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

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

On Saturday, March 02, 2024 01:55 EET, Kees Cook <keescook@chromium.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
> 
> nit: I think "appeared" can be dropped here.
> 
> > /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.
> 
> Everything above here is good to keep in the commit log, but it's all
> the "background". Please also write here what has been done to address
> the background above it. e.g.:
> 
> "Introduce a CONFIG and a __ro_after_init runtime toggle to make
> it so only processes that are already tracing the task to write to
> /proc/<pid>/mem." etc
> 
> > 
> > [1] https://lwn.net/Articles/476947/
> > [2] https://issues.chromium.org/issues/40089045
> 
> These can be:
> 
> Link: https://lwn.net/Articles/476947/ [1]
> Link: https://issues.chromium.org/issues/40089045 [2]
> 
> > 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
> 
> Can you mention in the commit log what behaviors have been tested with
> this patch? For example, I assume gdb still works with
> restrict_proc_mem_write=y ?
> 

Thanks, I will address all the above commit message feedback in v3.

Yes, gdb and gdbserver work with restrict_proc_mem_write=y. My testing is
focused on the correct functioning of GDB and gdbserver (lldb/server use 
ptrace POKEDATA so they work regardless of restrict_proc_mem_write).

This all started from my attempt to fix gdbserver by adding a ptrace fallback
in case /proc/pid/mem writes are blocked without any exception, because
that breaks basic functionality like setting breakpoints.

GDB upstream NAK'ed my ptrace fallback approach because it's doesn't
work well with their /proc/pid/mem focused design required for non-stop 
mode (the default all-stop mode is emulated on top of non-stop), as well
as ptrace peek/poke requiring a live task which can cause memory access
problems if the ptraced task dies.

Other solutions were considered by GDB upstream, including using the 
process_vm_writev & co, but they respect page permissions and GDB has
to write RO pages to set breakpoints.

In the end GDB maintainers directed me to do a proper kernel fix with an
exception for tasks already ptracing others, because from a security
perspective, they can already access tracee memory regardless of
/proc/pid/mem restrictions, so here we are. :)

> When this is enabled, what _does_ break that people might expect to
> work?

With the current iteration, all things I tested work as expected. It is rather
hard to come up with things that break with restrict_proc_mem_write=y,
because other than debuggers and exploits I don't have other use-cases.

Obvious things like "echo >/proc/self/mem" get permission denied, but
that is expected with restrict_proc_mem_write=y, so I wouldn't classify
it as breakage.

In theory there might be some weird/legacy apps which might break, so
that is why I suggest we land the mechanism as default off, and later,
after it gets tested in various distributions, pull the trigger to make it
default on.

> 
> > ---
> >  .../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]
> 
> Please add here:
> 
> 			Format: <bool>
> 

Ack, will do in v3.

> > +			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
> 
> Please drop this CONFIG entirely -- it should be always available for
> all builds of the kernel. Only CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON
> needs to remain.
> 

Ack, will do in v3.

> > +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
> 
> PROC_PID_MEM_MODE will need to be a __ro_after_init variable, set by
> early_restrict_proc_mem_write, otherwise the mode won't change based on
> the runtime setting. e.g.:
> 
> #ifdef CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON
> mode_t proc_pid_mem_mode __ro_after_init = S_IRUSR;
> #else
> mode_t proc_pid_mem_mode __ro_after_init = (S_IRUSR|S_IWUSR);
> #endif
> 
> DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON,
> 			   restrict_proc_mem_write);
> ...
> 	if (bool_result) {
> 		static_branch_enable(&restrict_proc_mem_write);
> 		proc_pid_mem_mode = S_IRUSR;
> 	} else {
> 		static_branch_disable(&restrict_proc_mem_write);
> 		proc_pid_mem_mode = (S_IRUSR|S_IWUSR);
> 	}
> ...
> 	REG("mem",        proc_pid_mem_mode, proc_mem_operations),
> 
> 

Ack, will do in v3.

> > +
> >  /*
> >   * 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
> 
> Drop this ifdef (as mentioned above).
> 
> > +	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)) {
> 
> Do we need to also do an mm_access() on the task to verify that the task
> we're about to check has its mm still matching file->private_data? The
> PID can change out from under us (but the mm cannot).
> 

Likely yes, will look into this for v3.

> > +			rcu_read_lock();
> > +			if (!ptracer_capable(current, mm->user_ns) ||
> > +			    current != ptrace_parent(task))
> 
> If you're just allowing "already ptracing", why include the
> ptracer_capable() check?
> 

It is a very good observation that the check is redundant. :)

It is a remnant from a previous iteration of this patch, from
when I was proposing solutions to GDB upstream. I left it there
because it doesn't do much harm to verify capability as well,
more of a precaution / test invariant than anything else.

I'll remove it in v3 since it might cause confusion.

> > +				ret = -EACCES;
> > +			rcu_read_unlock();
> > +		}
> > +		put_task_struct(task);
> > +	}
> > +#endif
> > +
> >  	/* OK to pass negative loff_t, we can catch out-of-range */
> >  	file->f_mode |= FMODE_UNSIGNED_OFFSET;
> >  
> > @@ -3281,7 +3324,7 @@ static const struct pid_entry tgid_base_stuff[] = {
> >  #ifdef CONFIG_NUMA
> >  	REG("numa_maps",  S_IRUGO, proc_pid_numa_maps_operations),
> >  #endif
> > -	REG("mem",        S_IRUSR|S_IWUSR, proc_mem_operations),
> > +	REG("mem",        PROC_PID_MEM_MODE, proc_mem_operations),
> >  	LNK("cwd",        proc_cwd_link),
> >  	LNK("root",       proc_root_link),
> >  	LNK("exe",        proc_exe_link),
> > @@ -3631,7 +3674,7 @@ static const struct pid_entry tid_base_stuff[] = {
> >  #ifdef CONFIG_NUMA
> >  	REG("numa_maps", S_IRUGO, proc_pid_numa_maps_operations),
> >  #endif
> > -	REG("mem",       S_IRUSR|S_IWUSR, proc_mem_operations),
> > +	REG("mem",       PROC_PID_MEM_MODE, proc_mem_operations),
> >  	LNK("cwd",       proc_cwd_link),
> >  	LNK("root",      proc_root_link),
> >  	LNK("exe",       proc_exe_link),
> > diff --git a/security/Kconfig b/security/Kconfig
> > index 412e76f1575d..ffee9e847ed9 100644
> > --- a/security/Kconfig
> > +++ b/security/Kconfig
> > @@ -19,6 +19,28 @@ config SECURITY_DMESG_RESTRICT
> >  
> >  	  If you are unsure how to answer this question, answer N.
> >  
> > +config SECURITY_PROC_MEM_RESTRICT_WRITE
> > +	bool "Restrict /proc/*/mem write access"
> > +	default n
> > +	help
> > +	  This restricts writes to /proc/<pid>/mem, except when the current
> > +	  process ptraces the /proc/<pid>/mem task, because a ptracer already
> > +	  has write access to the tracee memory.
> > +
> > +	  Write access to this file allows bypassing memory map permissions,
> > +	  such as modifying read-only code.
> > +
> > +	  If you are unsure how to answer this question, answer N.
> > +
> > +config SECURITY_PROC_MEM_RESTRICT_WRITE_DEFAULT_ON
> > +	bool "Default state of /proc/*/mem write restriction"
> > +	depends on SECURITY_PROC_MEM_RESTRICT_WRITE
> > +	default y
> > +	help
> > +	  /proc/*/mem write access is controlled by kernel boot param
> > +	  "restrict_proc_mem_write" and this config chooses the default
> > +	  boot state.
> 
> As mentioned, I'd say merge the help texts here, but drop
> SECURITY_PROC_MEM_RESTRICT_WRITE.
> 

Ack, will do in v3.

> > +
> >  config SECURITY
> >  	bool "Enable different security models"
> >  	depends on SYSFS
> > -- 
> > 2.30.2
> > 
> 
> Thanks for this! I look forward to turning it on. :)
>

Thank you very much for all your feedback!
It is much appreciated.

I'll wait a few more days before sending v3 to let others
comment, then address everything.
 
> -Kees
> 
> -- 
> Kees Cook