Re: [PATCH] Fix overflow in vma length when copying mmap on clone

[Hugh Dickins <hughd@google.com>]

On Tue, 24 Apr 2012, Siddhesh Poyarekar wrote:
> The vma length in dup_mmap is calculated and stored in a unsigned int,
> which is insufficient and hence overflows for very large maps (beyond
> 16TB). The following program demonstrates this:
> 
> \#include <stdio.h>
> \#include <unistd.h>
> \#include <sys/mman.h>
> 
> \#define GIG 1024 * 1024 * 1024L
> \#define EXTENT 16393
> 
> int main(void)
> {
>         int i, r;
>         void *m;
>         char buf[1024];
> 
>         for (i = 0; i < EXTENT; i++) {
>                 m = mmap(NULL, (size_t) 1 * 1024 * 1024 * 1024L,
>                          PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
> 
>                 if (m == (void *)-1)
>                         printf("MMAP Failed: %d\n", m);
>                 else
>                         printf("%d : MMAP returned %p\n", i, m);
> 
>                 r = fork();
> 
>                 if (r == 0) {
>                         printf("%d: successed\n", i);
>                         return 0;
>                 } else if (r < 0)
>                         printf("FORK Failed: %d\n", r);
>                 else if (r > 0)
>                         wait(NULL);
>         }
>         return 0;
> }
> 
> This trivial patch increases the storage size of the result to
> unsigned long, which should be sufficient for storing the difference
> between addresses.
> 
> Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>

Good catch, thank you, remarkable that's survived for so long.
For the patch,

Acked-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org

But I didn't (try very hard to) work out what your demo program shows
- though I am amused by your sense of humour in using %d for a pointer
there!  I wonder what setting of /proc/sys/vm/overcommit_memory is
needed for it to behave as you intend?

Personally, I wouldn't bother with the demo and describing it more fully,
I'd just be glad to get that fix in at last.

> ---
>  kernel/fork.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b9372a0..7acaee1 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -355,7 +355,8 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
>  		}
>  		charge = 0;
>  		if (mpnt->vm_flags & VM_ACCOUNT) {
> -			unsigned int len = (mpnt->vm_end - mpnt->vm_start) >> PAGE_SHIFT;
> +			unsigned long len;
> +			len = (mpnt->vm_end - mpnt->vm_start) >> PAGE_SHIFT;
>  			if (security_vm_enough_memory_mm(oldmm, len)) /* sic */
>  				goto fail_nomem;
>  			charge = len;
> -- 
> 1.7.7.6

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