[Qemu-devel] [PATCH] 9pfs: disallow / in path components

[Greg Kurz]

At various places in 9pfs, full paths are created by concatenating a guest
originated string to the export path. A malicious guest could forge a
relative path and access files outside the export path.

A tentative fix was sent recently by Prasad J Pandit, but it was only
focused on the local backend and did not get a positive review. This patch
tries to address the issue more globally, based on the official 9P spec.

The walk request described in the 9P spec [1] clearly shows that the client
is supposed to send individual path components: the official linux client
never sends portions of path containing the / character for example.

Moreover, the 9P spec [2] also states that a system can decide to restrict
the set of supported characters used in path components, with an explicit
mention "to remove slashes from name components".

This patch introduces a new name_has_illegal_characters() helper that looks
for such unwanted characters in strings sent by the client. Since 9pfs is
only supported on linux hosts, only the / character is checked at the
moment. When support for other hosts (AKA. win32) is added, other chars
may need to be blacklisted as well.

If a client sends a path component with an illegal character, the request
will fail and EINVAL is returned to the client.

For the sake of simplicity and consistency, the check is done at top-level
for all affected 9P requests:

- xattrwalk
- xattrcreate
- mknod
- rename
- renameat
- unlinkat
- mkdir
- walk
- link
- symlink
- create
- lcreate

[1] http://man.cat-v.org/plan_9/5/walk
[2] http://man.cat-v.org/plan_9/5/intro

Reported-by: Felix Wilhelm <address@hidden>
Suggested-by: Peter Maydell <address@hidden>
Signed-off-by: Greg Kurz <address@hidden>
---

Since the linux client does not send / in path components and I don't
have enough time to write an appropriate qtest, I choosed to do manual
testing of the mkdir request with GDB:

address@hidden host]$ mkdir ...foo
(then turning ...foo into ../foo in QEMU with GDB)
mkdir: cannot create directory ‘...foo’: Invalid argument

I also could run the POSIX file system test suite from TUXERA:

http://www.tuxera.com/community/open-source-posix/

and did not observe any regression with this patch.

 hw/9pfs/9p.c |   67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index b6b02b46a9da..1c008814509c 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -1256,6 +1256,11 @@ static int v9fs_walk_marshal(V9fsPDU *pdu, uint16_t 
nwnames, V9fsQID *qids)
     return offset;
 }
 
+static bool name_has_illegal_characters(const char *name)
+{
+    return strchr(name, '/') != NULL;
+}
+
 static void v9fs_walk(void *opaque)
 {
     int name_idx;
@@ -1289,6 +1294,10 @@ static void v9fs_walk(void *opaque)
             if (err < 0) {
                 goto out_nofid;
             }
+            if (name_has_illegal_characters(wnames[i].data)) {
+                err = -EINVAL;
+                goto out_nofid;
+            }
             offset += err;
         }
     } else if (nwnames > P9_MAXWELEM) {
@@ -1483,6 +1492,11 @@ static void v9fs_lcreate(void *opaque)
     }
     trace_v9fs_lcreate(pdu->tag, pdu->id, dfid, flags, mode, gid);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     fidp = get_fid(pdu, dfid);
     if (fidp == NULL) {
         err = -ENOENT;
@@ -2077,6 +2091,11 @@ static void v9fs_create(void *opaque)
     }
     trace_v9fs_create(pdu->tag, pdu->id, fid, name.data, perm, mode);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     fidp = get_fid(pdu, fid);
     if (fidp == NULL) {
         err = -EINVAL;
@@ -2242,6 +2261,11 @@ static void v9fs_symlink(void *opaque)
     }
     trace_v9fs_symlink(pdu->tag, pdu->id, dfid, name.data, symname.data, gid);
 
+    if (name_has_illegal_characters(symname.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     dfidp = get_fid(pdu, dfid);
     if (dfidp == NULL) {
         err = -EINVAL;
@@ -2316,6 +2340,11 @@ static void v9fs_link(void *opaque)
     }
     trace_v9fs_link(pdu->tag, pdu->id, dfid, oldfid, name.data);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     dfidp = get_fid(pdu, dfid);
     if (dfidp == NULL) {
         err = -ENOENT;
@@ -2398,6 +2427,12 @@ static void v9fs_unlinkat(void *opaque)
     if (err < 0) {
         goto out_nofid;
     }
+
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     dfidp = get_fid(pdu, dfid);
     if (dfidp == NULL) {
         err = -EINVAL;
@@ -2504,6 +2539,12 @@ static void v9fs_rename(void *opaque)
     if (err < 0) {
         goto out_nofid;
     }
+
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     fidp = get_fid(pdu, fid);
     if (fidp == NULL) {
         err = -ENOENT;
@@ -2616,6 +2657,12 @@ static void v9fs_renameat(void *opaque)
         goto out_err;
     }
 
+    if (name_has_illegal_characters(old_name.data) ||
+        name_has_illegal_characters(new_name.data)) {
+        err = -EINVAL;
+        goto out_err;
+    }
+
     v9fs_path_write_lock(s);
     err = v9fs_complete_renameat(pdu, olddirfid,
                                  &old_name, newdirfid, &new_name);
@@ -2826,6 +2873,11 @@ static void v9fs_mknod(void *opaque)
     }
     trace_v9fs_mknod(pdu->tag, pdu->id, fid, mode, major, minor);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     fidp = get_fid(pdu, fid);
     if (fidp == NULL) {
         err = -ENOENT;
@@ -2977,6 +3029,11 @@ static void v9fs_mkdir(void *opaque)
     }
     trace_v9fs_mkdir(pdu->tag, pdu->id, fid, name.data, mode, gid);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     fidp = get_fid(pdu, fid);
     if (fidp == NULL) {
         err = -ENOENT;
@@ -3020,6 +3077,11 @@ static void v9fs_xattrwalk(void *opaque)
     }
     trace_v9fs_xattrwalk(pdu->tag, pdu->id, fid, newfid, name.data);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     file_fidp = get_fid(pdu, fid);
     if (file_fidp == NULL) {
         err = -ENOENT;
@@ -3126,6 +3188,11 @@ static void v9fs_xattrcreate(void *opaque)
     }
     trace_v9fs_xattrcreate(pdu->tag, pdu->id, fid, name.data, size, flags);
 
+    if (name_has_illegal_characters(name.data)) {
+        err = -EINVAL;
+        goto out_nofid;
+    }
+
     file_fidp = get_fid(pdu, fid);
     if (file_fidp == NULL) {
         err = -EINVAL;