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

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Message-ID: <patch-1.thread-41c676.git-41c676c2d153.your-ad-here.call-01675030179-ext-9637@work.hours>
Date:   Sun, 29 Jan 2023 23:47:23 +0100
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nick Terrell <terrelln@...com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org
Subject: [PATCH 1/1] s390/decompressor: specify __decompress() buf len to
 avoid overflow

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@...ux.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