Re: [PATCH] Btrfs: fix an integer overflow check

[David Sterba <dsterba@suse.cz>]

On Fri, Mar 17, 2017 at 11:51:20PM +0300, Dan Carpenter wrote:
> This isn't super serious because you need CAP_ADMIN to run this code.
> 
> I added this integer overflow check last year but apparently I am
> rubbish at writing integer overflow checks...  There are two issues.
> First, access_ok() works on unsigned long type and not u64 so on 32 bit
> systems the access_ok() could be checking a truncated size.  The other
> issue is that we should be using a stricter limit so we don't overflow
> the kzalloc() setting ctx->clone_roots later in the function after the
> access_ok():
> 
> 	alloc_size = sizeof(struct clone_root) * (arg->clone_sources_count + 1);
> 	sctx->clone_roots = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN);
> 
> Fixes: f5ecec3ce21f ("btrfs: send: silence an integer overflow warning")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

Reviewed-by: David Sterba <dsterba@suse.com>

I'll copy parts of the changelog and add as comments, it's not obvious
what the check does.