Re: [PATCH] proc: revert /proc/*/cmdline rewrite

[Linus Torvalds]

[ Apparently this thread wasn't on the lists, and I didn't even
notice. So re-sending the patches ]

On Sat, Jul 13, 2019 at 9:16 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Given that this fixes a regression and a bug.
>
> Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

I'd much rather start from scratch. Like the attached.

Alexey Izbyshev has a third patch that then limits the setproctitle()
case to only allow looking into the env[] area, which looks
reasonable.

           Linus
From 5563a2fb39fe0ad42f239d2f583128d50155002b Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Sat, 13 Jul 2019 13:40:13 -0700
Subject: [PATCH 1/2] /proc/<pid>/cmdline: remove all the special cases

Start off with a clean slate that only reads exactly from arg_start to
arg_end, without any oddities.

We'll add back the setproctitle() special case very differently in the
next commit.

Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 fs/proc/base.c | 71 ++++++--------------------------------------------
 1 file changed, 8 insertions(+), 63 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 255f6754c70d..8040f9d1cf07 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -212,7 +212,7 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
 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 arg_start, arg_end;
 	unsigned long pos, len;
 	char *page;
 
@@ -223,36 +223,18 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 	spin_lock(&mm->arg_lock);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
-	env_start = mm->env_start;
-	env_end = mm->env_end;
 	spin_unlock(&mm->arg_lock);
 
 	if (arg_start >= arg_end)
 		return 0;
 
-	/*
-	 * We have traditionally allowed the user to re-write
-	 * the argument strings and overflow the end result
-	 * into the environment section. But only do that if
-	 * the environment area is contiguous to the arguments.
-	 */
-	if (env_start != arg_end || env_start >= env_end)
-		env_start = env_end = arg_end;
-
-	/* .. and limit it to a maximum of one page of slop */
-	if (env_end >= arg_end + PAGE_SIZE)
-		env_end = arg_end + PAGE_SIZE - 1;
-
 	/* We're not going to care if "*ppos" has high bits set */
-	pos = arg_start + *ppos;
-
 	/* .. but we do check the result is in the proper range */
-	if (pos < arg_start || pos >= env_end)
+	pos = arg_start + *ppos;
+	if (pos < arg_start || pos >= arg_end)
 		return 0;
-
-	/* .. and we never go past env_end */
-	if (env_end - pos < count)
-		count = env_end - pos;
+	if (count > arg_end - pos)
+		count = arg_end - pos;
 
 	page = (char *)__get_free_page(GFP_KERNEL);
 	if (!page)
@@ -262,48 +244,11 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 	while (count) {
 		int got;
 		size_t size = min_t(size_t, PAGE_SIZE, count);
-		long offset;
 
-		/*
-		 * Are we already starting past the official end?
-		 * We always include the last byte that is *supposed*
-		 * to be NUL
-		 */
-		offset = (pos >= arg_end) ? pos - arg_end + 1 : 0;
-
-		got = access_remote_vm(mm, pos - offset, page, size + offset, FOLL_ANON);
-		if (got <= offset)
+		got = access_remote_vm(mm, pos, page, size, FOLL_ANON);
+		if (got <= 0)
 			break;
-		got -= offset;
-
-		/* Don't walk past a NUL character once you hit arg_end */
-		if (pos + got >= arg_end) {
-			int n = 0;
-
-			/*
-			 * If we started before 'arg_end' but ended up
-			 * at or after it, we start the NUL character
-			 * check at arg_end-1 (where we expect the normal
-			 * EOF to be).
-			 *
-			 * NOTE! This is smaller than 'got', because
-			 * pos + got >= arg_end
-			 */
-			if (pos < arg_end)
-				n = arg_end - pos - 1;
-
-			/* Cut off at first NUL after 'n' */
-			got = n + strnlen(page+n, offset+got-n);
-			if (got < offset)
-				break;
-			got -= offset;
-
-			/* Include the NUL if it existed */
-			if (got < size)
-				got++;
-		}
-
-		got -= copy_to_user(buf, page+offset, got);
+		got -= copy_to_user(buf, page, got);
 		if (unlikely(!got)) {
 			if (!len)
 				len = -EFAULT;
-- 
2.22.0.193.g083935f9a2

From 63dd14999c6b210fbe33f780fec53faefa867d95 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Sat, 13 Jul 2019 14:27:14 -0700
Subject: [PATCH 2/2] /proc/<pid>/cmdline: add back the setproctitle() special
 case

This makes the setproctitle() special case very explicit indeed, and
handles it with a separate helper function entirely.

This makes the logic about when we use the string lengths etc much more
obvious, and makes it easy to see what we do.

[ Fixed for missing 'count' check noted by Alexey Izbyshev ]
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 fs/proc/base.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 8040f9d1cf07..3ad3ff4cc12c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -209,12 +209,54 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
 	return result;
 }
 
+/*
+ * If the user used setproctitle(), we just get the string from
+ * user space at arg_start, and limit it to a maximum of one page.
+ */
+static ssize_t get_mm_proctitle(struct mm_struct *mm, char __user *buf,
+				size_t count, loff_t *ppos,
+				unsigned long arg_start)
+{
+	unsigned long pos = *ppos;
+	char *page;
+	int ret, got;
+
+	if (pos >= PAGE_SIZE)
+		return 0;
+
+	page = (char *)__get_free_page(GFP_KERNEL);
+	if (!page)
+		return -ENOMEM;
+
+	ret = 0;
+	got = access_remote_vm(mm, arg_start, page, PAGE_SIZE, FOLL_ANON);
+	if (got > 0) {
+		int len = strnlen(page, got);
+
+		/* Include the NUL character if it was found */
+		if (len < got)
+			len++;
+
+		if (len > pos) {
+			len -= pos;
+			if (len > count)
+				len = count;
+			len -= copy_to_user(buf, page+pos, len);
+			if (!len)
+				len = -EFAULT;
+			ret = len;
+		}
+	}
+	free_page((unsigned long)page);
+	return ret;
+}
+
 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;
 	unsigned long pos, len;
-	char *page;
+	char *page, c;
 
 	/* Check if process spawned far enough to have cmdline. */
 	if (!mm->env_end)
@@ -228,6 +270,16 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
 	if (arg_start >= arg_end)
 		return 0;
 
+	/*
+	 * Magical special case: if the argv[] end byte is not
+	 * zero, the user has overwritten it with setproctitle(3).
+	 *
+	 * Possible future enhancement: do this only once when
+	 * pos is 0, and set a flag in the 'struct file'.
+	 */
+	if (access_remote_vm(mm, arg_end-1, &c, 1, FOLL_ANON) == 1 && c)
+		return get_mm_proctitle(mm, buf, count, ppos, arg_start);
+
 	/* We're not going to care if "*ppos" has high bits set */
 	/* .. but we do check the result is in the proper range */
 	pos = arg_start + *ppos;
-- 
2.22.0.193.g083935f9a2