[PATCH] mm: shmem: avoid overflowing in shmem_fallocate

[PATCH] mm: shmem: avoid overflowing in shmem_fallocate

[Sasha Levin]

"offset + len" has the potential of overflowing. Validate this user input
first to avoid undefined behaviour.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
---
 mm/shmem.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 185836b..5a0e344 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2098,6 +2098,9 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 	}
 
 	/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
+	error = -EOVERFLOW;
+	if ((u64)len + offset < (u64)len)
+		goto out;
 	error = inode_newsize_ok(inode, offset + len);
 	if (error)
 		goto out;
-- 
1.7.10.4

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Re: [PATCH] mm: shmem: avoid overflowing in shmem_fallocate

[Naoya Horiguchi]

On Wed, Dec 03, 2014 at 07:24:07PM -0500, Sasha Levin wrote:
> "offset + len" has the potential of overflowing. Validate this user input
> first to avoid undefined behaviour.
> 
> Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
> ---
>  mm/shmem.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 185836b..5a0e344 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2098,6 +2098,9 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
>  	}
>  
>  	/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
> +	error = -EOVERFLOW;
> +	if ((u64)len + offset < (u64)len)
> +		goto out;

Hi Sasha,

It seems to me that we already do some overflow check in common path,
do_fallocate():

        /* Check for wrap through zero too */
        if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
                return -EFBIG;

Do we really need another check?

And this patch changes the return value of fallocate(2), so you need
update man document.

BTW, when I'm reading your patch, I noticed that returning -EOVERFLOW
(rather than -EFBIG) looks better when ((offset + len) < 0) in
do_fallocate() is true.

Thanks,
Naoya Horiguchi
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Re: [PATCH] mm: shmem: avoid overflowing in shmem_fallocate

[Sasha Levin]

On 12/03/2014 08:51 PM, Naoya Horiguchi wrote:
> On Wed, Dec 03, 2014 at 07:24:07PM -0500, Sasha Levin wrote:
>> > "offset + len" has the potential of overflowing. Validate this user input
>> > first to avoid undefined behaviour.
>> > 
>> > Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
>> > ---
>> >  mm/shmem.c |    3 +++
>> >  1 file changed, 3 insertions(+)
>> > 
>> > diff --git a/mm/shmem.c b/mm/shmem.c
>> > index 185836b..5a0e344 100644
>> > --- a/mm/shmem.c
>> > +++ b/mm/shmem.c
>> > @@ -2098,6 +2098,9 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
>> >  	}
>> >  
>> >  	/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
>> > +	error = -EOVERFLOW;
>> > +	if ((u64)len + offset < (u64)len)
>> > +		goto out;
> Hi Sasha,
> 
> It seems to me that we already do some overflow check in common path,
> do_fallocate():
> 
>         /* Check for wrap through zero too */
>         if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
>                 return -EFBIG;
> 
> Do we really need another check?

It looks like we actually need to fix this snippet you pasted rather than shmem_fallocate().

We can't check for ((offset + len) < 0) since both offset and length are signed integers. I'll
send a patch to deal with that rather that this shmem specific one. Thanks!


Thanks,
Sasha

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