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Re: [Qemu-stable] [Qemu-devel] [PATCH] fw_cfg: fix memory corruption whe
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-stable] [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used |
Date: |
Tue, 9 Jan 2018 14:09:00 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 |
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
> *filename,
> * 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. 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
> *filename,
> assert(s->files);
>
> 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
> *filename,
> 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.)
Thanks
Laszlo
Re: [Qemu-stable] [Qemu-devel] [PATCH] fw_cfg: fix memory corruption when all fw_cfg slots are used, Eduardo Habkost, 2018/01/12