[PATCH] jffs2: Fix integer underflow in jffs2_rtime_compress

[PATCH] jffs2: Fix integer underflow in jffs2_rtime_compress

[Richard Weinberger]

The rtime compressor assumes that at least two bytes are
compressed.
If we try to compress just one byte, the loop condition will
wrap around and an out-of-bounds write happens.

Cc: <stable@vger.kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
---
 fs/jffs2/compr_rtime.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/jffs2/compr_rtime.c b/fs/jffs2/compr_rtime.c
index 406d9cc84ba8..cbf700001fc9 100644
--- a/fs/jffs2/compr_rtime.c
+++ b/fs/jffs2/compr_rtime.c
@@ -39,6 +39,9 @@ static int jffs2_rtime_compress(unsigned char *data_in,
 
 	memset(positions,0,sizeof(positions));
 
+	if (*dstlen < 2)
+		return -1;
+
 	while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
 		int backpos, runlen=0;
 		unsigned char value;
-- 
2.20.0

Re: [PATCH] jffs2: Fix integer underflow in jffs2_rtime_compress

[Hou Tao]

On 2018/12/16 0:23, Richard Weinberger wrote:
> The rtime compressor assumes that at least two bytes are
> compressed.
> If we try to compress just one byte, the loop condition will
> wrap around and an out-of-bounds write happens.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
>  fs/jffs2/compr_rtime.c | 3 +++
>  1 file changed, 3 insertions(+)
> It seems that it doesn't incur any harm because the minimal allocated
size will be 8-bytes and jffs2_rtime_compress() will write 2-bytes into
the allocated buffer.

> diff --git a/fs/jffs2/compr_rtime.c b/fs/jffs2/compr_rtime.c
> index 406d9cc84ba8..cbf700001fc9 100644
> --- a/fs/jffs2/compr_rtime.c
> +++ b/fs/jffs2/compr_rtime.c
> @@ -39,6 +39,9 @@ static int jffs2_rtime_compress(unsigned char *data_in,
>  
>  	memset(positions,0,sizeof(positions));
>  
> +	if (*dstlen < 2)
> +		return -1;
> +
>  	while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
>  		int backpos, runlen=0;
>  		unsigned char value;
> 

Re: [PATCH] jffs2: Fix integer underflow in jffs2_rtime_compress

[Richard Weinberger]

Am Donnerstag, 20. Dezember 2018, 11:43:08 CET schrieb Hou Tao:
> 
> On 2018/12/16 0:23, Richard Weinberger wrote:
> > The rtime compressor assumes that at least two bytes are
> > compressed.
> > If we try to compress just one byte, the loop condition will
> > wrap around and an out-of-bounds write happens.
> > 
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Richard Weinberger <richard@nod.at>
> > ---
> >  fs/jffs2/compr_rtime.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > It seems that it doesn't incur any harm because the minimal allocated
> size will be 8-bytes and jffs2_rtime_compress() will write 2-bytes into
> the allocated buffer.

Are you sure about that? I saw odd kernel behavior and KASAN complained too.

Thanks,
//richard

Re: [PATCH] jffs2: Fix integer underflow in jffs2_rtime_compress

[Hou Tao]

Hi Richard,

On 2018/12/20 18:45, Richard Weinberger wrote:
> Am Donnerstag, 20. Dezember 2018, 11:43:08 CET schrieb Hou Tao:
>>
>> On 2018/12/16 0:23, Richard Weinberger wrote:
>>> The rtime compressor assumes that at least two bytes are
>>> compressed.
>>> If we try to compress just one byte, the loop condition will
>>> wrap around and an out-of-bounds write happens.
>>>
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Richard Weinberger <richard@nod.at>
>>> ---
>>>  fs/jffs2/compr_rtime.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>> It seems that it doesn't incur any harm because the minimal allocated
>> size will be 8-bytes and jffs2_rtime_compress() will write 2-bytes into
>> the allocated buffer.
> 
> Are you sure about that? I saw odd kernel behavior and KASAN complained too.
> 

Sorry for the later reply.

Yes. KASAN complains but it doesn't incur any harm because the minimal allocated
size returned by kmalloc() will be 8-bytes.

But we better fix it, because it's bad to depend on the implementation of kmalloc().

It seems that mtd-utils has already fixed it years ago. Maybe we can use it directly ?

And your fix also looks good to me, so

Reviewed-by: Hou Tao <houtao1@huawei.com>

commit e8457f16306ad6e2c8708275bf42b5dfff40fffd
Author: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Date:   Thu Jun 24 15:02:40 2010 +0200

    mkfs.jffs2: fix integer underflow in jffs2_rtime_compress()

    When '*dstlen' is 0 or 1, comparison will return wrong result.  Reported
    by valgrind as

    ==5919== Invalid write of size 1
    ==5919==    at 0x40564E: jffs2_rtime_compress (compr_rtime.c:51)
    ==5919==    by 0x40676B: jffs2_compress (compr.c:246)
    ==5919==    by 0x403EE4: recursive_populate_directory (mkfs.jffs2.c:884)
    ==5919==  Address 0x4e1bdb1 is 0 bytes after a block of size 1 alloc'd
    ==5919==    at 0x4A0515D: malloc (vg_replace_malloc.c:195)
    ==5919==    by 0x40671C: jffs2_compress (compr.c:229)
    ==5919==    by 0x403EE4: recursive_populate_directory (mkfs.jffs2.c:884)

    Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
    Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --git a/compr_rtime.c b/compr_rtime.c
index 131536c..5613963 100644
--- a/compr_rtime.c
+++ b/compr_rtime.c
@@ -32,7 +32,7 @@ static int jffs2_rtime_compress(unsigned char *data_in, unsigned char *cpage_out

        memset(positions,0,sizeof(positions));

-       while (pos < (*sourcelen) && outpos <= (*dstlen)-2) {
+       while (pos < (*sourcelen) && outpos+2 <= (*dstlen)) {
                int backpos, runlen=0;
                unsigned char value;



> Thanks,
> //richard




> 
> 
> 
> .
>