From: Li Qiang <liqiang6-s@360.cn>
Date: Tue, 1 Nov 2016 12:00:40 +0100
Subject: [PATCH] 9pfs: fix integer overflow issue in xattr read/write
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The v9fs_xattr_read() and v9fs_xattr_write() are passed a guest
originated offset: they must ensure this offset does not go beyond
the size of the extended attribute that was set in v9fs_xattrcreate().
Unfortunately, the current code implement these checks with unsafe
calculations on 32 and 64 bit values, which may allow a malicious
guest to cause OOB access anyway.

Fix this by comparing the offset and the xattr size, which are
both uint64_t, before trying to compute the effective number of bytes
to read or write.

Suggested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-By: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
(cherry picked from commit 7e55d65c56a03dcd2c5d7c49d37c5a74b55d4bd6)
---
 hw/9pfs/9p.c | 32 ++++++++++++--------------------
 1 file changed, 12 insertions(+), 20 deletions(-)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index ad57123aaf..9c18322945 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -1629,20 +1629,17 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
 {
     ssize_t err;
     size_t offset = 7;
-    int read_count;
-    int64_t xattr_len;
+    uint64_t read_count;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
 
-    xattr_len = fidp->fs.xattr.len;
-    read_count = xattr_len - off;
+    if (fidp->fs.xattr.len < off) {
+        read_count = 0;
+    } else {
+        read_count = fidp->fs.xattr.len - off;
+    }
     if (read_count > max_count) {
         read_count = max_count;
-    } else if (read_count < 0) {
-        /*
-         * read beyond XATTR value
-         */
-        read_count = 0;
     }
     err = pdu_marshal(pdu, offset, "d", read_count);
     if (err < 0) {
@@ -1970,23 +1967,18 @@ static int v9fs_xattr_write(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
 {
     int i, to_copy;
     ssize_t err = 0;
-    int write_count;
-    int64_t xattr_len;
+    uint64_t write_count;
     size_t offset = 7;
 
 
-    xattr_len = fidp->fs.xattr.len;
-    write_count = xattr_len - off;
-    if (write_count > count) {
-        write_count = count;
-    } else if (write_count < 0) {
-        /*
-         * write beyond XATTR value len specified in
-         * xattrcreate
-         */
+    if (fidp->fs.xattr.len < off) {
         err = -ENOSPC;
         goto out;
     }
+    write_count = fidp->fs.xattr.len - off;
+    if (write_count > count) {
+        write_count = count;
+    }
     err = pdu_marshal(pdu, offset, "d", write_count);
     if (err < 0) {
         return err;