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@xxxxxxxxxx>
>> > ---
>> >  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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