From ee9652031491326736714a988fbbaeab8ef9255c Mon Sep 17 00:00:00 2001

From: Daniel Axtens <dja@axtens.net>

Date: Mon, 20 Sep 2021 01:12:24 +1000

Subject: [PATCH 4/8] net/tftp: Prevent a UAF and double-free from a failed

 seek

A malicious tftp server can cause UAFs and a double free.

An attempt to read from a network file is handled by grub_net_fs_read(). If

the read is at an offset other than the current offset, grub_net_seek_real()

is invoked.

In grub_net_seek_real(), if a backwards seek cannot be satisfied from the

currently received packets, and the underlying transport does not provide

a seek method, then grub_net_seek_real() will close and reopen the network

protocol layer.

For tftp, the ->close() call goes to tftp_close() and frees the tftp_data_t

file->data. The file->data pointer is not nulled out after the free.

If the ->open() call fails, the file->data will not be reallocated and will

continue point to a freed memory block. This could happen from a server

refusing to send the requisite ack to the new tftp request, for example.

The seek and the read will then fail, but the grub_file continues to exist:

the failed seek does not necessarily cause the entire file to be thrown

away (e.g. where the file is checked to see if it is gzipped/lzio/xz/etc.,

a read failure is interpreted as a decompressor passing on the file, not as

an invalidation of the entire grub_file_t structure).

This means subsequent attempts to read or seek the file will use the old

file->data after free. Eventually, the file will be close()d again and

file->data will be freed again.

Mark a net_fs file that doesn't reopen as broken. Do not permit read() or

close() on a broken file (seek is not exposed directly to the file API -

it is only called as part of read, so this blocks seeks as well).

As an additional defence, null out the ->data pointer if tftp_open() fails.

That would have lead to a simple null pointer dereference rather than

a mess of UAFs.

This may affect other protocols, I haven't checked.

Signed-off-by: Daniel Axtens <dja@axtens.net>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

---

 grub-core/net/net.c  | 11 +++++++++--

 grub-core/net/tftp.c |  1 +

 include/grub/net.h   |  1 +

 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/grub-core/net/net.c b/grub-core/net/net.c

index 2b6771523..9f09f8e48 100644

--- a/grub-core/net/net.c

+++ b/grub-core/net/net.c

@@ -1521,7 +1521,8 @@ grub_net_fs_close (grub_file_t file)

       grub_netbuff_free (file->device->net->packs.first->nb);

       grub_net_remove_packet (file->device->net->packs.first);

     }

-  file->device->net->protocol->close (file);

+  if (!file->device->net->broken)

+    file->device->net->protocol->close (file);

   grub_free (file->device->net->name);

   return GRUB_ERR_NONE;

 }

@@ -1744,7 +1745,10 @@ grub_net_seek_real (struct grub_file *file, grub_off_t offset)

     file->device->net->stall = 0;

     err = file->device->net->protocol->open (file, file->device->net->name);

     if (err)

-      return err;

+      {

+	file->device->net->broken = 1;

+	return err;

+      }

     grub_net_fs_read_real (file, NULL, offset);

     return grub_errno;

   }

@@ -1753,6 +1757,9 @@ grub_net_seek_real (struct grub_file *file, grub_off_t offset)

 static grub_ssize_t

 grub_net_fs_read (grub_file_t file, char *buf, grub_size_t len)

 {

+  if (file->device->net->broken)

+    return -1;

+

   if (file->offset != file->device->net->offset)

     {

       grub_err_t err;

diff --git a/grub-core/net/tftp.c b/grub-core/net/tftp.c

index ebbafe7a1..ee305e18a 100644

--- a/grub-core/net/tftp.c

+++ b/grub-core/net/tftp.c

@@ -400,6 +400,7 @@ tftp_open (struct grub_file *file, const char *filename)

     {

       grub_net_udp_close (data->sock);

       grub_free (data);

+      file->data = NULL;

       return grub_errno;

     }

diff --git a/include/grub/net.h b/include/grub/net.h

index db21e792f..a64a04cc8 100644

--- a/include/grub/net.h

+++ b/include/grub/net.h

@@ -276,6 +276,7 @@ typedef struct grub_net

   grub_fs_t fs;

   int eof;

   int stall;

+  int broken;

 } *grub_net_t;

 extern grub_net_t (*EXPORT_VAR (grub_net_open)) (const char *name);

-- 

2.33.5