proc: don't use FOLL_FORCE for reading cmdline and environment

BugLink: https://bugs.launchpad.net/bugs/1811080

commit 272ddc8b upstream.

Now that Lorenzo cleaned things up and made the FOLL_FORCE users
explicit, it becomes obvious how some of them don't really need
FOLL_FORCE at all.

So remove FOLL_FORCE from the proc code that reads the command line and
arguments from user space.

The mem_rw() function actually does want FOLL_FORCE, because gdd (and
possibly many other debuggers) use it as a much more convenient version
of PTRACE_PEEKDATA, but we should consider making the FOLL_FORCE part
conditional on actually being a ptracer.  This does not actually do
that, just moves adds a comment to that effect and moves the gup_flags
settings next to each other.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

fs/proc/base.c

@@ -254,7 +254,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	 * Inherently racy -- command line shares address space
 	 * with code and data.
 	 */
-	rv = access_remote_vm(mm, arg_end - 1, &c, 1, FOLL_FORCE);
+	rv = access_remote_vm(mm, arg_end - 1, &c, 1, 0);
 	if (rv <= 0)
 		goto out_free_page;
 
@@ -272,8 +272,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			int nr_read;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count,
-					FOLL_FORCE);
+			nr_read = access_remote_vm(mm, p, page, _count, 0);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -308,8 +307,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			bool final;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count,
-					FOLL_FORCE);
+			nr_read = access_remote_vm(mm, p, page, _count, 0);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -358,8 +356,7 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			bool final;
 
 			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count,
-					FOLL_FORCE);
+			nr_read = access_remote_vm(mm, p, page, _count, 0);
 			if (nr_read < 0)
 				rv = nr_read;
 			if (nr_read <= 0)
@@ -878,7 +875,7 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	unsigned long addr = *ppos;
 	ssize_t copied;
 	char *page;
-	unsigned int flags = FOLL_FORCE;
+	unsigned int flags;
 
 	if (!mm)
 		return 0;
@@ -891,6 +888,8 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 	if (!atomic_inc_not_zero(&mm->mm_users))
 		goto free;
 
+	/* Maybe we should limit FOLL_FORCE to actual ptrace users? */
+	flags = FOLL_FORCE;
 	if (write)
 		flags |= FOLL_WRITE;
 
@@ -1014,8 +1013,7 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 		max_len = min_t(size_t, PAGE_SIZE, count);
 		this_len = min(max_len, this_len);
 
-		retval = access_remote_vm(mm, (env_start + src),
-			page, this_len, FOLL_FORCE);
+		retval = access_remote_vm(mm, (env_start + src), page, this_len, 0);
 
 		if (retval <= 0) {
 			ret = retval;