Patchwork FS: ext4: fix integer overflow in alloc_flex_gd()

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Submitter Haogang Chen
Date Feb. 20, 2012, 10:41 p.m.
Message ID <1329777684-18396-1-git-send-email-haogangchen@gmail.com>
Download mbox | patch
Permalink /patch/142200/
State Accepted
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Comments

Haogang Chen - Feb. 20, 2012, 10:41 p.m.
In alloc_flex_gd(), when flexbg_size is large, kmalloc size would
overflow and flex_gd->groups would point to a buffer smaller than
expected, causing OOB accesses when it is used.

Note that in ext4_resize_fs(), flexbg_size is calculated using
sbi->s_log_groups_per_flex, which is read from the disk and only bounded
to [1, 31]. The patch returns NULL for too large flexbg_size.

Signed-off-by: Haogang Chen <haogangchen@gmail.com>
---
 fs/ext4/resize.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
Eric Sandeen - Feb. 20, 2012, 11:47 p.m.
On 2/20/12 4:41 PM, Haogang Chen wrote:
> In alloc_flex_gd(), when flexbg_size is large, kmalloc size would
> overflow and flex_gd->groups would point to a buffer smaller than
> expected, causing OOB accesses when it is used.
> 
> Note that in ext4_resize_fs(), flexbg_size is calculated using
> sbi->s_log_groups_per_flex, which is read from the disk and only bounded
> to [1, 31]. The patch returns NULL for too large flexbg_size

Hm this raises a few questions I think.

On the one hand, making sure the kmalloc arg doesn't overflow here is
certainly a good thing and probably the right thing to do in the short term.

So I guess:

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

for that, to close the hole.

But the types are a mess; alloc_flex_gd() takes an unsigned long; 
it's passed an int, and assigns to flex_gd->count, an ext4_group_t
(which is an unsigned int). They should probably all be ext4_group_t
for consistency.

But that's not the worst of it...

Doesn't this also mean that a valid s_log_groups_per_flex (i.e. 31)
will fail in this resize code?  That would be an unexpected outcome.
2^31 groups per flex is a little crazy, but still technically valid
according to the limits in the code.

So really, trying to allocate an array of all possible groups-per-flex
in the resize code is probably a really bad idea to start with, and the
resize code has got serious problems if kmalloc(UINT_MAX-1) is expected
to work...

-Eric

> Signed-off-by: Haogang Chen <haogangchen@gmail.com>
> ---
>  fs/ext4/resize.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index f9d948f..8601f4b 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -161,6 +161,8 @@ static struct ext4_new_flex_group_data *alloc_flex_gd(unsigned long flexbg_size)
>  	if (flex_gd == NULL)
>  		goto out3;
>  
> +	if (flexbg_size >= UINT_MAX / sizeof(struct ext4_new_flex_group_data))
> +		goto out2;
>  	flex_gd->count = flexbg_size;
>  
>  	flex_gd->groups = kmalloc(sizeof(struct ext4_new_group_data) *

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Xi Wang - Feb. 21, 2012, 1:55 p.m.
On Feb 20, 2012, at 6:47 PM, Eric Sandeen wrote:
> Hm this raises a few questions I think.
> 
> On the one hand, making sure the kmalloc arg doesn't overflow here is
> certainly a good thing and probably the right thing to do in the short term.
> 
> So I guess:
> 
> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
> 
> for that, to close the hole.

Another possibility is to wait for knalloc/kmalloc_array in the -mm
tree, which is basically the non-zeroing version of kcalloc that
performs overflow checking.

> Doesn't this also mean that a valid s_log_groups_per_flex (i.e. 31)
> will fail in this resize code?  That would be an unexpected outcome.
> 2^31 groups per flex is a little crazy, but still technically valid
> according to the limits in the code.

Or we could limit s_log_groups_per_flex/groups_per_flex to a
reasonable upper bound in ext4_fill_flex_info(), right?

- xi

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Eric Sandeen - Feb. 21, 2012, 4:36 p.m.
On 02/21/2012 07:55 AM, Xi Wang wrote:
> On Feb 20, 2012, at 6:47 PM, Eric Sandeen wrote:
>> Hm this raises a few questions I think.
>>
>> On the one hand, making sure the kmalloc arg doesn't overflow here is
>> certainly a good thing and probably the right thing to do in the short term.
>>
>> So I guess:
>>
>> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
>>
>> for that, to close the hole.
> 
> Another possibility is to wait for knalloc/kmalloc_array in the -mm
> tree, which is basically the non-zeroing version of kcalloc that
> performs overflow checking.
> 
>> Doesn't this also mean that a valid s_log_groups_per_flex (i.e. 31)
>> will fail in this resize code?  That would be an unexpected outcome.
>> 2^31 groups per flex is a little crazy, but still technically valid
>> according to the limits in the code.
> 
> Or we could limit s_log_groups_per_flex/groups_per_flex to a
> reasonable upper bound in ext4_fill_flex_info(), right?

Depends on the "flex_bg" design intent, I guess.

I don't know if the 2^31 was an intended design limit, or just a
mathematical limit that based on container sizes etc...

I'd have to look at the resize code more carefully but I can't imagine
that it's imperative to allocate this stuff all at once.

-Eric

> - xi
> 

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Andreas Dilger - Feb. 21, 2012, 5:19 p.m.
On 2012-02-21, at 9:36 AM, Eric Sandeen wrote:
> On 02/21/2012 07:55 AM, Xi Wang wrote:
>> On Feb 20, 2012, at 6:47 PM, Eric Sandeen wrote:
>>> Hm this raises a few questions I think.
>>> 
>>> On the one hand, making sure the kmalloc arg doesn't overflow here is
>>> certainly a good thing and probably the right thing to do in the short term.
>>> 
>>> So I guess:
>>> 
>>> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
>>> 
>>> for that, to close the hole.
>> 
>> Another possibility is to wait for knalloc/kmalloc_array in the -mm
>> tree, which is basically the non-zeroing version of kcalloc that
>> performs overflow checking.
>> 
>>> Doesn't this also mean that a valid s_log_groups_per_flex (i.e. 31)
>>> will fail in this resize code?  That would be an unexpected outcome.
>>> 2^31 groups per flex is a little crazy, but still technically valid
>>> according to the limits in the code.
>> 
>> Or we could limit s_log_groups_per_flex/groups_per_flex to a
>> reasonable upper bound in ext4_fill_flex_info(), right?
> 
> Depends on the "flex_bg" design intent, I guess.
> 
> I don't know if the 2^31 was an intended design limit, or just a
> mathematical limit that based on container sizes etc...
> 
> I'd have to look at the resize code more carefully but I can't imagine
> that it's imperative to allocate this stuff all at once.

We previously tried to use a large flex_bg size to put all metadata into a
single group so it could easily be allocated on a separate SSD device, but
that didn't work very well.  Once the number of bitmaps in group 0 is more
than the number of free blocks in that group (below 16k groups, due to group
descriptors) then they need to overflow into group 1 and collide with the
group descriptors there.  Then mke2fs chokes, AFAIR.

It may be different with bigalloc, since the number of blocks in a group can
be very large, I haven't tried that.

In any case, I don't think anyone expects vmalloc(2^32 * struct size) to work,
but I wouldn't sweat fixing this until there is some real reason to do so.

Cheers, Andreas





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Theodore Ts'o - May 28, 2012, 6:24 p.m.
On Mon, Feb 20, 2012 at 05:41:24PM -0500, Haogang Chen wrote:
> In alloc_flex_gd(), when flexbg_size is large, kmalloc size would
> overflow and flex_gd->groups would point to a buffer smaller than
> expected, causing OOB accesses when it is used.
> 
> Note that in ext4_resize_fs(), flexbg_size is calculated using
> sbi->s_log_groups_per_flex, which is read from the disk and only bounded
> to [1, 31]. The patch returns NULL for too large flexbg_size.
> 
> Signed-off-by: Haogang Chen <haogangchen@gmail.com>

Thanks, applied.  Apologies for missing this during the last cycle.

		  	    		     - Ted
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Patch

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index f9d948f..8601f4b 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -161,6 +161,8 @@  static struct ext4_new_flex_group_data *alloc_flex_gd(unsigned long flexbg_size)
 	if (flex_gd == NULL)
 		goto out3;
 
+	if (flexbg_size >= UINT_MAX / sizeof(struct ext4_new_flex_group_data))
+		goto out2;
 	flex_gd->count = flexbg_size;
 
 	flex_gd->groups = kmalloc(sizeof(struct ext4_new_group_data) *