[PATCH] proc: add missing '\0' back to /proc/$pid/cmdline

[Michal Kubecek]

Recent rewrite introduced a regression, /proc/$pid/cmdline is missing the
trailing null character:

mike@lion:/tmp> cat /proc/self/cmdline | od -t c
0000000   c   a   t  \0   /   p   r   o   c   /   s   e   l   f   /   c
0000020   m   d   l   i   n   e
0000026

This is because strnlen() is used to search for the null character but it
returns the length without it so that we need to copy one byte more (but
only if we actually found a null character). And once we pass the null
character to userspace we need to make sure next read returns 0 (EOF).

This is another problem, even if rather theoretical one: if userspace seeks
to arg_end or past it, we only start checking for null character from that
position so that we may return random segment of environment rather then
EOF.

Resolve both by always starting no later than at arg_end-1 (so that we
always catch the right null character) but don't copy data until we reach
the requested start position.

Fixes: 5ab827189965 ("fs/proc: simplify and clarify get_mm_cmdline() function")
Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
---
 fs/proc/base.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b6572944efc3..8faafc70ee83 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -209,7 +209,8 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 			      size_t count, loff_t *ppos)
 {
 	unsigned long arg_start, arg_end, env_start, env_end;
-	unsigned long pos, len;
+	unsigned long req_pos, pos, len;
+	bool end_found = false;
 	char *page;
 
 	/* Check if process spawned far enough to have cmdline. */
@@ -236,25 +237,27 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 		env_start = env_end = arg_end;
 
 	/* We're not going to care if "*ppos" has high bits set */
-	pos = arg_start + *ppos;
+	req_pos = arg_start + *ppos;
 
 	/* .. but we do check the result is in the proper range */
-	if (pos < arg_start || pos >= env_end)
+	if (req_pos < arg_start || req_pos >= env_end)
 		return 0;
 
 	/* .. and we never go past env_end */
-	if (env_end - pos < count)
+	if (env_end - req_pos < count)
 		count = env_end - pos;
 
+	pos = min_t(unsigned long, req_pos, arg_end - 1);
 	page = (char *)__get_free_page(GFP_KERNEL);
 	if (!page)
 		return -ENOMEM;
 
 	len = 0;
-	while (count) {
+	while (count && !end_found) {
 		int got;
-		size_t size = min_t(size_t, PAGE_SIZE, count);
+		size_t size = count + (pos < req_pos ? req_pos - pos : 0);
 
+		size = min_t(size_t, PAGE_SIZE, size);
 		got = access_remote_vm(mm, pos, page, size, FOLL_ANON);
 		if (got <= 0)
 			break;
@@ -276,12 +279,26 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 				n = arg_end - pos - 1;
 
 			/* Cut off at first NUL after 'n' */
-			got = n + strnlen(page+n, got-n);
+			n += strnlen(page + n, got - n);
+			got = min_t(int, got, n + 1);
+			end_found = !page[n];
 			if (!got)
 				break;
 		}
 
-		got -= copy_to_user(buf, page, got);
+		if (pos + got <= req_pos) {
+			/* got > 0 here so that pos always advances */
+			pos += got;
+			continue;
+		}
+
+		if (pos < req_pos) {
+			got -= (req_pos - pos);
+			got -= copy_to_user(buf, page + req_pos - pos, got);
+			pos = req_pos;
+		} else {
+			got -= copy_to_user(buf, page, got);
+		}
 		if (unlikely(!got)) {
 			if (!len)
 				len = -EFAULT;
-- 
2.17.1