Subject: Re: [PATCH] packfile: avoid overflowing shift during decode

From: Junio C Hamano <[email-masked]>
Date: Wed, 12 Jan 2022 12:12:34 -0800

Junio C Hamano <[email-masked]> writes:

> Marc Strapetz <[email-masked]> writes:
>
>> On 11/11/2021 02:58, Junio C Hamano wrote:
>>> Jonathan Tan <[email-masked]> writes:
>>> 
>>>> diff --git a/packfile.c b/packfile.c
>>>> index 89402cfc69..972c327e29 100644
>>>> --- a/packfile.c
>>>> +++ b/packfile.c
>>>> @@ -1068,7 +1068,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
>>>>   	size = c & 15;
>>>>   	shift = 4;
>>>>   	while (c & 0x80) {
>>>> -		if (len <= used || bitsizeof(long) <= shift) {
>>>> +		if (len <= used || (bitsizeof(long) - 7) <= shift) {
>>
>> This seems to cause troubles now for 32-bit systems (in my case Git
>> for Windows 32-Bit): `shift` will go through 4, 11, 18 and for 25 it
>> finally errors out. This means that objects >= 32MB can't be processed
>> anymore. The condition should probably be changed to:
>>
>> +		if (len <= used || (bitsizeof(long) - 7) < shift) {
>>
>> This still ensures that the shift can never overflow and on 32-bit
>> systems restores the maximum size of 4G with a final shift of 127<<25 
>> (the old condition `bitsizeof(long) <= shift` was perfectly valid for
>> 32-bit systems).
>
> Jonathan?

----- >8 --------- >8 --------- >8 --------- >8 --------- >8 -----
Date: Wed, 12 Jan 2022 12:11:42 -0800
Subject: [PATCH] packfile: fix off-by-one error in decoding logic

shift count being exactly at 7-bit smaller than the long is OK; on
32-bit architecture, shift count starts at 4 and goes through 11, 18
and 25, at which point the guard triggers one iteration too early.

Reported-by: Marc Strapetz <[email-masked]>
Signed-off-by: Junio C Hamano <[email-masked]>
---
 packfile.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packfile.c b/packfile.c
index d3820c780b..667e21ce97 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1067,7 +1067,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 	size = c & 15;
 	shift = 4;
 	while (c & 0x80) {
-		if (len <= used || (bitsizeof(long) - 7) <= shift) {
+		if (len <= used || (bitsizeof(long) - 7) < shift) {
 			error("bad object header");
 			size = used = 0;
 			break;
-- 
2.35.0-rc0-170-g6a31d082e5