[PATCH] fs: ubifs: fix bugs involving symlinks in ubifs_findfile

[Peter Collingbourne]

When encountering a symlink pointing to an absolute path, ubifs_findfile
would return the target of the symlink as the result instead of resolving
any following components in the original path. Fix it by following the
same code path that is used for relative paths except that we set the
next inode to the root if we see a leading slash.

The existing code used memcpy and sprintf to copy the symlink target
into a fixed size stack buffer and was therefore vulnerable to buffer
overflows with a sufficiently long symlink target. Fix it by using a
heap buffer for the temporary path during path resolution.

Signed-off-by: Peter Collingbourne <peter at pcc.me.uk>
Fixes: 9d7952e4c636 ("ubifs: Add support for looking up directory and relative symlinks")
---
 fs/ubifs/ubifs.c | 70 +++++++++++++++++++++++++++++++-----------------
 1 file changed, 45 insertions(+), 25 deletions(-)

diff --git a/fs/ubifs/ubifs.c b/fs/ubifs/ubifs.c
index b0cc0d2e1b2..aafbd01a028 100644
--- a/fs/ubifs/ubifs.c
+++ b/fs/ubifs/ubifs.c
@@ -505,26 +505,32 @@ static unsigned long ubifs_findfile(struct super_block *sb, char *filename)
 {
 	int ret;
 	char *next;
-	char fpath[128];
-	char symlinkpath[128];
-	char *name = fpath;
+	char *buf;
+	char *name;
 	unsigned long root_inum = 1;
 	unsigned long inum;
 	int symlink_count = 0; /* Don't allow symlink recursion */
-	char link_name[64];
-
-	strcpy(fpath, filename);
+	size_t filenamelen;
 
 	/* Remove all leading slashes */
-	while (*name == '/')
-		name++;
+	while (*filename == '/')
+		filename++;
+
+	filenamelen = strlen(filename);
+	buf = kmalloc(filenamelen + 1, GFP_NOFS);
+	if (!buf)
+		return -ENOMEM;
+	memcpy(buf, filename, filenamelen + 1);
+	name = buf;
 
 	/*
 	 * Handle root-direcoty ('/')
 	 */
 	inum = root_inum;
-	if (!name || *name == '\0')
+	if (!name || *name == '\0') {
+		kfree(buf);
 		return inum;
+	}
 
 	for (;;) {
 		struct inode *inode;
@@ -537,41 +543,53 @@ static unsigned long ubifs_findfile(struct super_block *sb, char *filename)
 			while (*next == '/')
 				*(next++) = '\0';
 		}
-
 		ret = ubifs_finddir(sb, name, root_inum, &inum);
-		if (!ret)
+		if (!ret) {
+			kfree(buf);
 			return 0;
+		}
 		inode = ubifs_iget(sb, inum);
 
-		if (!inode)
+		if (!inode) {
+			kfree(buf);
 			return 0;
+		}
 		ui = ubifs_inode(inode);
 
 		if ((inode->i_mode & S_IFMT) == S_IFLNK) {
-			char buf[128];
+			size_t newbufsize;
+			char *newbuf;
+			char *linkdata = ui->data;
+			size_t linklen = ui->data_len;
 
 			/* We have some sort of symlink recursion, bail out */
 			if (symlink_count++ > 8) {
 				ubifs_iput(inode);
 				printf("Symlink recursion, aborting\n");
+				kfree(buf);
 				return 0;
 			}
-			memcpy(link_name, ui->data, ui->data_len);
-			link_name[ui->data_len] = '\0';
 
-			if (link_name[0] == '/') {
-				/* Absolute path, redo everything without
-				 * the leading slash */
-				next = name = link_name + 1;
+			while (linklen && *linkdata == '/') {
+				/* Absolute path, i.e. relative to root. */
 				root_inum = 1;
+				linkdata++;
+				linklen--;
+			}
+			newbufsize =
+				linklen + 1 + (next ? strlen(next) : 0) + 1;
+			newbuf = kmalloc(newbufsize, GFP_NOFS);
+			if (!newbuf) {
+				kfree(buf);
 				ubifs_iput(inode);
-				continue;
+				return -ENOMEM;
 			}
-			/* Relative to cur dir */
-			sprintf(buf, "%s/%s",
-					link_name, next == NULL ? "" : next);
-			memcpy(symlinkpath, buf, sizeof(buf));
-			next = name = symlinkpath;
+
+			memcpy(newbuf, linkdata, linklen);
+			sprintf(newbuf + linklen, "/%s", next ?: "");
+			kfree(buf);
+			buf = newbuf;
+			name = newbuf;
 			ubifs_iput(inode);
 			continue;
 		}
@@ -583,6 +601,7 @@ static unsigned long ubifs_findfile(struct super_block *sb, char *filename)
 		/* Found the node!  */
 		if (!next || *next == '\0') {
 			ubifs_iput(inode);
+			kfree(buf);
 			return inum;
 		}
 
@@ -590,6 +609,7 @@ static unsigned long ubifs_findfile(struct super_block *sb, char *filename)
 		name = next;
 	}
 
+	kfree(buf);
 	return 0;
 }
 
-- 
2.54.0