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net/tftp: Prevent a UAF and double-free from a failed seek

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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>
This commit is contained in:
Daniel Axtens 2021-09-20 01:12:24 +10:00 committed by Daniel Kiper
parent 96abf4fb9d
commit ee96520314
3 changed files with 11 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
grub-core/net/net.c
View File

@ -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;

1
grub-core/net/tftp.c
View File

@ -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;
}

1
include/grub/net.h
View File

@ -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);