2021-12-21 02:05:47

by Hao Peng

Subject: [PATCH] fs/squashfs: handle possible null pointer

in squashfs_fill_super:

msblk->decompressor = supported_squashfs_filesystem(
fc,
le16_to_cpu(sblk->s_major),
le16_to_cpu(sblk->s_minor),
le16_to_cpu(sblk->compression));
if (msblk->decompressor == NULL)
goto failed_mount;
...

failed_mount:
...
squashfs_decompressor_destroy(msblk);

in squashfs_decompressor_destroy:
if (stream) {
msblk->decompressor->free(stream->stream);
msblk->decompressor is NULL.

so add a judgment whether a null pointer.

Signed-off-by: Peng Hao <[email protected]>
---
fs/squashfs/decompressor_single.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/decompressor_single.c b/fs/squashfs/decompressor_single.c
index 4eb3d083d45e..a155452bbc54 100644
--- a/fs/squashfs/decompressor_single.c
+++ b/fs/squashfs/decompressor_single.c
@@ -54,7 +54,8 @@ void squashfs_decompressor_destroy(struct squashfs_sb_info *msblk)
struct squashfs_stream *stream = msblk->stream;

if (stream) {
- msblk->decompressor->free(stream->stream);
+ if (msblk->decompressor)
+ msblk->decompressor->free(stream->stream);
kfree(stream);
}
}
--
2.27.0

2021-12-22 01:46:21

by Phillip Lougher

Subject: Re: [PATCH] fs/squashfs: handle possible null pointer

On 21/12/2021 02:03, Peng Hao wrote:
> in squashfs_fill_super:
>
> msblk->decompressor = supported_squashfs_filesystem(
> fc,
> le16_to_cpu(sblk->s_major),
> le16_to_cpu(sblk->s_minor),
> le16_to_cpu(sblk->compression));
> if (msblk->decompressor == NULL)
> goto failed_mount;
> ...
>
> failed_mount:
> ...
> squashfs_decompressor_destroy(msblk);
>
> in squashfs_decompressor_destroy:
> if (stream) {
> msblk->decompressor->free(stream->stream);
> msblk->decompressor is NULL.
>
> so add a judgment whether a null pointer.

NACK.

The NULL pointer dereference (msblk->decompressor) will not happen
because stream (msblk->stream) is NULL (thus the code in question will
not be executed).

At the start of the initialisation phase msblk is zeroed out

sb->s_fs_info = kzalloc(sizeof(*msblk), GFP_KERNEL);
if (sb->s_fs_info == NULL) {
ERROR("Failed to allocate squashfs_sb_info\n");
return -ENOMEM;
}
msblk = sb->s_fs_info;

Making msblk->decompressor and msblk->stream NULL.

msblk->decompressor is allocated first

msblk->decompressor = supported_squashfs_filesystem(
fc,
le16_to_cpu(sblk->s_major),
le16_to_cpu(sblk->s_minor),
le16_to_cpu(sblk->compression));
if (msblk->decompressor == NULL)
goto failed_mount;

If that succeeds (msblk->decompressor != NULL), then msblk->stream is
allocated

msblk->stream = squashfs_decompressor_setup(sb, flags);
if (IS_ERR(msblk->stream)) {
err = PTR_ERR(msblk->stream);
msblk->stream = NULL;
goto insanity;
}

Thus, in squashfs_decompressor_destroy() if stream (msblk->stream) is
not NULL, then msblk->decompressor is also guaranteed to be not NULL.

Likewise if msblk->decompressor is NULL, then msblk->stream will also be
NULL.

Or to put it another way, the decompressor is only destroyed if it was
previously created (msblk->stream != NULL), and it will only have been
created if msblk->decompressor is != NULL.

Phillip