The decompression code parses a huffman tree and counts the number of
symbols for a given bit length.  In rare cases, there may be >= 256
symbols with a given bit length, causing the unsigned char to overflow.
This causes a decompression failure later when the code tries and fails to
find the bit length for a given symbol.

Since the maximum number of symbols is 258, use unsigned short instead.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
---

This issue was noticed in Linux decompressing initrds but since Xen has
the same decompression code, it is possible the issue occurs here too.

 xen/common/bunzip2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/bunzip2.c b/xen/common/bunzip2.c
index 4466426941e0..79f17162b138 100644
--- a/xen/common/bunzip2.c
+++ b/xen/common/bunzip2.c
@@ -221,7 +221,8 @@ static int __init get_next_block(struct bunzip_data *bd)
 	   RUNB) */
 	symCount = symTotal+2;
 	for (j = 0; j < groupCount; j++) {
-		unsigned char length[MAX_SYMBOLS], temp[MAX_HUFCODE_BITS+1];
+		unsigned char length[MAX_SYMBOLS];
+		unsigned short temp[MAX_HUFCODE_BITS+1];
 		int	minLen,	maxLen, pp;
 		/* Read Huffman code lengths for each symbol.  They're
 		   stored in a way similar to mtf; record a starting
-- 
2.45.2

On 30.07.2024 10:03, Ross Lagerwall wrote:
> The decompression code parses a huffman tree and counts the number of
> symbols for a given bit length.  In rare cases, there may be >= 256
> symbols with a given bit length, causing the unsigned char to overflow.
> This causes a decompression failure later when the code tries and fails to
> find the bit length for a given symbol.
> 
> Since the maximum number of symbols is 258, use unsigned short instead.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

Acked-by: Jan Beulich <jbeulich@suse.com>

A Fixes: tag and maybe an Origin: one would have been nice; the latter just
"maybe" because the Linux patch is one of yours anyway.

Jan

On Tue, Jul 30, 2024 at 10:25 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 30.07.2024 10:03, Ross Lagerwall wrote:
> > The decompression code parses a huffman tree and counts the number of
> > symbols for a given bit length.  In rare cases, there may be >= 256
> > symbols with a given bit length, causing the unsigned char to overflow.
> > This causes a decompression failure later when the code tries and fails to
> > find the bit length for a given symbol.
> >
> > Since the maximum number of symbols is 258, use unsigned short instead.
> >
> > Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
>
> Acked-by: Jan Beulich <jbeulich@suse.com>
>
> A Fixes: tag and maybe an Origin: one would have been nice; the latter just
> "maybe" because the Linux patch is one of yours anyway.
>

Indeed, I decided against an origin tag since I wasn't backporting
someone else's change from Linux, just fixing the same thing in
multiple places.

I should have added a fixes tag. Here it is:

Fixes: ab77e81f6521 ("x86/dom0: support bzip2 and lzma compressed
bzImage payloads")

Thanks,
Ross