linux-kernel - [PATCH v2] cifs: sanitize multiple delimiters in prepath

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Message-Id: <20211217182022.589857-1-trbecker@gmail.com>
Date:   Fri, 17 Dec 2021 15:20:22 -0300
From:   Thiago Rafael Becker <trbecker@...il.com>
To:     linux-cifs@...r.kernel.org
Cc:     Steve French <sfrench@...ba.org>, linux-kernel@...r.kernel.org,
        lsahlber@...hat.com, Thiago Rafael Becker <trbecker@...il.com>,
        stable@...r.kernel.org
Subject: [PATCH v2] cifs: sanitize multiple delimiters in prepath

mount.cifs can pass a device with multiple delimiters in it. This will
cause rename(2) to fail with ENOENT.

V2:
  - Make sanitize_path more readable.
  - Fix multiple delimiters between UNC and prepath.
  - Avoid a memory leak if a bad user starts putting a lot of delimiters
    in the path on purpose.

BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=2031200
Fixes: 24e0a1eff9e2 ("cifs: switch to new mount api")
Cc: stable@...r.kernel.org # 5.11+
Signed-off-by: Thiago Rafael Becker <trbecker@...il.com>
---
 fs/cifs/fs_context.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/fs/cifs/fs_context.c b/fs/cifs/fs_context.c
index 6a179ae753c1..e3ed25dc6f3f 100644
--- a/fs/cifs/fs_context.c
+++ b/fs/cifs/fs_context.c
@@ -434,6 +434,42 @@ int smb3_parse_opt(const char *options, const char *key, char **val)
 	return rc;
 }
 
+/*
+ * Remove duplicate path delimiters. Windows is supposed to do that
+ * but there are some bugs that prevent rename from working if there are
+ * multiple delimiters.
+ *
+ * Returns a sanitized duplicate of @path. The caller is responsible for
+ * cleaning up the original.
+ */
+#define IS_DELIM(c) ((c) == '/' || (c) == '\\')
+static char *sanitize_path(char *path)
+{
+	char *cursor1 = path, *cursor2 = path;
+
+	/* skip all prepended delimiters */
+	while (IS_DELIM(*cursor1))
+		cursor1++;
+
+	/* copy the first letter */
+	*cursor2 = *cursor1;
+
+	/* copy the remainder... */
+	while (*(cursor1++)) {
+		/* ... skipping all duplicated delimiters */
+		if (IS_DELIM(*cursor1) && IS_DELIM(*cursor2))
+			continue;
+		*(++cursor2) = *cursor1;
+	}
+
+	/* if the last character is a delimiter, skip it */
+	if (IS_DELIM(*(cursor2 - 1)))
+		cursor2--;
+
+	*(cursor2) = '\0';
+	return kstrdup(path, GFP_KERNEL);
+}
+
 /*
  * Parse a devname into substrings and populate the ctx->UNC and ctx->prepath
  * fields with the result. Returns 0 on success and an error otherwise
@@ -493,7 +529,7 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
 	if (!*pos)
 		return 0;
 
-	ctx->prepath = kstrdup(pos, GFP_KERNEL);
+	ctx->prepath = sanitize_path(pos);
 	if (!ctx->prepath)
 		return -ENOMEM;
 
-- 
2.31.1