[PATCH] proc: add config & param to block forcing mem writes

[PATCH] proc: add config & param to block forcing mem writes

[Adrian Ratiu]

This adds a Kconfig option and boot param to allow removing
the FOLL_FORCE flag from /proc/pid/mem write calls because
it can be abused.

The traditional forcing behavior is kept as default because
it can break GDB and some other use cases.

Previously we tried a more sophisticated approach allowing
distributions to fine-tune /proc/pid/mem behavior, however
that got NAK-ed by Linus [1], who prefers this simpler
approach with semantics also easier to understand for users.

Link: https://lore.kernel.org/lkml/CAHk-=wiGWLChxYmUA5HrT5aopZrB7_2VTa0NLZcxORgkUe5tEQ@mail.gmail.com/ [1]
Cc: Doug Anderson <dianders@chromium.org>
Cc: Jeff Xu <jeffxu@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
---
 .../admin-guide/kernel-parameters.txt         | 10 ++++
 fs/proc/base.c                                | 58 ++++++++++++++++++-
 security/Kconfig                              | 32 ++++++++++
 3 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c1134ad5f06d..793301f360ec 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4791,6 +4791,16 @@
 	printk.time=	Show timing data prefixed to each printk message line
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
 
+	proc_mem.force_override= [KNL]
+			Format: {always | ptrace | never}
+			Traditionally /proc/pid/mem allows users to override memory
+			permissions. This allows people to limit that.
+			Can be one of:
+			- 'always' traditional behavior always allows mem overrides.
+			- 'ptrace' only allow for active ptracers.
+			- 'never'  never allow mem permission overrides.
+			If not specified, default is always.
+
 	processor.max_cstate=	[HW,ACPI]
 			Limit processor to maximum C-state
 			max_cstate=9 overrides any DMI blacklist limit.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 72a1acd03675..5ef14ba953a2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -117,6 +117,40 @@
 static u8 nlink_tid __ro_after_init;
 static u8 nlink_tgid __ro_after_init;
 
+enum proc_mem_force_state {
+	PROC_MEM_FORCE_ALWAYS,
+	PROC_MEM_FORCE_PTRACE,
+	PROC_MEM_FORCE_NEVER
+};
+
+#if defined(CONFIG_PROC_MEM_ALWAYS_FORCE)
+static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_ALWAYS;
+#elif defined(CONFIG_PROC_MEM_FORCE_PTRACE)
+static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_PTRACE;
+#else
+static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_NEVER;
+#endif
+
+static int __init early_proc_mem_force_override(char *buf)
+{
+	if (!buf)
+		return -EINVAL;
+
+	if (strcmp(buf, "always") == 0) {
+		proc_mem_force_override = PROC_MEM_FORCE_ALWAYS;
+	} else if (strcmp(buf, "ptrace") == 0) {
+		proc_mem_force_override = PROC_MEM_FORCE_PTRACE;
+	} else if (strcmp(buf, "never") == 0) {
+		proc_mem_force_override = PROC_MEM_FORCE_NEVER;
+	} else {
+		pr_warn("proc_mem.force_override: ignoring unknown option '%s'\n", buf);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+early_param("proc_mem.force_override", early_proc_mem_force_override);
+
 struct pid_entry {
 	const char *name;
 	unsigned int len;
@@ -835,6 +869,26 @@ static int mem_open(struct inode *inode, struct file *file)
 	return ret;
 }
 
+static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
+{
+	switch (proc_mem_force_override) {
+	case PROC_MEM_FORCE_NEVER:
+		return false;
+	case PROC_MEM_FORCE_PTRACE: {
+		bool ptrace_active = false;
+		struct task_struct *task = get_proc_task(file_inode(file));
+
+		if (task) {
+			ptrace_active = task->ptrace && task->mm == mm && task->parent == current;
+			put_task_struct(task);
+		}
+		return ptrace_active;
+	}
+	default:
+		return true;
+	}
+}
+
 static ssize_t mem_rw(struct file *file, char __user *buf,
 			size_t count, loff_t *ppos, int write)
 {
@@ -855,7 +909,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	if (!mmget_not_zero(mm))
 		goto free;
 
-	flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
+	flags = write ? FOLL_WRITE : 0;
+	if (proc_mem_foll_force(file, mm))
+		flags |= FOLL_FORCE;
 
 	while (count > 0) {
 		size_t this_len = min_t(size_t, count, PAGE_SIZE);
diff --git a/security/Kconfig b/security/Kconfig
index 412e76f1575d..a93c1a9b7c28 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -19,6 +19,38 @@ config SECURITY_DMESG_RESTRICT
 
 	  If you are unsure how to answer this question, answer N.
 
+choice
+	prompt "Allow /proc/pid/mem access override"
+	default PROC_MEM_ALWAYS_FORCE
+	help
+	  Traditionally /proc/pid/mem allows users to override memory
+	  permissions for users like ptrace, assuming they have ptrace
+	  capability.
+
+	  This allows people to limit that - either never override, or
+	  require actual active ptrace attachment.
+
+	  Defaults to the traditional behavior (for now)
+
+config PROC_MEM_ALWAYS_FORCE
+	bool "Traditional /proc/pid/mem behavior"
+	help
+	  This allows /proc/pid/mem accesses to override memory mapping
+	  permissions if you have ptrace access rights.
+
+config PROC_MEM_FORCE_PTRACE
+	bool "Require active ptrace() use for access override"
+	help
+	  This allows /proc/pid/mem accesses to override memory mapping
+	  permissions for active ptracers like gdb.
+
+config PROC_MEM_NO_FORCE
+	bool "Never"
+	help
+	  Never override memory mapping permissions
+
+endchoice
+
 config SECURITY
 	bool "Enable different security models"
 	depends on SYSFS
-- 
2.44.2

Re: [PATCH] proc: add config & param to block forcing mem writes

[Linus Torvalds]

On Tue, 23 Jul 2024 at 10:18, Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
>
> This adds a Kconfig option and boot param to allow removing
> the FOLL_FORCE flag from /proc/pid/mem write calls because
> it can be abused.

Ack, this looks much simpler.

That said, I think this can be prettied up some more:

> +enum proc_mem_force_state {
> +       PROC_MEM_FORCE_ALWAYS,
> +       PROC_MEM_FORCE_PTRACE,
> +       PROC_MEM_FORCE_NEVER
> +};
> +
> +#if defined(CONFIG_PROC_MEM_ALWAYS_FORCE)
> +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_ALWAYS;
> +#elif defined(CONFIG_PROC_MEM_FORCE_PTRACE)
> +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_PTRACE;
> +#else
> +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_NEVER;
> +#endif

I think instead of that forest of #if defined(), we can just do

  enum proc_mem_force {
        PROC_MEM_FORCE_ALWAYS,
        PROC_MEM_FORCE_PTRACE,
        PROC_MEM_FORCE_NEVER
  };

  static enum proc_mem_force proc_mem_force_override __ro_after_init =
      IS_ENABLED(CONFIG_PROC_MEM_ALWAYS_FORCE) ? PROC_MEM_FORCE_ALWAYS :
      IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE :
      PROC_MEM_FORCE_NEVER;

I also really thought we had some parser helper for this pattern:

> +static int __init early_proc_mem_force_override(char *buf)
> +{
> +       if (!buf)
> +               return -EINVAL;
> +
> +       if (strcmp(buf, "always") == 0) {
> +               proc_mem_force_override = PROC_MEM_FORCE_ALWAYS;
 ....

but it turns out we only really "officially" have it for filesystem
superblock parsing.

Oh well. We could do

  #include <linux/fs_parser.h>
 ...
  struct constant_table proc_mem_force_table[] {
        { "ptrace", PROC_MEM_FORCE_PTRACE },
        { "never", PROC_MEM_FORCE_NEVER },
        { }
  };
  ...
  proc_mem_force_override = lookup_constant(
        proc_mem_force_table, buf, PROC_MEM_FORCE_NEVER);

but while that looks a bit prettier, the whole "fs_parser.h" thing is
admittedly odd.

Anyway, I think the patch is ok, although I also happen to think it
could be a bit prettier.

            Linus

Re: [PATCH] proc: add config & param to block forcing mem writes

[Linus Torvalds]

On Tue, 23 Jul 2024 at 11:30, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> but while that looks a bit prettier, the whole "fs_parser.h" thing is
> admittedly odd.

.. don't get me wrong - /proc obviously *is* a filesystem, but in this
context it's a boot command line parameter, not a mount option.

The "constant_table" thing obviously does work outside of mount
options too, it's just that it's documented and used in the context of
the mount API.

                  Linus

Re: [PATCH] proc: add config & param to block forcing mem writes

[Adrian Ratiu]

On Tuesday, July 23, 2024 21:30 EEST, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, 23 Jul 2024 at 10:18, Adrian Ratiu <adrian.ratiu@collabora.com> wrote:
> >
> > This adds a Kconfig option and boot param to allow removing
> > the FOLL_FORCE flag from /proc/pid/mem write calls because
> > it can be abused.
> 
> Ack, this looks much simpler.
> 
> That said, I think this can be prettied up some more:
> 
> > +enum proc_mem_force_state {
> > +       PROC_MEM_FORCE_ALWAYS,
> > +       PROC_MEM_FORCE_PTRACE,
> > +       PROC_MEM_FORCE_NEVER
> > +};
> > +
> > +#if defined(CONFIG_PROC_MEM_ALWAYS_FORCE)
> > +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_ALWAYS;
> > +#elif defined(CONFIG_PROC_MEM_FORCE_PTRACE)
> > +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_PTRACE;
> > +#else
> > +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_NEVER;
> > +#endif
> 
> I think instead of that forest of #if defined(), we can just do
> 
>   enum proc_mem_force {
>         PROC_MEM_FORCE_ALWAYS,
>         PROC_MEM_FORCE_PTRACE,
>         PROC_MEM_FORCE_NEVER
>   };
> 
>   static enum proc_mem_force proc_mem_force_override __ro_after_init =
>       IS_ENABLED(CONFIG_PROC_MEM_ALWAYS_FORCE) ? PROC_MEM_FORCE_ALWAYS :
>       IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE :
>       PROC_MEM_FORCE_NEVER;
> 
> I also really thought we had some parser helper for this pattern:
> 
> > +static int __init early_proc_mem_force_override(char *buf)
> > +{
> > +       if (!buf)
> > +               return -EINVAL;
> > +
> > +       if (strcmp(buf, "always") == 0) {
> > +               proc_mem_force_override = PROC_MEM_FORCE_ALWAYS;
>  ....
> 
> but it turns out we only really "officially" have it for filesystem
> superblock parsing.
> 
> Oh well. We could do
> 
>   #include <linux/fs_parser.h>
>  ...
>   struct constant_table proc_mem_force_table[] {
>         { "ptrace", PROC_MEM_FORCE_PTRACE },
>         { "never", PROC_MEM_FORCE_NEVER },
>         { }
>   };
>   ...
>   proc_mem_force_override = lookup_constant(
>         proc_mem_force_table, buf, PROC_MEM_FORCE_NEVER);
> 
> but while that looks a bit prettier, the whole "fs_parser.h" thing is
> admittedly odd.
> 
> Anyway, I think the patch is ok, although I also happen to think it
> could be a bit prettier.

Thanks again, I am perfectly fine with using fs_parser.h.

I'll wait a few days to give others a chance to review/respond,
then apply your changes and send a v3.

(this was actually v2, however git format-patch removed my
"Changes in v2" blurb and v2 title; will add them next time)