Re: [PATCH] mm/mempolicy: Fix set_mempolicy_home_node() previous VMA pointer

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Thu, Sep 28, 2023 at 01:24:32PM -0400, Liam R. Howlett wrote:
> The two users of mbind_range() are expecting that mbind_range() will
> update the pointer to the previous VMA, or return an error.  However,
> set_mempolicy_home_node() does not call mbind_range() if there is no VMA
> policy.  The fix is to update the pointer to the previous VMA prior to
> continuing iterating the VMAs when there is no policy.
>
> Users may experience a WARN_ON() during VMA policy updates when updating
> a range of VMAs on the home node.
>
> Reported-by: Yikebaer Aizezi <yikebaer61@gmail.com>
> Closes: https://lore.kernel.org/linux-mm/CALcu4rbT+fMVNaO_F2izaCT+e7jzcAciFkOvk21HGJsmLcUuwQ@mail.gmail.com/
> Link: https://lore.kernel.org/linux-mm/CALcu4rbT+fMVNaO_F2izaCT+e7jzcAciFkOvk21HGJsmLcUuwQ@mail.gmail.com/
> Fixes: f4e9e0e69468 ("mm/mempolicy: fix use-after-free of VMA iterator")
> Cc: stable@vger.kernel.org
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  mm/mempolicy.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
>
> For completeness, here is the syzbot reproducer so that it is available
> from the mailing list:
>
> #define _GNU_SOURCE
>
> #include <endian.h>
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/syscall.h>
> #include <sys/types.h>
> #include <unistd.h>
>
> #ifndef __NR_set_mempolicy_home_node
> #define __NR_set_mempolicy_home_node 450
> #endif
>
> int main(void)
> {
>                 syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
>         syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul, /*prot=*/7ul, /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
>         syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
>
> *(uint64_t*)0x20000000 = 0xffffffffffffff81;
>         syscall(__NR_mbind, /*addr=*/0x20ffa000ul, /*len=*/0x4000ul, /*mode=*/2ul, /*nodemask=*/0x20000000ul, /*maxnode=*/7ul, /*flags=*/0ul);
>         syscall(__NR_mbind, /*addr=*/0x20ff9000ul, /*len=*/0x3000ul, /*mode=*/0ul, /*nodemask=*/0ul, /*maxnode=*/0ul, /*flags=*/0ul);
>         syscall(__NR_set_mempolicy_home_node, /*addr=*/0x20ffa000ul, /*len=*/0x4000ul, /*home_node=*/0ul, /*flags=*/0ul);
>         return 0;
> }
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 42b5567e3773..717d93c175f2 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1544,8 +1544,10 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
>  		 * the home node for vmas we already updated before.
>  		 */
>  		old = vma_policy(vma);
> -		if (!old)
> +		if (!old) {
> +			prev = vma;
>  			continue;
> +		}
>  		if (old->mode != MPOL_BIND && old->mode != MPOL_PREFERRED_MANY) {
>  			err = -EOPNOTSUPP;
>  			break;
> --
> 2.40.1
>

It feels a bit like the prev assignment is in the wrong place, however
looking at mbind_range() it's because of the possible merge that this is so
I guess. Just a pity the two bits get separated, as obviously it is at this
upper loop where the assignment of prev is most meaningful.

But definitely looks correct,

Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>