[PATCH] hugetlb: fix null-ptr-deref in hugetlb_vma_lock_write

[Mike Kravetz <mike.kravetz@oracle.com>]

The routine __vma_private_lock tests for the existence of a reserve map
associated with a private hugetlb mapping.  A pointer to the reserve map
is in vma->vm_private_data.  __vma_private_lock was checking the pointer
for NULL.  However, it is possible that the low bits of the pointer
could be used as flags.  In such instances, vm_private_data is not NULL
and not a valid pointer.  This results in the null-ptr-deref
reported by syzbot:

general protection fault, probably for non-canonical address 0xdffffc000000001d:
 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000000e8-0x00000000000000ef]
CPU: 0 PID: 5048 Comm: syz-executor139 Not tainted 6.6.0-rc7-syzkaller-00142-g88
8cf78c29e2 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 1
0/09/2023
RIP: 0010:__lock_acquire+0x109/0x5de0 kernel/locking/lockdep.c:5004
...
Call Trace:
 <TASK>
 lock_acquire kernel/locking/lockdep.c:5753 [inline]
 lock_acquire+0x1ae/0x510 kernel/locking/lockdep.c:5718
 down_write+0x93/0x200 kernel/locking/rwsem.c:1573
 hugetlb_vma_lock_write mm/hugetlb.c:300 [inline]
 hugetlb_vma_lock_write+0xae/0x100 mm/hugetlb.c:291
 __hugetlb_zap_begin+0x1e9/0x2b0 mm/hugetlb.c:5447
 hugetlb_zap_begin include/linux/hugetlb.h:258 [inline]
 unmap_vmas+0x2f4/0x470 mm/memory.c:1733
 exit_mmap+0x1ad/0xa60 mm/mmap.c:3230
 __mmput+0x12a/0x4d0 kernel/fork.c:1349
 mmput+0x62/0x70 kernel/fork.c:1371
 exit_mm kernel/exit.c:567 [inline]
 do_exit+0x9ad/0x2a20 kernel/exit.c:861
 __do_sys_exit kernel/exit.c:991 [inline]
 __se_sys_exit kernel/exit.c:989 [inline]
 __x64_sys_exit+0x42/0x50 kernel/exit.c:989
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Mask off low bit flags before checking for NULL pointer.  In addition,
the reserve map only 'belongs' to the OWNER (parent in parent/child
relationships) so also check for the OWNER flag.

Reported-by: syzbot+6ada951e7c0f7bc8a71e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-mm/00000000000078d1e00608d7878b@google.com/
Fixes: bf4916922c60 ("hugetlbfs: extend hugetlb_vma_lock to private VMAs")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 include/linux/hugetlb.h | 5 +----
 mm/hugetlb.c            | 7 +++++++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index d3acecc5db4b..236ec7b63c54 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1268,10 +1268,7 @@ static inline bool __vma_shareable_lock(struct vm_area_struct *vma)
 	return (vma->vm_flags & VM_MAYSHARE) && vma->vm_private_data;
 }
 
-static inline bool __vma_private_lock(struct vm_area_struct *vma)
-{
-	return (!(vma->vm_flags & VM_MAYSHARE)) && vma->vm_private_data;
-}
+bool __vma_private_lock(struct vm_area_struct *vma);
 
 /*
  * Safe version of huge_pte_offset() to check the locks.  See comments
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1169ef2f2176..6feb3e0630d1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1182,6 +1182,13 @@ static int is_vma_resv_set(struct vm_area_struct *vma, unsigned long flag)
 	return (get_vma_private_data(vma) & flag) != 0;
 }
 
+bool __vma_private_lock(struct vm_area_struct *vma)
+{
+	return !(vma->vm_flags & VM_MAYSHARE) &&
+		get_vma_private_data(vma) & ~HPAGE_RESV_MASK &&
+		is_vma_resv_set(vma, HPAGE_RESV_OWNER);
+}
+
 void hugetlb_dup_vma_private(struct vm_area_struct *vma)
 {
 	VM_BUG_ON_VMA(!is_vm_hugetlb_page(vma), vma);
-- 
2.41.0