[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg s

From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used
Date: Tue, 9 Jan 2018 15:18:45 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 09/01/2018 15:09, Laszlo Ersek wrote:

Hi Laszlo,

I'll respond first to this mail' I'll take my time with the rest :)

On 01/08/18 22:50, Marcel Apfelbaum wrote:
When all the fw_cfg slots are used, a write is made outside the
bounds of the fw_cfg files array as part of the sort algorithm.

Fix it by avoiding an unnecessary array element move.
Fix also an assert while at it.

Signed-off-by: Marcel Apfelbaum <address@hidden>
  hw/nvram/fw_cfg.c | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index 753ac0e4ea..4313484b21 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -784,7 +784,7 @@ void fw_cfg_add_file_callback(FWCfgState *s,  const char 
       * index and "i - 1" is the one being copied from, thus the
       * unusual start and end in the for statement.
-    for (i = count + 1; i > index; i--) {
+    for (i = count; i > index; i--) {
          s->files->f[i] = s->files->f[i - 1];
          s->files->f[i].select = cpu_to_be16(FW_CFG_FILE_FIRST + i);
          s->entries[0][FW_CFG_FILE_FIRST + i] =

This hunk looks correct to me.

After my change or before?

I think I am right.
At this point we have "count" elements in the array.
That means the last element in the array is at arr[count - 1].
We want to make room for the new element at index, so we move
all the elements from index to index + 1.

The first element we should move is arr[count - 1] to arr[count].
But the code moved arr[count] to arr [count + 1].
This move is not needed.

 We currently have count elements in the
array, so we cannot normally access the element *at* count. However, we
are extending the array right now, therefore we can assign (store) the
element at count (and then we'll increment count later). But accessing
an element at (count+1) is wrong.

@@ -833,7 +833,6 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
index = be32_to_cpu(s->files->count);
-    assert(index < fw_cfg_file_slots(s));
for (i = 0; i < index; i++) {
          if (strcmp(filename, s->files->f[i].name) == 0) {
@@ -843,6 +842,9 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
              return ptr;
+    assert(index < fw_cfg_file_slots(s));
      /* add new one */
      fw_cfg_add_file_callback(s, filename, NULL, NULL, NULL, data, len, true);
      return NULL;

I think I agree with Marc-André here, when I say, replace the assert
with a comment instead? (About the fact that fw_cfg_add_file_callback()
will assert(), *if* we reach that far.)

Hmm, what should we add to the comment? "We lost, brace for impact :)"

My point, if we are going to abort, let's abort as early as we can.
But if is a consensus, I'll get rid of it.



reply via email to

[Prev in Thread] Current Thread [Next in Thread]