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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

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