proc: add config & param to block forcing mem writes

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>

1 files changed, 60 insertions, 1 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 72a1acd03675..f389c69767fa 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);