Re: [PATCH v2] proc: allow restricting /proc/pid/mem writes

["Adrian Ratiu" <adrian.ratiu@collabora.com>]

On Monday, March 04, 2024 16:05 EET, Christian Brauner <brauner@kernel.org> wrote:

> On Mon, Mar 04, 2024 at 01:48:19PM +0000, Adrian Ratiu wrote:
> > On Monday, March 04, 2024 15:20 EET, Christian Brauner <brauner@kernel.org> wrote:
> > 
> > > On Fri, Mar 01, 2024 at 11:34:42PM +0200, Adrian Ratiu 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>
> > > > Cc: Kees Cook <keescook@chromium.org>
> > > > Cc: Jann Horn <jannh@google.com>
> > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > Cc: Randy Dunlap <rdunlap@infradead.org>
> > > > Cc: Christian Brauner <brauner@kernel.org>
> > > > Co-developed-by: Mike Frysinger <vapier@chromium.org>
> > > > Signed-off-by: Mike Frysinger <vapier@chromium.org>
> > > > Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
> > > > ---
> > > > Changes in v2:
> > > >  * Added boot time parameter with default kconfig option
> > > >  * Moved check earlier in mem_open() instead of mem_write()
> > > >  * Simplified implementation branching
> > > >  * Removed dependency on CONFIG_MEMCG
> > > > ---
> > > >  .../admin-guide/kernel-parameters.txt         |  4 ++
> > > >  fs/proc/base.c                                | 47 ++++++++++++++++++-
> > > >  security/Kconfig                              | 22 +++++++++
> > > >  3 files changed, 71 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > > index 460b97a1d0da..0647e2f54248 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -5618,6 +5618,10 @@
> > > >  	reset_devices	[KNL] Force drivers to reset the underlying device
> > > >  			during initialization.
> > > >  
> > > > +	restrict_proc_mem_write= [KNL]
> > > > +			Enable or disable write access to /proc/*/mem files.
> > > > +			Default is SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON.
> > > > +
> > > >  	resume=		[SWSUSP]
> > > >  			Specify the partition device for software suspend
> > > >  			Format:
> > > > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > > > index 98a031ac2648..92f668191312 100644
> > > > --- a/fs/proc/base.c
> > > > +++ b/fs/proc/base.c
> > > > @@ -152,6 +152,30 @@ struct pid_entry {
> > > >  		NULL, &proc_pid_attr_operations,	\
> > > >  		{ .lsmid = LSMID })
> > > >  
> > > > +#ifdef CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE
> > > > +DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON,
> > > > +			   restrict_proc_mem_write);
> > > > +static int __init early_restrict_proc_mem_write(char *buf)
> > > > +{
> > > > +	int ret;
> > > > +	bool bool_result;
> > > > +
> > > > +	ret = kstrtobool(buf, &bool_result);
> > > > +	if (ret)
> > > > +		return ret;
> > > > +
> > > > +	if (bool_result)
> > > > +		static_branch_enable(&restrict_proc_mem_write);
> > > > +	else
> > > > +		static_branch_disable(&restrict_proc_mem_write);
> > > > +	return 0;
> > > > +}
> > > > +early_param("restrict_proc_mem_write", early_restrict_proc_mem_write);
> > > > +# define PROC_PID_MEM_MODE S_IRUSR
> > > > +#else
> > > > +# define PROC_PID_MEM_MODE (S_IRUSR|S_IWUSR)
> > > > +#endif
> > > > +
> > > >  /*
> > > >   * Count the number of hardlinks for the pid_entry table, excluding the .
> > > >   * and .. links.
> > > > @@ -829,6 +853,25 @@ static int mem_open(struct inode *inode, struct file *file)
> > > >  {
> > > >  	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACH);
> > > >  
> > > > +#ifdef CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE
> > > > +	struct mm_struct *mm = file->private_data;
> > > > +	struct task_struct *task = get_proc_task(inode);
> > > > +
> > > > +	if (mm && task) {
> > > > +		/* Only allow writes by processes already ptracing the target task */
> > > > +		if (file->f_mode & FMODE_WRITE &&
> > > > +		    static_branch_maybe(CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON,
> > > > +					&restrict_proc_mem_write)) {
> > > > +			rcu_read_lock();
> > > > +			if (!ptracer_capable(current, mm->user_ns) ||
> > > > +			    current != ptrace_parent(task))
> > > > +				ret = -EACCES;
> > > 
> > > Uhm, this will break the seccomp notifier, no? So you can't turn on
> > > SECURITY_PROC_MEM_RESTRICT_WRITE when you want to use the seccomp
> > > notifier to do system call interception and rewrite memory locations of
> > > the calling task, no? Which is very much relied upon in various
> > > container managers and possibly other security tools.
> > > 
> > > Which means that you can't turn this on in any of the regular distros.
> > > 
> > > So you need to either account for the calling task being a seccomp
> > > supervisor for the task whose memory it is trying to access or you need
> > > to provide a migration path by adding an api that let's caller's perform
> > > these writes through the seccomp notifier.
> > 
> > Thanks for raising this!
> > 
> > I did test seccomp filtering/blocking functionality which seemed to
> > work but I'll make sure to also test syscall interception before
> > sending v3, to confirm whether it breaks.
> > 
> > The simplest solution is to add an exception for seccomp supervisors
> > just like we did for tracers, yes, so I'm inclined to go with that if
> > needed. :)
> 
> Ok. Note that your patch also doesn't cover process_vm_writev() which
> means that you can still use that as an alternative to write to memory -
> albeit with a lote more raciness. IOW, a seccomp notifier can do the
> dance of:
> 
> pidfd = clone3(CLONE_PIDFD)
> // handle error
> int fd_mem = open("/proc/$pid/mem", O_RDWR);:
> // handle error
> if (pidfd_send_signal(pidfd, 0, ...) == 0)
>         write(fd_mem, ...);
> 
> which lets it avoid the raciness. That's not possible with
> process_vm_writev() especially if it's received via AF_UNIX sockets
> which happens if the seccomp notifier lives in a proxy process. I know
> that happens in the wild.
> 
> So overall, it seems a bit odd to me because why block /proc/<pid>/mem
> specifically and not also cover process_vm_writev()? Because that's easy
> to block via regular seccomp system call filtering?

Yes, easy to block and also respect page permissions (can't write read-only memory) as well as require ptrace access anyway by checking PTRACE_MODE_ATTACH_REALCREDS.