[3.5.y.z extended stable] Patch "lib/decompressors: fix "no limit" output buffer length" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    lib/decompressors: fix "no limit" output buffer length

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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>From 883cd7b16d4bae102bfadad5547ba0e07c5a79e7 Mon Sep 17 00:00:00 2001
From: Alexandre Courbot <acourbot at nvidia.com>
Date: Wed, 11 Sep 2013 14:23:53 -0700
Subject: lib/decompressors: fix "no limit" output buffer length

commit 1431574a1c4c669a0c198e4763627837416e4443 upstream.

When decompressing into memory, the output buffer length is set to some
arbitrarily high value (0x7fffffff) to indicate the output is, virtually,
unlimited in size.

The problem with this is that some platforms have their physical memory at
high physical addresses (0x80000000 or more), and that the output buffer
address and its "unlimited" length cannot be added without overflowing.
An example of this can be found in inflate_fast():

/* next_out is the output buffer address */
out = strm->next_out - OFF;
/* avail_out is the output buffer size. end will overflow if the output
 * address is >= 0x80000104 */
end = out + (strm->avail_out - 257);

This has huge consequences on the performance of kernel decompression,
since the following exit condition of inflate_fast() will be always true:

} while (in < last && out < end);

Indeed, "end" has overflowed and is now always lower than "out".  As a
result, inflate_fast() will return after processing one single byte of
input data, and will thus need to be called an unreasonably high number of
times.  This probably went unnoticed because kernel decompression is fast
enough even with this issue.

Nonetheless, adjusting the output buffer length in such a way that the
above pointer arithmetic never overflows results in a kernel decompression
that is about 3 times faster on affected machines.

Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
Tested-by: Jon Medhurst <tixy at linaro.org>
Cc: Stephen Warren <swarren at wwwdotorg.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Mark Brown <broonie at kernel.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 lib/decompress_inflate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/decompress_inflate.c b/lib/decompress_inflate.c
index 19ff89e..d619b28 100644
--- a/lib/decompress_inflate.c
+++ b/lib/decompress_inflate.c
@@ -48,7 +48,7 @@ STATIC int INIT gunzip(unsigned char *buf, int len,
 		out_len = 0x8000; /* 32 K */
 		out_buf = malloc(out_len);
 	} else {
-		out_len = 0x7fffffff; /* no limit */
+		out_len = ((size_t)~0) - (size_t)out_buf; /* no limit */
 	}
 	if (!out_buf) {
 		error("Out of memory while allocating output buffer");
--
1.8.3.2