.../admin-guide/kernel-parameters.txt | 10 ++++ fs/proc/base.c | 54 ++++++++++++++++++- security/Kconfig | 32 +++++++++++ 3 files changed, 95 insertions(+), 1 deletion(-)
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.
+ 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
--
2.44.2On 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
>
>
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
© 2016 - 2026 Red Hat, Inc.