Date:   Sun, 17 Nov 2019 12:17:04 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Jonathan Corbet <corbet@....net>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        dev@...ncontainers.org, containers@...ts.linux-foundation.org,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-api@...r.kernel.org,
        libc-alpha@...rceware.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: [PATCH v17 04/13] namei: allow set_root() to produce errors

For LOOKUP_BENEATH and LOOKUP_IN_ROOT it is necessary to ensure that
set_root() is never called, and thus (for hardening purposes) it should
return an error rather than permit a breakout from the root. In
addition, move all of the repetitive set_root() calls to nd_jump_root().

Signed-off-by: Aleksa Sarai <cyphar@...har.com>
---
 fs/namei.c | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1024a641f075..74574a69a614 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -798,7 +798,7 @@ static int complete_walk(struct nameidata *nd)
 	return status;
 }
 
-static void set_root(struct nameidata *nd)
+static int set_root(struct nameidata *nd)
 {
 	struct fs_struct *fs = current->fs;
 
@@ -814,6 +814,7 @@ static void set_root(struct nameidata *nd)
 		get_fs_root(fs, &nd->root);
 		nd->flags |= LOOKUP_ROOT_GRABBED;
 	}
+	return 0;
 }
 
 static void path_put_conditional(struct path *path, struct nameidata *nd)
@@ -837,6 +838,11 @@ static inline void path_to_nameidata(const struct path *path,
 
 static int nd_jump_root(struct nameidata *nd)
 {
+	if (!nd->root.mnt) {
+		int error = set_root(nd);
+		if (error)
+			return error;
+	}
 	if (nd->flags & LOOKUP_RCU) {
 		struct dentry *d;
 		nd->path = nd->root;
@@ -1080,10 +1086,9 @@ const char *get_link(struct nameidata *nd)
 			return res;
 	}
 	if (*res == '/') {
-		if (!nd->root.mnt)
-			set_root(nd);
-		if (unlikely(nd_jump_root(nd)))
-			return ERR_PTR(-ECHILD);
+		error = nd_jump_root(nd);
+		if (unlikely(error))
+			return ERR_PTR(error);
 		while (unlikely(*++res == '/'))
 			;
 	}
@@ -1698,8 +1703,13 @@ static inline int may_lookup(struct nameidata *nd)
 static inline int handle_dots(struct nameidata *nd, int type)
 {
 	if (type == LAST_DOTDOT) {
-		if (!nd->root.mnt)
-			set_root(nd);
+		int error = 0;
+
+		if (!nd->root.mnt) {
+			error = set_root(nd);
+			if (error)
+				return error;
+		}
 		if (nd->flags & LOOKUP_RCU) {
 			return follow_dotdot_rcu(nd);
 		} else
@@ -2162,6 +2172,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 /* must be paired with terminate_walk() */
 static const char *path_init(struct nameidata *nd, unsigned flags)
 {
+	int error;
 	const char *s = nd->name->name;
 
 	if (!*s)
@@ -2194,11 +2205,13 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	nd->path.dentry = NULL;
 
 	nd->m_seq = read_seqbegin(&mount_lock);
+
+	/* Figure out the starting path and root (if needed). */
 	if (*s == '/') {
-		set_root(nd);
-		if (likely(!nd_jump_root(nd)))
-			return s;
-		return ERR_PTR(-ECHILD);
+		error = nd_jump_root(nd);
+		if (unlikely(error))
+			return ERR_PTR(error);
+		return s;
 	} else if (nd->dfd == AT_FDCWD) {
 		if (flags & LOOKUP_RCU) {
 			struct fs_struct *fs = current->fs;
-- 
2.24.0

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