Re: [PATCH] initramfs: prevent buffer overflow when unpacking torootfs

From: Aaro Koskinen
Date: Sat Apr 03 2010 - 16:41:29 EST

On Fri, 2 Apr 2010, Andrew Morton wrote:
On Thu, 1 Apr 2010 12:45:46 +0300
Aaro Koskinen <aaro.koskinen@xxxxxx> wrote:

Garbage in the initrd memory area may result in the unpack routine
accessing memory outside the buffer. The patch adds a check that the
specified area size is not exceeded.

Signed-off-by: Aaro Koskinen <aaro.koskinen@xxxxxx>
Cc: stable <stable@xxxxxxxxxx>

The patch prevents the following kernel panic on Amstrad E3:

Unpacking initramfs...
Unable to handle kernel paging request at virtual address c20121a7


--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -460,6 +460,8 @@ static char * __init unpack_to_rootfs(char *buf, unsigned len)
if (state != Reset)
error("junk in compressed archive");
+ if (my_inptr >= len)
+ break;
this_header = saved_offset + my_inptr;
buf += my_inptr;
len -= my_inptr;

OK, so if I'm understanding this right, the call to

decompress(buf, len, NULL, flush_buffer, NULL, &my_inptr, error);

has gone and generated more output data than it was asked to generate?

If so, isn't that a bug in the decompressor? Which one is your system using?

The decompressor is gzip, and it returned correct buffer position.

The problem was that the loop in unpack_to_rootfs() continued
to process the remaining buffer. my_inptr was never again updated
(decompress_method() did not recognize the remaining garbage data, so
there was no further calls to decompressor). The loop kept subtracting
my_inptr from len eventually exceeding it. Perhaps there should be some
better logic to give up earlier.

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