Re: [PATCH 1/1] pstore/ram: Check member of buffers during the initialization phase of the pstore

From: Kees Cook
Date: Fri Aug 04 2023 - 04:10:29 EST


On Tue, Aug 01, 2023 at 02:04:32PM +0800, Yunlong Xing wrote:
> From: Enlin Mu <enlin.mu@xxxxxxxxxx>
>
> The commit 30696378f68a("pstore/ram: Do not treat empty buffers as valid")
> would introduce the following issue:
>
> When finding the buffer_size is zero, it would return directly.However, at
> the same time, if the buffer's start is a illegal value, the others would
> panic if access the buffer.

Which "others" do you mean?

> To avoid these happenning, check if the members are legal during the
> initialization phase of the pstore.
>
> Fixes: 30696378f68a ("pstore/ram: Do not treat empty buffers as valid")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Enlin Mu <enlin.mu@xxxxxxxxxx>
> ---
> fs/pstore/ram_core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index 85aaf0fc6d7d..eb6df190d752 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -519,7 +519,7 @@ static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 sig,
> sig ^= PERSISTENT_RAM_SIG;
>
> if (prz->buffer->sig == sig) {
> - if (buffer_size(prz) == 0) {
> + if (buffer_size(prz) == 0 && buffer_start(prz) == 0) {
> pr_debug("found existing empty buffer\n");
> return 0;
> }

And in the case of "buffer_size(prz) == 0" but "buffer_start(prz) != 0",
this will be caught by:

if (buffer_size(prz) > prz->buffer_size ||
buffer_start(prz) > buffer_size(prz)) {
pr_info("found existing invalid buffer, size %zu, start %zu\n",
buffer_size(prz), buffer_start(prz));
zap = true;
}

i.e. it will be detected and zapped back to a sane state.

That sounds correct to me, though I wonder if reporting it as an
"invalid buffer" is inaccurate? Perhaps we should have a separate case:

if (buffer_size(prz) == 0) {
if (buffer_start(prz) == 0)
pr_debug("found existing empty buffer\n");
else {
pr_debug("found existing empty buffer with non-zero start\n");
zap = true;
}
} else if ...

What do you think?

--
Kees Cook