Re: [PATCH] mm: fix arithmetic overflow in __vm_enough_memory()

[Andrew Shewmaker <agshew@gmail.com>]

On Thu, Jan 29, 2015 at 04:06:03PM +0300, Roman Gushchin wrote:
> I noticed, that "allowed" can easily overflow by falling below 0,
> because (total_vm / 32) can be larger than "allowed". The problem
> occurs in OVERCOMMIT_NONE mode.
> 
> In this case, a huge allocation can success and overcommit the system
> (despite OVERCOMMIT_NONE mode). All subsequent allocations will fall
> (system-wide), so system become unusable.
> 
> The problem was masked out by commit c9b1d0981fcc
> ("mm: limit growth of 3% hardcoded other user reserve"),
> but it's easy to reproduce it on older kernels:
> 1) set overcommit_memory sysctl to 2
> 2) mmap() large file multiple times (with VM_SHARED flag)
> 3) try to malloc() large amount of memory
> 
> It also can be reproduced on newer kernels, but miss-configured
> sysctl_user_reserve_kbytes is required.
> 
> Fix this issue by switching to signed arithmetic here.
> 
> Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andrew Shewmaker <agshew@gmail.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Cc: stable@vger.kernel.org
> ---
>  mm/mmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7f684d5..5aa8dfe 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -152,7 +152,7 @@ EXPORT_SYMBOL_GPL(vm_memory_committed);
>   */
>  int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>  {
> -	unsigned long free, allowed, reserve;
> +	long free, allowed, reserve;
>  
>  	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
>  			-(s64)vm_committed_as_batch * num_online_cpus(),
> @@ -220,7 +220,7 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>  	 */
>  	if (mm) {
>  		reserve = sysctl_user_reserve_kbytes >> (PAGE_SHIFT - 10);
> -		allowed -= min(mm->total_vm / 32, reserve);
> +		allowed -= min((long)mm->total_vm / 32, reserve);
>  	}
>  
>  	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
> -- 
> 2.1.0
> 
Makes sense to me. Please fix mm/nommu.c also.

If a caller passes in a big negative value for pages,
then vm_acct_memory() would decrement vm_committed_as, possibly 
causing percpu_counter_read_positive(&vm_committed_as) and
__vm_enough_memory to return 0. Maybe that's okay? Callers
won't be passing in a negative pages anyway. Is there a reason
to let them, though?

-Andrew

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