[PATCH] hfs: fix array out of bounds read of array extent

[PATCH] hfs: fix array out of bounds read of array extent

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

Currently extent and index i are both being incremented causing
an array out of bounds read on extent[i]. Fix this by removing
the extraneous increment of extent.

Detected by CoverityScan, CID#711541 ("Out of bounds read")

Fixes: d1081202f1d0 ("HFS rewrite")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 fs/hfs/extent.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/hfs/extent.c b/fs/hfs/extent.c
index 5d0182654580..636cdfcecb26 100644
--- a/fs/hfs/extent.c
+++ b/fs/hfs/extent.c
@@ -300,7 +300,7 @@ int hfs_free_fork(struct super_block *sb, struct hfs_cat_file *file, int type)
 		return 0;
 
 	blocks = 0;
-	for (i = 0; i < 3; extent++, i++)
+	for (i = 0; i < 3; i++)
 		blocks += be16_to_cpu(extent[i].count);
 
 	res = hfs_free_extents(sb, extent, blocks, blocks);
-- 
2.17.1

Re: [PATCH] hfs: fix array out of bounds read of array extent

[Ernesto A. Fernández]

On Fri, Aug 31, 2018 at 03:05:38PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> Currently extent and index i are both being incremented causing
> an array out of bounds read on extent[i]. Fix this by removing
> the extraneous increment of extent.
> 
> Detected by CoverityScan, CID#711541 ("Out of bounds read")
> 
> Fixes: d1081202f1d0 ("HFS rewrite")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>

I don't think this got picked up yet; let's see if I can help.

Reviewed-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>

> ---
>  fs/hfs/extent.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/hfs/extent.c b/fs/hfs/extent.c
> index 5d0182654580..636cdfcecb26 100644
> --- a/fs/hfs/extent.c
> +++ b/fs/hfs/extent.c
> @@ -300,7 +300,7 @@ int hfs_free_fork(struct super_block *sb, struct hfs_cat_file *file, int type)
>  		return 0;
>  
>  	blocks = 0;
> -	for (i = 0; i < 3; extent++, i++)
> +	for (i = 0; i < 3; i++)
>  		blocks += be16_to_cpu(extent[i].count);
>  
>  	res = hfs_free_extents(sb, extent, blocks, blocks);
> -- 
> 2.17.1
> 

Re: [PATCH] hfs: fix array out of bounds read of array extent

[Al Viro]

On Wed, Oct 17, 2018 at 03:01:17PM -0700, Andrew Morton wrote:
> On Fri, 31 Aug 2018 15:05:38 +0100 Colin King <colin.king@canonical.com> wrote:
> 
> > From: Colin Ian King <colin.king@canonical.com>
> > 
> > Currently extent and index i are both being incremented causing
> > an array out of bounds read on extent[i]. Fix this by removing
> > the extraneous increment of extent.
> > 
> > Detected by CoverityScan, CID#711541 ("Out of bounds read")
> > 
> > Fixes: d1081202f1d0 ("HFS rewrite")
> 
> No such commit here.  I assume this is 7cb74be6fd827e314f8.
> 
> > --- a/fs/hfs/extent.c
> > +++ b/fs/hfs/extent.c
> > @@ -300,7 +300,7 @@ int hfs_free_fork(struct super_block *sb, struct hfs_cat_file *file, int type)
> >  		return 0;
> >  
> >  	blocks = 0;
> > -	for (i = 0; i < 3; extent++, i++)
> > +	for (i = 0; i < 3; i++)
> >  		blocks += be16_to_cpu(extent[i].count);
> >  
> >  	res = hfs_free_extents(sb, extent, blocks, blocks);
> 
> Well, that's quite the bug.  Question is, why didn't anyone notice it. 
> What are the runtime effects?  A disk space leak, perhaps?
> 
> I worry a bit that, given the fs was evidently working "ok", perhaps
> this error was corrected elsewhere in the code and that "fixing" this
> site will have unexpected and undesirable runtime effects.  Can someone
> help me out here?

hfs_free_extents() seems to expect the 'offset' argument to be the
sum of ->count of 1--3 starting elements of extent array.  In case of
mismatch, it returns -EIO and that's it - hfs_free_fork() will bugger
off with -EIO at that point.  If it does match, block_nr is supposed
to be in range 0..offset and blocks offset - block_nr .. offset - 1
are freed.

So at a guess, that sucker mostly ends up leaking blocks.  Said that,
it means that the rest of hfs_free_fork() has never been tested.

I'd suggest somebody to turn that
        /* panic? */
        return -EIO;
in hfs_free_extents() into
	printk(KERN_ERR "hfs_free_extents is fucked");
	return -EIO;
and see if it's triggerable.  Then check if there's a block leak in
the reproducer, whatever it is.

Re: [PATCH] hfs: fix array out of bounds read of array extent

[Ernesto A. Fernández]

On Wed, Oct 17, 2018 at 03:01:17PM -0700, Andrew Morton wrote:
> On Fri, 31 Aug 2018 15:05:38 +0100 Colin King <colin.king@canonical.com> wrote:
> 
> > From: Colin Ian King <colin.king@canonical.com>
> > 
> > Currently extent and index i are both being incremented causing
> > an array out of bounds read on extent[i]. Fix this by removing
> > the extraneous increment of extent.
> > 
> > Detected by CoverityScan, CID#711541 ("Out of bounds read")
> > 
> > Fixes: d1081202f1d0 ("HFS rewrite")
> 
> No such commit here.  I assume this is 7cb74be6fd827e314f8.

Sorry, I missed that.  This bug has actually been here since before the
first git commit.

> 
> > --- a/fs/hfs/extent.c
> > +++ b/fs/hfs/extent.c
> > @@ -300,7 +300,7 @@ int hfs_free_fork(struct super_block *sb, struct hfs_cat_file *file, int type)
> >  		return 0;
> >  
> >  	blocks = 0;
> > -	for (i = 0; i < 3; extent++, i++)
> > +	for (i = 0; i < 3; i++)
> >  		blocks += be16_to_cpu(extent[i].count);
> >  
> >  	res = hfs_free_extents(sb, extent, blocks, blocks);
> 
> Well, that's quite the bug.  Question is, why didn't anyone notice it. 
> What are the runtime effects?

This is only triggered when deleting a file with a resource fork.  I may
be wrong because the documentation isn't clear, but I don't think you can
create those under linux.  So I guess nobody was testing them.

> A disk space leak, perhaps?

That's what it looks like in general.  hfs_free_extents() won't do anything
if the block count doesn't add up, and the error will be ignored.  Now, if
the block count randomly does add up, we could see some corruption.
 
> I worry a bit that, given the fs was evidently working "ok", perhaps
> this error was corrected elsewhere in the code and that "fixing" this
> site will have unexpected and undesirable runtime effects.  Can someone
> help me out here?

I don't think so.  This bug also makes extent point to the wrong place on
the following call to hfs_free_extents().  There is no way this can work
correctly in general.

Re: [PATCH] hfs: fix array out of bounds read of array extent

[Viacheslav Dubeyko]

On Wed, 2018-10-17 at 15:01 -0700, Andrew Morton wrote:
> On Fri, 31 Aug 2018 15:05:38 +0100 Colin King <colin.king@canonical.com> wrote:
> 
> > From: Colin Ian King <colin.king@canonical.com>
> > 
> > Currently extent and index i are both being incremented causing
> > an array out of bounds read on extent[i]. Fix this by removing
> > the extraneous increment of extent.
> > 
> > Detected by CoverityScan, CID#711541 ("Out of bounds read")
> > 
> > Fixes: d1081202f1d0 ("HFS rewrite")
> 
> No such commit here.  I assume this is 7cb74be6fd827e314f8.
> 
> > --- a/fs/hfs/extent.c
> > +++ b/fs/hfs/extent.c
> > @@ -300,7 +300,7 @@ int hfs_free_fork(struct super_block *sb, struct hfs_cat_file *file, int type)
> >  		return 0;
> >  
> >  	blocks = 0;
> > -	for (i = 0; i < 3; extent++, i++)


By the way, the hfs_free_extents() has the same logic [1] of for (i = 0;
i < 3; extent++, i++). It looks like that the bug is not fixed yet. Did
anyone test this patch? What's the real reproduction path for the bug?

Thanks,
Vyacheslav Dubeyko.

[1] https://elixir.bootlin.com/linux/latest/source/fs/hfs/extent.c#L251

> > +	for (i = 0; i < 3; i++)
> >  		blocks += be16_to_cpu(extent[i].count);
> >  
> >  	res = hfs_free_extents(sb, extent, blocks, blocks);
> 
> Well, that's quite the bug.  Question is, why didn't anyone notice it. 
> What are the runtime effects?  A disk space leak, perhaps?
> 
> I worry a bit that, given the fs was evidently working "ok", perhaps
> this error was corrected elsewhere in the code and that "fixing" this
> site will have unexpected and undesirable runtime effects.  Can someone
> help me out here?