diff mbox series

[v3] proc: add config & param to block forcing mem writes

Message ID 20240726090858.71541-1-adrian.ratiu@collabora.com (mailing list archive)
State Handled Elsewhere
Headers show
Series [v3] proc: add config & param to block forcing mem writes | expand

Commit Message

Adrian Ratiu July 26, 2024, 9:08 a.m. UTC
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>
---
Changes in v3:
* Simplified code to use shorthand ifs and a
  lookup_constant() table.

Changes in v2:
* Added bootparam on top of Linus' patch.
* Slightly reworded commit msg.
---
 .../admin-guide/kernel-parameters.txt         | 10 ++++
 fs/proc/base.c                                | 54 ++++++++++++++++++-
 security/Kconfig                              | 32 +++++++++++
 3 files changed, 95 insertions(+), 1 deletion(-)

Comments

Ard Biesheuvel July 26, 2024, 10:18 a.m. UTC | #1
On Fri, 26 Jul 2024 at 11:11, 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.
>
> 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>
> ---
> Changes in v3:
> * Simplified code to use shorthand ifs and a
>   lookup_constant() table.
>
> Changes in v2:
> * Added bootparam on top of Linus' patch.
> * Slightly reworded commit msg.
> ---
>  .../admin-guide/kernel-parameters.txt         | 10 ++++
>  fs/proc/base.c                                | 54 ++++++++++++++++++-
>  security/Kconfig                              | 32 +++++++++++
>  3 files changed, 95 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.

Better to use passive tense here rather than referring to 'users' and 'people'.

'Traditionally, /proc/pid/mem allows memory permissions to be
overridden without restrictions.
This option may be set to restrict that'

> +                       Can be one of:
> +                       - 'always' traditional behavior always allows mem overrides.

punctuation please

> +                       - 'ptrace' only allow for active ptracers.
> +                       - 'never'  never allow mem permission overrides.

Please be consistent: 'mem overrides' or 'mem permission overrides' in
both instances.

> +                       If not specified, default is always.

'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..0ca3fc3d9e0e 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -85,6 +85,7 @@
>  #include <linux/elf.h>
>  #include <linux/pid_namespace.h>
>  #include <linux/user_namespace.h>
> +#include <linux/fs_parser.h>
>  #include <linux/fs_struct.h>
>  #include <linux/slab.h>
>  #include <linux/sched/autogroup.h>
> @@ -117,6 +118,35 @@
>  static u8 nlink_tid __ro_after_init;
>  static u8 nlink_tgid __ro_after_init;
>
> +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;
> +
> +struct constant_table proc_mem_force_table[] = {

This can be static const __initconst

> +       { "always", PROC_MEM_FORCE_ALWAYS },
> +       { "ptrace", PROC_MEM_FORCE_PTRACE },
> +       { }
> +};
> +
> +static int __init early_proc_mem_force_override(char *buf)
> +{
> +       if (!buf)
> +               return -EINVAL;
> +

Can this ever happen?

> +       proc_mem_force_override = lookup_constant(proc_mem_force_table,
> +                                                 buf, PROC_MEM_FORCE_NEVER);
> +
> +       return 0;
> +}
> +early_param("proc_mem.force_override", early_proc_mem_force_override);
> +
>  struct pid_entry {
>         const char *name;
>         unsigned int len;
> @@ -835,6 +865,26 @@ 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;
> +       }

This indentation looks dodgy. If you move the local var declarations
out of this block, and use assignments instead, you don't need to  { }
at all.


> +       default:
> +               return true;
> +       }
> +}
> +
>  static ssize_t mem_rw(struct file *file, char __user *buf,
>                         size_t count, loff_t *ppos, int write)
>  {
> @@ -855,7 +905,9 @@ 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
>
>
Adrian Ratiu July 26, 2024, 4:15 p.m. UTC | #2
On Friday, July 26, 2024 13:18 EEST, Ard Biesheuvel <ardb@kernel.org> wrote:

> On Fri, 26 Jul 2024 at 11:11, 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.
> >
> > 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>
> > ---
> > Changes in v3:
> > * Simplified code to use shorthand ifs and a
> >   lookup_constant() table.
> >
> > Changes in v2:
> > * Added bootparam on top of Linus' patch.
> > * Slightly reworded commit msg.
> > ---
> >  .../admin-guide/kernel-parameters.txt         | 10 ++++
> >  fs/proc/base.c                                | 54 ++++++++++++++++++-
> >  security/Kconfig                              | 32 +++++++++++
> >  3 files changed, 95 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.
> 
> Better to use passive tense here rather than referring to 'users' and 'people'.
> 
> 'Traditionally, /proc/pid/mem allows memory permissions to be
> overridden without restrictions.
> This option may be set to restrict that'
> 
> > +                       Can be one of:
> > +                       - 'always' traditional behavior always allows mem overrides.
> 
> punctuation please
> 
> > +                       - 'ptrace' only allow for active ptracers.
> > +                       - 'never'  never allow mem permission overrides.
> 
> Please be consistent: 'mem overrides' or 'mem permission overrides' in
> both instances.
> 
> > +                       If not specified, default is always.
> 
> '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..0ca3fc3d9e0e 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -85,6 +85,7 @@
> >  #include <linux/elf.h>
> >  #include <linux/pid_namespace.h>
> >  #include <linux/user_namespace.h>
> > +#include <linux/fs_parser.h>
> >  #include <linux/fs_struct.h>
> >  #include <linux/slab.h>
> >  #include <linux/sched/autogroup.h>
> > @@ -117,6 +118,35 @@
> >  static u8 nlink_tid __ro_after_init;
> >  static u8 nlink_tgid __ro_after_init;
> >
> > +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;
> > +
> > +struct constant_table proc_mem_force_table[] = {
> 
> This can be static const __initconst
> 
> > +       { "always", PROC_MEM_FORCE_ALWAYS },
> > +       { "ptrace", PROC_MEM_FORCE_PTRACE },
> > +       { }
> > +};
> > +
> > +static int __init early_proc_mem_force_override(char *buf)
> > +{
> > +       if (!buf)
> > +               return -EINVAL;
> > +
> 
> Can this ever happen?

Not sure, many calls simply ignore this case while others
like this [1] printk example do test it. I'm inclined to think
it can't happen however it's still to good to error check.

Thanks for all the suggestions, I'll leave this a bit for others
to get a chance to review, then send another iteration.

[1] https://elixir.bootlin.com/linux/v6.10.1/source/kernel/printk/printk.c#L1051
diff mbox series

Patch

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..0ca3fc3d9e0e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -85,6 +85,7 @@ 
 #include <linux/elf.h>
 #include <linux/pid_namespace.h>
 #include <linux/user_namespace.h>
+#include <linux/fs_parser.h>
 #include <linux/fs_struct.h>
 #include <linux/slab.h>
 #include <linux/sched/autogroup.h>
@@ -117,6 +118,35 @@ 
 static u8 nlink_tid __ro_after_init;
 static u8 nlink_tgid __ro_after_init;
 
+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;
+
+struct constant_table proc_mem_force_table[] = {
+	{ "always", PROC_MEM_FORCE_ALWAYS },
+	{ "ptrace", PROC_MEM_FORCE_PTRACE },
+	{ }
+};
+
+static int __init early_proc_mem_force_override(char *buf)
+{
+	if (!buf)
+		return -EINVAL;
+
+	proc_mem_force_override = lookup_constant(proc_mem_force_table,
+						  buf, PROC_MEM_FORCE_NEVER);
+
+	return 0;
+}
+early_param("proc_mem.force_override", early_proc_mem_force_override);
+
 struct pid_entry {
 	const char *name;
 	unsigned int len;
@@ -835,6 +865,26 @@  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 +905,9 @@  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