >  security/Kconfig | 13 +++++++++++++
> >  2 files changed, 37 insertions(+), 2 deletions(-)
> 
> Thanks for posting this! This looks reasonable to me, but I'm nowhere
> near an expert on this so I won't add a Reviewed-by tag.
> 
> This feels like the kind of thing that Kees might be interested in
> reviewing, so adding him to the "To" list.

I'd love to make /proc/$pid/mem more strict. A few comments:

> [...]
> +	if (ptracer_capable(current, mm->user_ns) &&

It really looks like you're trying to do a form of ptrace_may_access(),
but _without_ the introspection exception?

Also, using "current" in the write path can lead to problems[1], so this
should somehow use file->f_cred, or limit write access during the open()
instead?

> [...]
> +config SECURITY_PROC_MEM_RESTRICT_WRITES

Instead of a build-time CONFIG, I'd prefer a boot-time config (or a
sysctl, but that's be harder given the perms). That this is selectable
by distro users, etc, and they don't need to rebuild their kernel to
benefit from it.

Jann Horn has tried to restrict access to this file in the past as well,
so he may have some additional advice about it.

-Kees

[1] https://docs.kernel.org/security/credentials.html#open-file-credentials

-- 
Kees Cook

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Mike Frysinger]

(lemme try this again as plain text)

On Mon, Feb 26, 2024 at 2:24 PM Kees Cook <keescook@chromium.org> wrote:
> On Mon, Feb 26, 2024 at 09:10:54AM -0800, Doug Anderson wrote:
> > On Wed, Feb 21, 2024 at 1:06 PM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
> > +     if (ptracer_capable(current, mm->user_ns) &&
>
> It really looks like you're trying to do a form of ptrace_may_access(),
> but _without_ the introspection exception?

to be clear, we want the check to be "ptracer is attached, and the
process attempting the write is the ptracer", not "does the writer
pass ptrace access checks".  the latter opens up more angles,
including shellcode self-modification, that we don't want.  the only
use case we have for writable mem files is for debuggers, and those
should already be attached.
-mike

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Adrian Ratiu]

Hello

On Monday, February 26, 2024 21:24 EET, Kees Cook <keescook@chromium.org> wrote:

> [sorry for the duplicate, fixing Jann's email address]
> 
> On Mon, Feb 26, 2024 at 09:10:54AM -0800, Doug Anderson wrote:
> > Hi,
> > 
> > On Wed, Feb 21, 2024 at 1:06 PM Adrian Ratiu <adrian.ratiu@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 appeared started causing drama like [1]. The
> > > /proc/*/mem exploits 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.
> > >
> > > As part of hardening against these types of attacks, distrbutions
> > > can restrict /proc/*/mem to only allow writes when they makes sense,
> > > like in case of debuggers which have ptrace permissions, as they
> > > are able to access memory anyway via PTRACE_POKEDATA and friends.
> > >
> > > Dropping the mode bits disables write access for non-root users.
> > > Trying to `chmod` the paths back fails as the kernel rejects it.
> > >
> > > For users with CAP_DAC_OVERRIDE (usually just root) we have to
> > > disable the mem_write callback to avoid bypassing the mode bits.
> > >
> > > Writes can be used to bypass permissions on memory maps, even if a
> > > memory region is mapped r-x (as is a program's executable pages),
> > > the process can open its own /proc/self/mem file and write to the
> > > pages directly.
> > >
> > > Even if seccomp filters block mmap/mprotect calls with W|X perms,
> > > they often cannot block open calls as daemons want to read/write
> > > their own runtime state and seccomp filters cannot check file paths.
> > > Write calls also can't be blocked in general via seccomp.
> > >
> > > 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 on layer fails.
> > >
> > > [1] https://lwn.net/Articles/476947/
> > > [2] https://issues.chromium.org/issues/40089045
> > >
> > > Based on an initial patch by Mike Frysinger <vapier@chromium.org>.
> > >
> > > Cc: Guenter Roeck <groeck@chromium.org>
> > > Cc: Doug Anderson <dianders@chromium.org>
> > > Signed-off-by: Mike Frysinger <vapier@chromium.org>
> 
> This should have a "Co-developed-by: Mike..." tag, since you're making
> changes and not just passing it along directly.

Thanks, I'll address this in v2.

> 
> > > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
> > > ---
> > > Tested on next-20240220.
> > >
> > > I would really like to avoid depending on CONFIG_MEMCG which is
> > > required for the struct mm_stryct "owner" pointer.
> > >
> > > Any suggestions how check the ptrace owner without MEMCG?
> > > ---
> > >  fs/proc/base.c   | 26 ++++++++++++++++++++++++--
> > >  security/Kconfig | 13 +++++++++++++
> > >  2 files changed, 37 insertions(+), 2 deletions(-)
> > 
> > Thanks for posting this! This looks reasonable to me, but I'm nowhere
> > near an expert on this so I won't add a Reviewed-by tag.
> > 
> > This feels like the kind of thing that Kees might be interested in
> > reviewing, so adding him to the "To" list.
> 
> I'd love to make /proc/$pid/mem more strict. A few comments:
> 
> > [...]
> > +	if (ptracer_capable(current, mm->user_ns) &&
> 
> It really looks like you're trying to do a form of ptrace_may_access(),
> but _without_ the introspection exception?
> 
> Also, using "current" in the write path can lead to problems[1], so this
> should somehow use file->f_cred, or limit write access during the open()
> instead?

I think Mike explained pretty well why we need to check if current already
is a ptracer. The point you raise is valid (thanks again) so we need to check
a bit earlier, during open().

> 
> > [...]
> > +config SECURITY_PROC_MEM_RESTRICT_WRITES
> 
> Instead of a build-time CONFIG, I'd prefer a boot-time config (or a
> sysctl, but that's be harder given the perms). That this is selectable
> by distro users, etc, and they don't need to rebuild their kernel to
> benefit from it.

Ack, I'll implement a cmdline arg in v2.

> 
> Jann Horn has tried to restrict access to this file in the past as well,
> so he may have some additional advice about it.

I'll leave this a few more days in case others have more ideas, then will
send v2 and also add Jann to the "To:" list.

> 
> -Kees
> 
> [1] https://docs.kernel.org/security/credentials.html#open-file-credentials
> 
> -- 
> Kees Cook

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Doug Anderson]

Hi,

On Mon, Feb 26, 2024 at 2:33 PM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
>
> > > [...]
> > > +config SECURITY_PROC_MEM_RESTRICT_WRITES
> >
> > Instead of a build-time CONFIG, I'd prefer a boot-time config (or a
> > sysctl, but that's be harder given the perms). That this is selectable
> > by distro users, etc, and they don't need to rebuild their kernel to
> > benefit from it.
>
> Ack, I'll implement a cmdline arg in v2.

Any objections to doing both? Have a CONFIG option for a default and a
cmdline to override it? This way if a distro wants to restrict writes
by default then don't need to jam more stuff into the kernel command
line.

-Doug

Re: [PATCH] proc: allow restricting /proc/pid/mem writes

[Kees Cook]

On Mon, Feb 26, 2024 at 02:37:29PM -0800, Doug Anderson wrote:
> Hi,
> 
> On Mon, Feb 26, 2024 at 2:33 PM Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
> >
> > > > [...]
> > > > +config SECURITY_PROC_MEM_RESTRICT_WRITES
> > >
> > > Instead of a build-time CONFIG, I'd prefer a boot-time config (or a
> > > sysctl, but that's be harder given the perms). That this is selectable
> > > by distro users, etc, and they don't need to rebuild their kernel to
> > > benefit from it.
> >
> > Ack, I'll implement a cmdline arg in v2.
> 
> Any objections to doing both? Have a CONFIG option for a default and a
> cmdline to override it? This way if a distro wants to restrict writes
> by default then don't need to jam more stuff into the kernel command
> line.

For an example, take a look at randomize_kstack_offset and
CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT.

-- 
Kees Cook