[PATCH AUTOSEL 6.11 064/244] proc: add config & param to block forcing mem writes

Sasha Levin
2024-09-25T11:39:15Z

From: Adrian Ratiu <adrian.ratiu@collabora.com>

[ Upstream commit 41e8149c8892ed1962bd15350b3c3e6e90cba7f4 ]

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: Ard Biesheuvel <ardb@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>
Link: https://lore.kernel.org/r/20240802080225.89408-1-adrian.ratiu@collabora.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
.../admin-guide/kernel-parameters.txt | 10 +++
fs/proc/base.c | 61 ++++++++++++++++++-
security/Kconfig | 32 ++++++++++
3 files changed, 102 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 09126bb8cc9ff..be010fec76541 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4788,6 +4788,16 @@
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 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.
+ - 'ptrace': only allow mem overrides for active ptracers.
+ - 'never': never allow mem overrides.
+ If not specified, default is the CONFIG_PROC_MEM_* choice.
+
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 72a1acd03675c..f389c69767fa5 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 +118,40 @@
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_NO_FORCE) ? PROC_MEM_FORCE_NEVER :
+ IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE :
+ PROC_MEM_FORCE_ALWAYS;
+
+static const struct constant_table proc_mem_force_table[] __initconst = {
+ { "always", PROC_MEM_FORCE_ALWAYS },
+ { "ptrace", PROC_MEM_FORCE_PTRACE },
+ { "never", PROC_MEM_FORCE_NEVER },
+ { }
+};
+
+static int __init early_proc_mem_force_override(char *buf)
+{
+ if (!buf)
+ return -EINVAL;
+
+ /*
+ * lookup_constant() defaults to proc_mem_force_override to preseve
+ * the initial Kconfig choice in case an invalid param gets passed.
+ */
+ proc_mem_force_override = lookup_constant(proc_mem_force_table,
+ buf, proc_mem_force_override);
+
+ return 0;
+}
+early_param("proc_mem.force_override", early_proc_mem_force_override);
+
struct pid_entry {
const char *name;
unsigned int len;
@@ -835,6 +870,28 @@ 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)
+{
+ struct task_struct *task;
+ bool ptrace_active = false;
+
+ switch (proc_mem_force_override) {
+ case PROC_MEM_FORCE_NEVER:
+ return false;
+ case PROC_MEM_FORCE_PTRACE:
+ task = get_proc_task(file_inode(file));
+ if (task) {
+ ptrace_active = READ_ONCE(task->ptrace) &&
+ READ_ONCE(task->mm) == mm &&
+ READ_ONCE(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 +912,9 @@ 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 412e76f1575d0..a93c1a9b7c283 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
--
2.43.0