[PATCH 0/1] s390: fix initrd corruption in decompressor with new zstd version

[PATCH 0/1] s390: fix initrd corruption in decompressor with new zstd version

[Vasily Gorbik]

The new version of zstd library integrated in the kernel since v6.2-rc1
https://lore.kernel.org/all/20221024202606.404049-1-nickrterrell@gmail.com/
contains commit
https://github.com/facebook/zstd/commit/6a7ede3dfccb
which introduces a side effect for historical usage of __decompress() function,
i.e. not specifying "out_len" parameter and expecting that no writes beyond
uncompressed kernel image are performed. More details are in follow up fix.

From architectures which claim HAVE_KERNEL_ZSTD, s390 and MIPS use
__decompress() without specifying "out_len". On s390 this leads to
initrd corruption.
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2003348

I haven't looked in details for MIPS but I've added Thomas as the
maintainer and MIPS list in Cc.

The follow up fix addresses that for s390.

Vasily Gorbik (1):
  s390/decompressor: specify __decompress() buf len to avoid overflow

 arch/s390/boot/decompressor.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.38.1

[PATCH 1/1] s390/decompressor: specify __decompress() buf len to avoid overflow

[Vasily Gorbik]

Historically calls to __decompress() didn't specify "out_len" parameter
on many architectures including s390, expecting that no writes beyond
uncompressed kernel image are performed. This has changed since commit
2aa14b1ab2c4 ("zstd: import usptream v1.5.2") which includes zstd library
commit 6a7ede3dfccb ("Reduce size of dctx by reutilizing dst buffer
(#2751)"). Now zstd decompression code might store literal buffer in
the unwritten portion of the destination buffer. Since "out_len" is
not set, it is considered to be unlimited and hence free to use for
optimization needs. On s390 this might corrupt initrd or ipl report
which are often placed right after the decompressor buffer. Luckily the
size of uncompressed kernel image is already known to the decompressor,
so to avoid the problem simply specify it in the "out_len" parameter.

Link: https://github.com/facebook/zstd/commit/6a7ede3dfccb
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
---
 arch/s390/boot/decompressor.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/boot/decompressor.c b/arch/s390/boot/decompressor.c
index 090621b98d95..d762733a0753 100644
--- a/arch/s390/boot/decompressor.c
+++ b/arch/s390/boot/decompressor.c
@@ -81,6 +81,6 @@ void *decompress_kernel(void)
 	void *output = (void *)decompress_offset;
 
 	__decompress(_compressed_start, _compressed_end - _compressed_start,
-		     NULL, NULL, output, 0, NULL, error);
+		     NULL, NULL, output, vmlinux.image_size, NULL, error);
 	return output;
 }
-- 
2.38.1

Re: [PATCH 1/1] s390/decompressor: specify __decompress() buf len to avoid overflow

[Alexander Egorenkov]

Hi Vasily,

thanks for the fix, tested on s390 + KVM + buildroot + linux-next.

Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>

Regards
Alex

Re: [PATCH 1/1] s390/decompressor: specify __decompress() buf len to avoid overflow

[Heiko Carstens]

On Sun, Jan 29, 2023 at 11:47:23PM +0100, Vasily Gorbik wrote:
> Historically calls to __decompress() didn't specify "out_len" parameter
> on many architectures including s390, expecting that no writes beyond
> uncompressed kernel image are performed. This has changed since commit
> 2aa14b1ab2c4 ("zstd: import usptream v1.5.2") which includes zstd library
> commit 6a7ede3dfccb ("Reduce size of dctx by reutilizing dst buffer
> (#2751)"). Now zstd decompression code might store literal buffer in
> the unwritten portion of the destination buffer. Since "out_len" is
> not set, it is considered to be unlimited and hence free to use for
> optimization needs. On s390 this might corrupt initrd or ipl report
> which are often placed right after the decompressor buffer. Luckily the
> size of uncompressed kernel image is already known to the decompressor,
> so to avoid the problem simply specify it in the "out_len" parameter.
> 
> Link: https://github.com/facebook/zstd/commit/6a7ede3dfccb
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
> ---
>  arch/s390/boot/decompressor.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied, thanks!